Greening of the Earth

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From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change

More than two decades ago, Idso (1986) published a small item in Nature advancing the idea that the aerial fertilization effect of the CO2 that is liberated by the burning of coal, gas and oil was destined to dramatically enhance the productivity of earth’s vegetation. In a little book he had published four years earlier (Idso, 1982), he had predicted that “CO2 effects on both the managed and unmanaged biosphere will be overwhelmingly positive.” In a monograph based on a lecture he gave nine years later (Idso, 1995), he said “we appear to be experiencing the initial stages of what could truly be called a rebirth of the biosphere, the beginnings of a biological rejuvenation that is without precedent in all of human history.”

In light of the fact that Idso’s worldview is nearly the exact opposite of the apocalyptic vision promulgated by the IPCC, it is instructive to see what real-world observations reveal about the matter. In this section we review studies that show a CO2-induced greening of Africa, Asia, Europe, North America, the oceans, and the entire globe.

Additional information on this topic, including reviews not discussed here, can be found at http://www.co2science.org/subject/g/subject_g.php under the heading Greening of the Earth.


Contents

Africa

In an article by Fred Pearce that was posted on the website of New Scientist magazine on 16 September 2002 titled “Africa’s deserts are in ‘spectacular’ retreat,” we were told the story of vegetation reclaiming great tracts of barren land across the entire southern edge of the Sahara. This information likely came as a bit of a surprise to many, since the United Nations Environment Program had reported to the World Summit on Sustainable Development in Johannesburg, South Africa in August of that year that over 45 percent of the continent was experiencing severe desertification. The world of nature, however, told a vastly different story.

Pearce began by stating “the southern Saharan desert is in retreat, making farming viable again in what were some of the most arid parts of Africa,” noting that “Burkina Faso, one of the West African countries devastated by drought and advancing deserts 20 years ago, is growing so much greener that families who fled to wetter coastal regions are starting to go home.”

The good news was not confined to Burkina Faso. “Vegetation,” according to Pearce, “is ousting sand across a swathe of land stretching from Mauritania on the shores of the Atlantic to Eritrea 6000 kilometers away on the Red Sea coast.” Besides being widespread in space, the greening was widespread in time, having been happening since at least the mid-1980s.

Quoting Chris Reij of the Free University of Amsterdam, Pearce wrote that “aerial photographs taken in June show ‘quite spectacular regeneration of vegetation’ in northern Burkina Faso.” The data indicated the presence of more trees for firewood and more grassland for livestock. In addition, a survey that Reij was collating showed, according to Pearce, “a 70% increase in yields of local cereals such as sorghum and millet in one province in recent years.” Also studying the area was Kjeld Rasmussen of the University of Copenhagen, who reported that since the 1980s there had been a “steady reduction in bare ground” with “vegetation cover, including bushes and trees, on the increase on the dunes.”

Pearce also reported on the work of a team of geographers from Britain, Sweden and Denmark that had spent much of the prior summer analyzing archived satellite images of the Sahel. Citing Andrew Warren of University College London as a source of information on this study, he said the results showed “that ‘vegetation seems to have increased significantly’ in the past 15 years, with major regrowth in southern Mauritania, northern Burkina Faso, north-western Niger, central Chad, much of Sudan and parts of Eritrea.”

Should these findings take us by surprise? Not in the least, as Nicholson et al. (1998) reported in a study of a series of satellite images of the Central and Western Sahel that were taken from 1980 to 1995, they could find no evidence of any overall expansion of deserts and no drop in the rainfall use efficiency of native vegetation. In addition, in a satellite study of the entire Sahel from 1982 to 1990, Prince et al. (1998) detected a steady rise in rainfall use efficiency, suggesting that plant productivity and coverage of the desert had increased during this period.

That the greening phenomenon has continued apace is borne out by the study of Eklundh and Olsson (2003), who analyzed Normalized Difference Vegetation Index (NDVI) data obtained from the U.S. National Oceanic and Atmospheric Administration’s satellite-borne Advanced Very High Resolution Radiometer whenever it passed over the African Sahel for the period 1982-2000. As they describe their findings, “strong positive change in NDVI occurred in about 22% of the area, and weak positive change in 60% of the area,” while “weak negative change occurred in 17% of the area, and strong negative change in 0.6% of the area.” In addition, they report that “integrated NDVI has increased by about 80% in the areas with strong positive change,” while in areas with weak negative change, “integrated NDVI has decreased on average by 13%.” The primary story told by these data, therefore, is one of strong positive trends in NDVI for large areas of the African Sahel over the last two decades of the twentieth century. Eklundh and Olsson conclude that the “increased vegetation, as suggested by the observed NDVI trend, could be part of the proposed tropical sink of carbon.”

Due to the increase in vegetation over the past quarter-century in the Sahel, the African region was recently featured in a special issue of the Journal of Arid Environments titled “The ‘Greening’ of the Sahel.” Therein, Anyamba and Tucker (2005) describe their development of an NDVI history of the region for the period 1981-2003. Comparing this history with the precipitation history of the Sahel developed by Nicholson (2005), they found that “the persistence and spatial coherence of drought conditions during the 1980s is well represented by the NDVI anomaly patterns and corresponds with the documented rainfall anomalies across the region during this time period.” In addition, they report that “the prevalence of greener than normal conditions during the 1990s to 2003 follows a similar increase in rainfall over the region during the last decade.”

In another analysis of NDVI and rainfall data in the same issue of the Journal of Arid Environments, Olsson et al. (2005) report finding “a consistent trend of increasing vegetation greenness in much of the region,” which they describe as “remarkable.” They say increasing rainfall over the last few years “is certainly one reason” for the greening phenomenon. However, they find the increase in rainfall “does not fully explain” the increase in greenness.

For one thing, the three Swedish scientists note that “only eight out of 40 rainfall observations showed a statistically significant increase between 1982-1990 and 1991-1999.” In addition, they report that “further analysis of this relationship does not indicate an overall relationship between rainfall increase and vegetation trend.” So what else could be driving the increase in greenness?

Olsson et al. suggest that “another potential explanation could be improved land management, which has been shown to cause similar changes in vegetation response elsewhere (Runnstrom, 2003).” However, in more detailed analyses of Burkina Faso and Mali, where production of millet rose by 55 percent and 35 percent, respectively, since 1980, they could find “no clear relationship” between agricultural productivity and NDVI, which argues against the land management explanation.

A third speculation of Olsson et al. is that the greening of the Sahel could be caused by increasing rural-to-urban migration. In this scenario, widespread increases in vegetation occur as a result of “reduced area under cultivation,” due to a shortage of rural laborers, and/or “increasing inputs on cropland,” such as seeds, machinery and fertilizers made possible by an increase in money sent home to rural households by family members working in cities. However, Olsson et al. note that “more empirical research is needed to verify this [hypothesis].”

About the only thing left is what Idso (1982, 1986, 1995) has suggested, i.e., that the aerial fertilization effect of the ongoing rise in the air’s CO2 concentration (which greatly enhances vegetative productivity) and its anti-transpiration effect (which enhances plant water-use efficiency and enables plants to grow in areas that were once too dry for them) are the major players in the greening phenomenon. Whatever was the reason for the greening of the Sahel over the past quarter-century, it is clear that in spite of what the IPCC claims were unprecedented increases in anthropogenic CO2 emissions and global temperatures, the Sahel experienced an increase in vegetative cover that was truly, as Olsson et al. write, “remarkable.”

Disputing the mainstream opinion that climate change will negatively impact Africa, Kim et al. (2014) determined that biases have impacted modelling of African climate change, as "the systematic variations in [regional climate model data] skill may indicate common weaknesses in physics parameterizations used in these models."

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/g/africagreen.php.

Asia

We begin a review of Asia with the modeling work of Liu et al. (2004), who derived detailed estimates of the economic impact of predicted climate change on agriculture in China, utilizing county-level agricultural, climate, social, economic and edaphic data for 1275 agriculture-dominated counties for the period 1985-1991, together with the outputs of three general circulation models of the atmosphere that were based on five different scenarios of anthropogenic CO2-induced climate change that yielded a mean countrywide temperature increase of 3.0°C and a mean precipitation increase of 3.9 percent for the 50-year period ending in AD 2050. In doing so, they determined that “all of China would benefit from climate change in most scenarios.” In addition, they state that “the effects of CO2 fertilization should [also] be included, for some studies indicate that this may produce a significant increase in yield.” The significance of these findings is readily grasped when it is realized, in Liu et al.’s words, that “China’s agriculture has to feed more than one-fifth of the world’s population, and, historically, China has been famine prone.” They report that “as recently as the late 1950s and early 1960s a great famine claimed about thirty million lives (Ashton et al., 1984; Cambridge History of China, 1987).”

Moving from agro-ecosystems to natural ones, Su et al. (2004) used an ecosystem process model to explore the sensitivity of the net primary productivity (NPP) of an oak forest near Beijing (China) to the global climate changes projected to result from a doubling of the atmosphere’s CO2 concentration from 355 to 710 ppm. The results of this work suggested that the aerial fertilization effect of the specified increase in the air’s CO2 content would raise the forest’s NPP by 14.0 percent, that a concomitant temperature increase of 2°C would boost the NPP increase to 15.7 percent, and that adding a 20 percent increase in precipitation would push the NPP increase to 25.7 percent. They calculated that a 20 percent increase in precipitation and a 4°C increase in temperature would also boost the forest’s NPP by 25.7 percent.

Grunzweig et al. (2003) tell the tale of the Yatir forest, a 2,800-hectare stand of Aleppo and other pine trees, that had been planted some 35 years earlier at the edge of the Negev Desert in Israel. An intriguing aspect of this particular forest, which they characterize as growing in poor soil of only 0.2 to 1.0 meter’s depth above chalk and limestone, is that although it is located in an arid part of Asia that receives less annual precipitation than all of the other scores of FluxNet stations in the global network of micrometeorological tower sites that use eddy covariance methods to measure exchanges of CO2, water vapor and energy between terrestrial ecosystems and the atmosphere (Baldocchi et al., 2001), the forest’s annual net ecosystem CO2 exchange was just as high as that of many high-latitude boreal forests and actually higher than that of most temperate forests. Grunzweig et al. note that the increase in atmospheric CO2 concentration that has occurred since pre-industrial times should have improved water use efficiency (WUE) in most plants by increasing the ratio of CO2 fixed to water lost via evapotranspiration. They report that “reducing water loss in arid regions improves soil moisture conditions, decreases water stress and extends water availability,” which “can indirectly increase carbon sequestration by influencing plant distribution, survival and expansion into water-limited environments.”

That this hypothesis is correct has been demonstrated by Leavitt et al. (2003) within the context of the long-term atmospheric CO2 enrichment experiment of Idso and Kimball (2001) on sour orange trees. It has also been confirmed in nature by Fang (2003), who obtained identical (to the study of Leavitt et al.) CO2-induced WUE responses for 23 groups of naturally occurring trees scattered across western North America over the period 1800-1985, which response, Fang concludes, “would have caused natural trees in arid environments to grow more rapidly, acting as a carbon sink for anthropogenic CO2,” which is exactly what Grunzweig et al. found to be happening in the Yatir forest on the edge of the Negev Desert.

Based primarily on satellite-derived Normalized Difference Vegetation Index (NDVI) data, Zhou et al. (2001) found that from July 1981 to December 1999, between 40 and 70° N latitude, there was a persistent increase in growing season vegetative productivity in excess of 12 percent over a broad contiguous swath of Asia stretching from Europe through Siberia to the Aldan plateau, where almost 58 percent of the land is forested. And in a companion study, Bogaert et al. (2002) determined that this productivity increase occurred at a time when this vast Asian region showed an overall warming trend “with negligible occurrence of cooling.”

In another study that included a portion of Europe, Lapenis et al. (2005) analyzed trends in forest biomass in all 28 ecoregions of the Russian territory, based on data collected from 1953 to 2002 within 3196 sample plots comprised of about 50,000 entries, which database, in their words, “contains all available archived and published data.” This work revealed that over the period 1961-1998, as they describe it, “aboveground wood, roots, and green parts increased by 4%, 21%, and 33%, respectively,” such that “the total carbon density of the living biomass stock of the Russian forests increased by ~9%.” They also report there was a concomitant increase of ~11 percent in the area of Russian forests. In addition, the team of U.S., Austrian and Russian scientists reported that “within the range of 50-65° of latitude [the range of 90 percent of Russian forests], the relationship between biomass density and the area-averaged NDVI is very close to a linear function, with a slope of ~1,” citing the work of Myneni et al. (2001). Therefore, as they continue, “changes in the carbon density of live biomass in Russian forests occur at about the same rate as the increase in the satellite-based estimate in the seasonally accumulated NDVI,” which observation strengthens the findings of all satellite-based NDVI studies.

Returning to China for several concluding reports, we begin with the work of Brogaard et al. (2005), who studied the dry northern and northwestern regions of the country—including the Inner Mongolia Autonomous Region (IMAR)—which had been thought to have experienced declining vegetative productivity over the past few decades due to “increasing livestock numbers, expansion of cultivated land on erosive soils and the gathering of fuel wood and herb digging,” which practices were believed to have been driven by rising living standards, which in combination with a growing population were assumed to have increased the pressure on these marginal lands. In the case of increasing grazing, for example, Brogaard et al. note that the total number of livestock in the IMAR increased from approximately 46 million head in 1980 to about 71 million in 1997.

To better assess the seriousness of this supposedly “ongoing land degradation process,” as they describe it, the researchers adapted a satellite-driven parametric model, originally developed for Sahelian conditions, to the central Asian steppe region of the IMAR by including “additional stress factors and growth efficiency computations.” The applied model, in their words, “uses satellite sensor-acquired reflectance in combination with climate data to generate monthly estimates of gross primary production.” To their great surprise, this work revealed that “despite a rapid increase in grazing animals on the steppes of the IMAR for the 1982-1999 period,” their model estimates did “not indicate declining biological production.”

Clearly, some strong positive influence compensated for the increased human and animal pressures on the lands of the IMAR over the period of Brogaard et al.’s study. In this regard, they mention the possibility of increasing productivity on the agricultural lands of the IMAR, but they note that crops are grown on “only a small proportion of the total land area.” Other potential contributing factors they mention are “an increase in precipitation, as well as afforestation projects.” Two things they do not mention are the aerial fertilization effect and the transpiration-reducing effect of the increase in the air’s CO2 concentration that was experienced over the study period. Applied together, the sum of these positive influences (and possibly others that remain unknown) was demonstrably sufficient to keep plant productivity from declining in the face of greatly increasing animal and human pressures on the lands of the IMAR from 1982 to 1999.

Piao et al. (2005a) used a time series of NDVI data from 1982 to 1999, together with precipitation and temperature data, to investigate variations of desert area in China by “identifying the climatic boundaries of arid area and semiarid area, and changes in NDVI in these areas.” In doing so, they discovered that “average rainy season NDVI in arid and semiarid regions both increased significantly during the period 1982-1999.” Specifically, they found that the NDVI increased for 72.3 percent of total arid regions and for 88.2 percent of total semiarid regions, such that the area of arid regions decreased by 6.9 percent and the area of semiarid regions decreased by 7.9 percent. They also report that by analyzing Thematic Mapper satellite images, “Zhang et al. (2003) documented that the process of desertification in the Yulin area, Shannxi Province showed a decreased trend between 1987 and 1999,” and that “according to the national monitoring data on desertification in western China (Shi, 2003), the annual desertification rate decreased from 1.2% in the 1950s to -0.2% at present.”

Further noting that “variations in the vegetation coverage of these regions partly affect the frequency of sand-dust storm occurrence (Zou and Zhai, 2004),” Piao et al. concluded that “increased vegetation coverage in these areas will likely fix soil, enhance its anti-wind-erosion ability, reduce the possibility of released dust, and consequently cause a mitigation of sand-dust storms.” They also reported that “recent studies have suggested that the frequencies of strong and extremely strong sand-dust storms in northern China have significantly declined from the early 1980s to the end of the 1990s (Qian et al., 2002; Zhao et al., 2004).”

Piao et al. (2006) investigated vegetation net primary production (NPP) derived from a carbon model (Carnegie-Ames-Stanford approach, CASA) and its interannual change in the Qinghai-Xizang (Tibetan) Plateau using 1982-1999 NDVI data and paired ground-based information on vegetation, climate, soil, and solar radiation. This work revealed that over the entire study period, NPP rose at a mean annual rate of 0.7 percent. However, Piao et al. report that “the NPP trends in the plateau over the two decades were divided into two distinguished periods: without any clear trend from 1982 to 1990 and significant increase from 1991 to 1999.”

The three researchers say their findings suggest that “vegetation growth on the plateau in the 1990s has been much enhanced compared to that in [the] 1980s, consistent with the trend in the northern latitudes indicated by Schimel et al. (2001).” In addition, they say that “previous observational and NPP modeling studies have documented substantial evidence that terrestrial photosynthetic activity has increased over the past two to three decades in the middle and high latitudes in the Northern Hemisphere,” and that “satellite-based NDVI data sets for the period of 1982-1999 also indicate consistent trends of NDVI increase,” citing multiple references in support of each of these statements. Piao et al.’s findings, therefore, add to the growing body of evidence that reveals a significant “greening of the earth” is occurring.

Applying the same techniques, Fang et al. (2003) looked at the whole of China, finding that its terrestrial NPP increased by 18.7 percent between 1982 and 1999. Referring to this result as “an unexpected aspect of biosphere dynamics,” they say that this increase “is much greater than would be expected to result from the fertilization effect of elevated CO2, and also greater than expected from climate, based on broad geographic patterns.” From 1982 to 1999, the atmosphere’s CO2 concentration rose by approximately 27.4 ppm. The aerial fertilization effect of this CO2 increase could be expected to have increased the NPP of the conglomerate of forest types found in China by about 7.3 percent. (See the procedures and reasoning described in a CO2 Science editorial, September 18, 2002, http://www.co2science.org/articles/V5/N38/ EDIT.php). But this increase is only a part of the total NPP increase we could expect, for Fang et al. note that “much of the trend in NPP appeared to reflect a change towards an earlier growing season,” which was driven by the 1.1°C increase in temperature they found to have occurred in their region of study between 1982 and 1999.

Following this lead, we learn from the study of White et al. (1999)—which utilized 88 years of data (1900-1987) that were obtained from 12 different locations within the eastern U.S. deciduous forest that stretches from Charleston, SC (32.8°N latitude) to Burlington, VT (44.5°N latitude)—that a 1°C increase in mean annual air temperature increases the length of the forest’s growing season by approximately five days. In addition, White et al. determined that a one-day extension in growing season length increased the mean forest NPP of the 12 sites they studied by an average of 1.6 percent. Hence, we could expect an additional NPP increase due to the warming-induced growing season expansion experienced in China from 1982 to 1999 of 1.6 percent/day x 5 days = 8.0 percent, which brings the total CO2-induced plus warming-induced increase in NPP to 15.3 percent.

Last, we note there is a well-documented positive synergism between increasing air temperature and CO2 concentration (Idso and Idso, 1994), such that the 1°C increase in temperature experienced in China between 1982 and 1999 could easily boost the initial CO2-induced 7.3 percent NPP enhancement to the 10.7 percent enhancement that when combined with the 8.0 percent enhancement caused by the warming-induced increase in growing season length would produce the 18.7 percent increase in NPP detected in the satellite data.

Research by Zhao et al. (2011) used satellite-sensed Normalized Difference Vegetation Index (NDVI) data from 1982 to 2003 to investigate spatio-temporal changes in vegetation growth in the grassland-oasis-desert complex of northwest China. Over the 22 years of their study, the annual mean temperature increased 0.06°C/year and about 30% of the total vegetated area showed an annual increase of 0.7% in growing season NDVI. The global air temperature change could have been responsible for the shifts in regional precipitation during this period, which appears to have driven the observed shifts in NDVI.

In view of these observations, the findings of Fang et al. are seen to be right in line with what would be expected to result from the increases in air temperature and atmospheric CO2 concentration that occurred between 1982 and 1999 in China: a stimulated terrestrial biosphere that is growing ever more productive with each passing year. This is the true observed consequence of rising CO2 and temperature, and it is about as far removed as one can get from the negative scenarios offered by the IPCC.

Analyzing the same set of data still further, Piao et al. (2005b) say their results suggest that “terrestrial NPP in China increased at a rate of 0.015 Pg C yr-1 over the period 1982-1999, corresponding to a total increase of 18.5%, or 1.03% annually.” They also found that “during the past 2 decades the amplitude of the seasonal curve of NPP has increased and the annual peak NPP has advanced,” which they say “may indirectly explain the enhanced amplitude and advanced timing of the seasonal cycle of atmospheric CO2 concentration (Keeling et al., 1996),” the former of which phenomena they further suggest “was probably due to the rise in atmospheric CO2 concentration, elevated temperature, and increased atmospheric N and P deposition,” while the latter phenomenon they attribute to “advanced spring onset and extended autumn growth owing to climate warming.” We are in basic agreement on most of these points, but note that the advanced onset of what may be called biological spring is also fostered by the enhancement of early spring growth that is provided by the ongoing rise in the air’s CO2 concentration.

Citing a total of 20 scientific papers at various places in the following sentence from their research report, Piao et al. conclude that “results from observed atmospheric CO2 and O2 concentrations, inventory data, remote sensing data, and carbon process models have all suggested that terrestrial vegetation NPP of the Northern Hemisphere has increased over the past 2 decades and, as a result, the northern terrestrial ecosystems have become important sinks for atmospheric CO2.”


In conclusion, the historical increases in the atmosphere’s CO2 concentration and temperature have fostered a significant greening of the earth, including that observed throughout the length and breadth of Asia. It would appear that the climatic change claimed by the IPCC to have been experienced by the globe over the latter part of the twentieth century either did not occur or was dwarfed by opposing phenomena that significantly benefited China, as its lands grew ever greener during this period and its increased vegetative cover helped to stabilize its soils and throw feared desertification into reverse.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/g/asiagreen.php.


Europe

Allen et al. (1999) analyzed sediment cores from a lake in southern Italy and from the Mediterranean Sea, developing high-resolution climate and vegetation data sets for this region over the last 102,000 years. These materials indicated that rapid changes in vegetation were well correlated with rapid changes in climate, such that complete shifts in natural ecosystems would sometimes occur over periods of less than 200 years. Over the warmest portion of the record (the Holocene), the total organic carbon content of the vegetation reached its highest level, more than doubling values experienced over the rest of the record, while other proxy indicators revealed that during the more productive woody-plant period of the Holocene, the increased vegetative cover also led to less soil erosion. The results of this study demonstrate that the biosphere can successfully respond to rapid changes in climate. As the 15 researchers involved in the work put it, “the biosphere was a full participant in these rapid fluctuations, contrary to widely held views that vegetation is unable to change with such rapidity.” Furthermore, their work revealed that warmer was always better in terms of vegetative productivity.

Osborne et al. (2000) used an empirically based mechanistic model of Mediterranean shrub vegetation to address two important questions: (1) Has recent climate change, especially increased drought, negatively impacted Mediterranean shrublands? and (2) Has the historical increase in the air’s CO2 concentration modified this impact? The data-based model they employed suggests that the warming and reduced precipitation experienced in the Mediterranean area over the past century should have had negative impacts on net primary production and leaf area index. When the measured increase in atmospheric CO2 concentration experienced over the period was factored into the calculation, however, these negative influences were overpowered, with the net effect that both measures of vegetative prowess increased: net primary productivity by 25 percent and leaf area index by 7 percent. These results, in their words, “indicate that the recent rise in atmospheric CO2 may already have had significant impacts on productivity, structure and water relations of sclerophyllous shrub vegetation, which tended to offset the detrimental effects of climate change in the region.”

How can we relate this observation to climate change predictions for the earth as a whole? For a nominal doubling of the air’s CO2 concentration from 300 to 600 ppm, earth’s mean surface air temperature is predicted by current climate models to rise by approximately 3°C, which equates to a temperature rise of 0.01°C per ppm CO2. In the case of the Mediterranean region here described, the temperature rise over the past century was quoted by Osborne et al. as being 0.75°C, over which period of time the air’s CO2 concentration rose by approximately 75 ppm, for an analogous climate response of exactly the same value: 0.01°C per ppm CO2.

With respect to model-predicted changes in earth’s precipitation regime, a doubling of the air’s CO2 content is projected to lead to a modest intensification of the planet’s hydrologic cycle. In the case of the Mediterranean region over the last century, however, there has been a recent tendency toward drier conditions. Hence, the specific case investigated by Osborne et al. represents a much-worse-case scenario than what is predicted by current climate models for the earth as a whole. Nevertheless, the area’s vegetation has done even better than it did before the climatic change, thanks to the over-powering beneficial biological effects of the concurrent rise in the air’s CO2 content.

Cheddadi et al. (2001) employed a standard biogeochemical model (BIOME3)—which uses monthly temperature and precipitation data, certain soil characteristics, cloudiness, and atmospheric CO2 concentration as inputs—to simulate the responses of various biomes in the region surrounding the Mediterranean Sea to changes in both climate (temperature and precipitation) and the air’s CO2 content. Their first step was to validate the model for two test periods: the present and 6000 years before present (BP). Recent instrumental records provided actual atmospheric CO2, temperature and precipitation data for the present period; while pollen data were used to reconstruct monthly temperature and precipitation values for 6000 years BP, and ice core records were used to determine the atmospheric CO2 concentration of that earlier epoch. These efforts suggested that winter temperatures 6000 years ago were about 2°C cooler than they are now, that annual rainfall was approximately 200 mm less than today, and that the air’s CO2 concentration averaged 280 ppm, which is considerably less than the value of 345 ppm the researchers used to represent the present, i.e., the mid-point of the period used for calculating 30-year climate normals at the time they wrote their paper. Applying the model to these two sets of conditions, they demonstrated that “BIOME3 can be used to simulate … the vegetation distribution under … different climate and [CO2] conditions than today,” where [CO2] is the abbreviation they use to represent “atmospheric CO2 concentration.”

Cheddadi et al.’s next step was to use their validated model to explore the vegetative consequences of an increase in anthropogenic CO2 emissions that pushes the air’s CO2 concentration to a value of 500 ppm and its mean annual temperature to a value 2°C higher than today’s mean value. The basic response of the vegetation to this change in environmental conditions was “a substantial southward shift of Mediterranean vegetation and a spread of evergreen and conifer forests in the northern Mediterranean.”

More specifically, in the words of the researchers, “when precipitation is maintained at its present-day level, an evergreen forest spreads in the eastern Mediterranean and a conifer forest in Turkey.” Current xerophytic woodlands in this scenario become “restricted to southern Spain and southern Italy and they no longer occur in southern France.” In northwest Africa, on the other hand, “Mediterranean xerophytic vegetation occupies a more extensive territory than today and the arid steppe/desert boundary shifts southward,” as each vegetation zone becomes significantly more verdant than it is currently.

What is the basis for these positive developments? Cheddadi et al. say “the replacement of xerophytic woodlands by evergreen and conifer forests could be explained by the enhancement of photosynthesis due to the increase of [CO2].” Likewise, they note that “under a high [CO2] stomata will be much less open which will lead to a reduced evapotranspiration and lower water loss, both for C3 and C4 plants,” adding that “such mechanisms may help plants to resist long-lasting drought periods that characterize the Mediterranean climate.”

Contrary to what is often predicted for much of the world’s moisture-challenged lands, therefore, the authors were able to report that “an increase of [CO2], jointly with an increase of ca. 2°C in annual temperature would not lead to desertification on any part of the Mediterranean unless annual precipitation decreased drastically,” where they define a drastic decrease as a decline of 30 percent or more. Equally important in this context is the fact that Hennessy et al. (1997) have indicated that a doubling of the air’s CO2 content would in all likelihood lead to a 5 to 10 percent increase in annual precipitation at Mediterranean latitudes, which is also what is predicted for most of the rest of the world. Hence, the results of the present study—where precipitation was held constant—may validly be considered to be a worst-case scenario, with the true vegetative response being even better than the good-news results reported by Cheddadi et al., even when utilizing what we believe to be erroneously inflated global warming predictions.

Julien et al. (2006) “used land surface temperature (LST) algorithms and NDVI [Normalized Difference Vegetation Index] values to estimate changes in vegetation in the European continent between 1982 and 1999 from the Pathfinder AVHRR [Advanced Very High Resolution Radiometer] Land (PAL) dataset.” This program revealed that arid and semi-arid areas (Northern Africa, Southern Spain and the Middle East) have seen their mean LST increase and NDVI decrease, while temperate areas (Western and Central Europe) have suffered a slight decrease in LST but a more substantial increase in NDVI, especially in Germany, the Czech Republic, Poland and Belarus. In addition, parts of continental and Northern Europe have experienced either slight increases or decreases in NDVI while LST values have decreased. Considering the results in their totality, the Dutch and Spanish researchers concluded that, over the last two decades of the twentieth century, “Europe as a whole has a tendency to greening,” and much of it is “seeing an increase in its wood land proportion.”

Working in the Komi Republic in the northeast European sector of Russia, Lopatin et al. (2006) (1) collected discs and cores from 151 Siberian spruce trees and 110 Scots pines from which they developed ring-width chronologies that revealed yearly changes in forest productivity, (2) developed satellite-based time series of NDVI for the months of June, July, August over the period 1982-2001, (3) correlated their site-specific ring-width-derived productivity histories with same-site NDVI time series, (4) used the resulting relationship to establish six regional forest productivity histories for the period 1982-2001, and (5) compared the six regional productivity trends over this period with corresponding-region temperature and precipitation trends. For all six vegetation zones of the Komi Republic, this work indicated that the 1982-2001 trends of integrated NDVI values from June to August were positive, and that the “increase in productivity reflected in [the] NDVI data [was] maximal on the sites with increased temperature and decreased precipitation.”

In discussing their findings, the three scientists state that “several studies (Riebsame et al., 1994; Myneni et al., 1998; Vicente-Serrano et al., 2004) have shown a recent increase in vegetation cover in different world ecosystems.” What is special about their study, as they describe it, is that “in Europe, most forests are managed, except for those in northwestern Russia [the location of their work], where old-growth natural forests are dominant (Aksenov et al., 2002).” Consequently, and because of their positive findings, they say we can now conclude that “productivity during recent decades also increased in relatively untouched forests,” where non-management-related “climate change with lengthening growing season, increasing CO2 and nitrogen deposition” are the primary determinants of changes in forest productivity.

Alcaraz-Segura et al. (2008) employed satellite-derived normalized difference vegetation index (NDVI) data to “evaluate the impact of global environmental change on terrestrial ecosystem functioning of [Spain's] national parks,” which provides a sound basis for determining what is to be expected throughout much of Europe. The researchers report that "most parks showed areas with positive NDVI trends that tended to have higher proportions of Mediterranean coniferous and mixed forests, oro-Mediterranean scrublands, heathlands, maquis and garrigues.” Thus, they conclude that, in terms of vegetation greenness, forests are changing in a directional way such that "a large part of the Spanish National Parks is intercepting more photosynthetically active radiation than in the past."

In conclusion, this brief review of pertinent studies conducted in Europe strongly contradicts today’s obsession with the ongoing rise in the atmosphere’s CO2 content, as well as the many environmental catastrophes it has been predicted to produce. The results of rising CO2 concentrations and temperatures in the twentieth century were overwhelmingly positive.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/g/europegreen.php.

North America

In a paper titled “Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999,” Zhou et al. (2001) determined that the magnitude of the satellite-derived normalized difference vegetation index (NDVI) rose by 8.44 percent in North America over this period. Noting that the NDVI “can be used to proxy the vegetation’s responses to climate changes because it is well correlated with the fraction of photosynthetically active radiation absorbed by plant canopies and thus leaf area, leaf biomass, and potential photosynthesis,” they went on to suggest that the increases in plant growth and vitality implied by their NDVI data were primarily driven by concurrent increases in near-surface air temperature, although temperatures may have actually declined throughout the eastern part of the United States over the period of their study.

Zhou et al.’s attribution of this “greening” of the continent to increases in near-surface air temperature was challenged by Ahlbeck (2002), who suggested that the observed upward trend in NDVI was primarily driven by the increase in the air’s CO2 concentration, and that fluctuations in temperature were primarily responsible for variations about the more steady upward trend defined by the increase in CO2. In replying to this challenge, Kaufmann et al. (2002) claimed Ahlbeck was wrong and reaffirmed their initial take on the issue. We believe it was Ahlbeck who was “clearly the ‘more correct’ of the two camps.” (See the discussion in CO2 Science at www.co2science.org/articles/V5/N38/EDIT.php.)

About the same time, Hicke et al. (2002) computed net primary productivity (NPP) over North America for the years 1982-1998 using the Carnegie-Ames-Stanford Approach (CASA) carbon cycle model, which was driven by a satellite NDVI record at 8-km spatial resolution. This effort revealed that NPP increases of 30 percent or more occurred across the continent from 1982 to 1998. During this period, the air’s CO2 concentration rose by 25.74 ppm, as calculated from the Mauna Loa data of Keeling and Whorf (1998), which amount is 8.58 percent of the 300 ppm increase often used in experiments on plant growth responses to atmospheric CO2 enrichment. Consequently, for herbaceous plants that display NPP increases of 30-40 percent in response to a 300-ppm increase in atmospheric CO2 concentration, the CO2-induced NPP increase experienced between 1982 and 1998 would be expected to have been 2.6-3.4 percent. Similarly, for woody plants that display NPP increases of 60-80 percent in response to a 300-ppm increase in atmospheric CO2 (Saxe et al., 1998; Idso and Kimball, 2001), the expected increase in productivity between 1982 and 1998 would have been 5.1-6.9 percent. Since both of these NPP increases are considerably less that the 30 percent or more observed by Hicke et al., additional factors must have helped to stimulate NPP over this period, some of which may have been concomitant increases in precipitation and air temperature, the tendency for warming to lengthen growing seasons and enhance the aerial fertilization effect of rising CO2 concentrations, increasingly intensive crop and forest management, increasing use of genetically improved plants, the regrowth of forests on abandoned cropland, and improvements in agricultural practices such as irrigation and fertilization. Whatever the mix might have been, one thing is clear: Its effect was overwhelmingly positive.

In a study based on a 48-year record derived from an average of 17 measurements per year, Raymond and Cole (2003) demonstrated that the export of alkalinity, in the form of bicarbonate ions, from the Mississippi River to the Gulf of Mexico had increased by approximately 60 percent since 1953. “This increased export,” as they described it, was “in part the result of increased flow resulting from higher rainfall in the Mississippi basin,” which had led to a 40 percent increase in annual Mississippi River discharge to the Gulf of Mexico over the same time period. The remainder, however, had to have been due to increased rates of chemical weathering of soil minerals. What factors might have been responsible for this phenomenon? The two researchers noted that potential mechanisms included “an increase in atmospheric CO2, an increase [in] rainwater throughput, or an increase in plant and microbial production of CO2 and organic acids in soils due to biological responses to increased rainfall and temperature.” Unfortunately, they forgot to mention the increase in terrestrial plant productivity that is produced by the increase in the aerial fertilization effect provided by the historical rise in the air’s CO2 content, which also leads to “an increase in plant and microbial production of CO2 and organic acids in soils.” This phenomenon should have led to an increase in Mississippi River alkalinity equivalent to that which they had observed since 1953.

In a study using data obtained from dominant stands of loblolly pine plantations growing at 94 locations spread across the southeastern United States, Westfall and Amateis (2003) employed mean height measurements made at three-year intervals over a period of 15 years to calculate a site index related to the mean growth rate for each of five three-year periods, which index would be expected to increase monotonically if growth rates were being enhanced above normal by some monotonically increasing factor that promotes growth. This work revealed, in their words, that “mean site index over the 94 plots consistently increased at each remeasurement period,” which would suggest, as they further state, that “loblolly pine plantations are realizing greater than expected growth rates,” and, we would add, that the growth rate increases are growing larger and larger with each succeeding three-year period. As to what could be causing the monotonically increasing growth rates of loblolly pine trees over the entire southeastern United States, Westfall and Amateis named increases in temperature and precipitation in addition to rising atmospheric CO2 concentrations. However, they report that a review of annual precipitation amounts and mean ground surface temperatures showed no trends in these factors over the period of their study. They also suggested that if increased nitrogen deposition were the cause, “such a factor would have to be acting on a regional scale to produce growth increases over the range of study plots.” Hence, they tended to favor the ever-increasing aerial fertilization effect of atmospheric CO2 enrichment as being responsible for the accelerating pine tree growth rates.

Returning to satellite studies, Lim et al. (2004) correlated the monthly rate of relative change in NDVI, which they derived from advanced very high resolution radiometer data, with the rate of change in atmospheric CO2 concentration during the natural vegetation growing season within three different eco-region zones of North America (Arctic and Sub-Arctic Zone, Humid Temperate Zone, and Dry and Desert Zone, which they further subdivided into 17 regions) over the period 1982-1992, after which they explored the temporal progression of annual minimum NDVI over the period 1982-2001 throughout the eastern humid temperate zone of North America. The result of these operations was that in all of the regions but one, according to the researchers, “δCO2 was positively correlated with the rate of change in vegetation greenness in the following month, and most correlations were high,” which they say is “consistent with a CO2 fertilization effect” of the type observed in “experimental manipulations of atmospheric CO2 that report a stimulation of photosynthesis and above-ground productivity at high CO2.” In addition, they determined that the yearly “minimum vegetation greenness increased over the period 1982-2001 for all the regions of the eastern humid temperate zone in North America.” As for the cause of this phenomenon, Lim et al. say that rising CO2 could “increase minimum greenness by stimulating photosynthesis at the beginning of the growing season,” citing the work of Idso et al. (2000).

In a somewhat similar study, but one that focused more intensely on climate change, Xiao and Moody (2004) examined the responses of the normalized difference vegetation index integrated over the growing season (gNDVI) to annual and seasonal precipitation, maximum temperature (Tmax) and minimum temperature (Tmin) over an 11-year period (1990-2000) for six biomes in the conterminous United States (Evergreen Needleleaf Forest, Deciduous Broadleaf Forest, Mixed Forest, Open Shrubland, Woody Savanna and Grassland), focusing on within- and across-biome variance in long-term average gNDVI and emphasizing the degree to which this variance is explained by spatial gradients in long-term average seasonal climate. The results of these protocols indicated that the greatest positive climate-change impacts on biome productivity were caused by increases in spring, winter and fall precipitation, as well as increases in fall and spring temperature, especially Tmin, which has historically increased at roughly twice the rate of Tmax in the United States. Hence, “if historical climatic trends and the biotic responses suggested in this analysis continue to hold true, we can anticipate further increases in productivity for both forested and nonforested ecoregions in the conterminous US, with associated implications for carbon budgets and woody proliferation.”

Goetz et al. (2005) transformed satellite-derived NDVI data obtained across boreal North America (Canada and Alaska) for the period 1982-2003 into photosynthetically active radiation absorbed by green vegetation and treated the result as a proxy for relative June-August gross photosynthesis (Pg), stratifying the results by vegetation type and comparing them with spatially matched concomitant trends in surface air temperature data. Over the course of the study, this work revealed that area-wide tundra experienced a significant increase in Pg in response to a similar increase in air temperature; and Goetz et al. say “this observation is supported by a wide and increasing range of local field measurements characterizing elevated net CO2 uptake (Oechel et al., 2000), greater depths of seasonal thaw (Goulden et al., 1998), changes in the composition and density of herbaceous vegetation (Chapin et al., 2000; Epstein et al., 2004), and increased woody encroachment in the tundra areas of North America (Sturm et al., 2001).” In the case of interior forest, on the other hand, there was no significant increase in air temperature and essentially no change in Pg, with the last data point of the series being essentially indistinguishable from the first. This latter seemingly aberrant observation is in harmony with the fact that at low temperatures the growth-promoting effects of increasing atmospheric CO2 levels are often very small or even non-existent (Idso and Idso, 1994), which is what appears to have been the case with North American boreal forests over the same time period. As a result, Canada’s and Alaska’s tundra ecosystems exhibited increasing productivity over the past couple of decades, while their boreal forests did not.

Also working in Alaska, Tape et al. (2006) analyzed repeat photography data from a photo study of the Colville River conducted between 1945 and 1953, as well as 202 new photos of the same sites that were obtained between 1999 and 2002, to determine the nature of shrub expansion in that region over the past half-century. This approach revealed, in their words, that “large shrubs have increased in size and abundance over the past 50 years, colonizing areas where previously there were no large shrubs.” In addition, they say their review of plot and remote sensing studies confirms that “shrubs in Alaska have expanded their range and grown in size” and that “a population of smaller, intertussock shrubs not generally sampled by the repeat photography, is also expanding and growing.” Taken together, they conclude that “these three lines of evidence allow us to infer a general increase in tundra shrubs across northern Alaska.” Tape et al. attribute this to large-scale pan-Arctic warming. From analyses of logistic growth curves, they estimate that the expansion began about 1900, “well before the current warming in Alaska (which started about 1970).” Hence, they conclude that “the expansion predates the most recent warming trend and is perhaps associated with the general warming since the Little Ice Age.” These inferences appear reasonable, although we would add that the 80-ppm increase in the atmosphere’s CO2 concentration since 1900 likely played a role in the shrub expansion as well. If continued, the researchers say the transition “will alter the fundamental architecture and function of this ecosystem with important ramifications,” the great bulk of which, in our opinion, will be positive.

Working at eight different sites within the Pacific Northwest of the United States, Soule and Knapp (2006) studied ponderosa pine trees to see how they may have responded to the increase in the atmosphere’s CO2 concentration that occurred after 1950. The two geographers say the sites they chose “fit several criteria designed to limit potential confounding influences associated with anthropogenic disturbance.” In addition, they selected locations with “a variety of climatic and topo-edaphic conditions, ranging from extremely water-limiting environments … to areas where soil moisture should be a limiting factor for growth only during extreme drought years.” They also say that all sites were located in areas “where ozone concentrations and nitrogen deposition are typically low.”

At all eight of the sites that met all of these criteria, Soule and Knapp obtained core samples from about 40 mature trees that included “the potentially oldest trees on each site,” so that their results would indicate, as they put it, “the response of mature, naturally occurring ponderosa pine trees that germinated before anthropogenically elevated CO2 levels, but where growth, particularly post-1950, has occurred under increasing and substantially higher atmospheric CO2 concentrations.” Utilizing meteorological evaluations of the Palmer Drought Severity Index, they thus compared ponderosa pine radial growth rates during matched wet and dry years pre- and post-1950. Overall, the two researchers found a post-1950 radial growth enhancement that was “more pronounced during drought years compared with wet years, and the greatest response occurred at the most stressed site.” As for the magnitude of the response, they determined that “the relative change in growth [was] upward at seven of our [eight] sites, ranging from 11 to 133%.” With respect to the significance of their observations, Soule and Knapp say their results show that “radial growth has increased in the post-1950s period … while climatic conditions have generally been unchanged,” which further suggests that “nonclimatic driving forces are operative.” In addition, they say the “radial growth responses are generally consistent with what has been shown in long-term open-top chamber (Idso and Kimball, 2001) and free-air CO2 enrichment (FACE) studies (Ainsworth and Long, 2005).” They conclude their findings “suggest that elevated levels of atmospheric CO2 are acting as a driving force for increased radial growth of ponderosa pine, but that the overall influence of this effect may be enhanced, reduced or obviated by site-specific conditions.”

Wang et al. (2006) examined ring-width development in cohorts of young and old white spruce trees in a mixed grass-prairie ecosystem in southwestern Manitoba, Canada, where a 1997 wildfire killed most of the older trees growing in high-density spruce islands, but where younger trees slightly removed from the islands escaped the ravages of the flames. There, “within each of a total of 24 burned islands,” in the words of the three researchers, “the largest dominant tree (dead) was cut down and a disc was then sampled from the stump height,” while “adjacent to each sampled island, a smaller, younger tree (live) was also cut down, and a disc was sampled from the stump height.”

After removing size-, age- and climate-related trends in radial growth from the ring-width histories of the trees, Wang et al. plotted the residuals as functions of time for the 30-year periods for which both the old and young trees would have been approximately the same age: 1900-1929 for the old trees and 1970-1999 for the young trees. During the first of these periods, the atmosphere’s CO2 concentration averaged 299 ppm; during the second it averaged 346 ppm. Also, the mean rate-of-rise of the atmosphere’s CO2 concentration was 0.37 ppm/year for first period and 1.43 ppm/year for the second.

The results of this exercise revealed that the slope of the linear regression describing the rate-of-growth of the ring-width residuals for the later period (when the air’s CO2 concentration was 15 percent greater and its rate-of-rise 285 percent greater) was more than twice that of the linear regression describing the rate-of-growth of the ring-width residuals during the earlier period. As the researchers describe it, these results show that “at the same developmental stage, a greater growth response occurred in the late period when atmospheric CO2 concentration and the rate of atmospheric CO2 increase were both relatively high,” and they say that “these results are consistent with expectations for CO2-fertilization effects.” In fact, they say “the response of the studied young trees can be taken as strong circumstantial evidence for the atmospheric CO2-fertilization effect.”

Another thing Wang et al. learned was that “postdrought growth response was much stronger for young trees (1970-1999) compared with old trees at the same development stage (1900-1929),” and they add that “higher atmospheric CO2 concentration in the period from 1970-1999 may have helped white spruce recover from severe drought.” In a similar vein, they also determined that young trees showed a weaker relationship to precipitation than did old trees, noting that “more CO2 would lead to greater water-use efficiency, which may be dampening the precipitation signal in young trees.”

In summary, Wang et al.’s unique study provides an exciting real-world example of the benefits the historical increase in the air’s CO2 content has likely conferred on nearly all of earth’s plants, and especially its long-lived woody species. Together with the results of the other North American studies we have reviewed, this body of research paints a picture of a significant greening of North America in the twentieth century.

In a 2011 study by Johnson et al., the effects of herbivorous brown lemmings (Lemmus trimucronatus) on the growth of tundra plants (including grasses, sedges, lichens and moss) was documented in the coastal Arctic location of Barrow, Alaska. In 2002 and 2010 (each two years after documented lemming population outbreaks), the researchers sampled 2m x 2m plots: 12 control plots and 12 "exclosure" plots (which excluded brown lemmings, the area's primary herbivore). For each of these time periods, the authors "examined the effects of sustained herbivory on plant community composition and the degree to which lemming herbivory may have contributed to the regional greening.” Results indicated that there were more sedges and grasses in the control plots than those that excluded lemmings. This suggests that lemming activity resulting from lemming population outbreaks promotes greater growth of grasses and sedges afterward. Because grasses and sedges were found to respond favorably to periodic lemming activity as well as to increased temperature, the conclusion that "greening" of coastal Arctic tundra is the direct result of warming due to reduced sea ice may well be premature.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/g/namergreen.php.

Oceans

Rising air temperatures and atmospheric CO2 concentrations in the twentieth century also have affected the productivity of earth’s seas. We begin with a study that takes a much longer view of the subject.

Elderfield and Rickaby (2000) note that the typically low atmospheric CO2 concentrations of glacial periods have generally been attributed to an increased oceanic uptake of CO2, “particularly in the southern oceans.” However, the assumption that intensified phytoplanktonic photosynthesis may have stimulated CO2 uptake rates during glacial periods has always seemed at odds with the observational fact that rates of photosynthesis are generally much reduced in environments of significantly lower-than-current atmospheric CO2 concentrations, such as typically prevail during glacial periods.

The two scientists provide a new interpretation of Cd/Ca systematics in sea water that helps to resolve this puzzle, as it allows them to more accurately estimate surface water phosphate conditions during glacial times and thereby determine the implications for concomitant atmospheric CO2 concentrations. What they found, in their words, is that “results from the Last Glacial Maximum [LGM] show similar phosphate utilization in the subantarctic to that of today, but much smaller utilization in the polar Southern Ocean,” which implies, according to Delaney (2000), that Antarctic productivity was lower at that time than it is now, but that subantarctic productivity was about the same as it has been in modern times, due perhaps to greater concentrations of bio-available iron compensating for the lower atmospheric CO2 concentrations of the LGM.

So what caused the much smaller utilization of phosphate in the polar Southern Ocean during the LGM? Noting that the area of sea-ice cover in the Southern Ocean during glacial periods may have been as much as double that of modern times, Elderfield and Rickaby suggest that “by restricting communication between the ocean and atmosphere, sea ice expansion also provides a mechanism for reduced CO2 release by the Southern Ocean and lower glacial atmospheric CO2.” Hence, it is possible that phytoplanktonic productivity in the glacial Southern Ocean may have been no higher than it is at the present time, notwithstanding the greater supply of bio-available iron typical of glacial epochs.

In the case of the interglacial period in which we currently live, Dupouy et al. (2000) say it has long been believed that N2 fixation in the world’s oceans is unduly low, in consequence of the present low supply of wind-blown iron compared to that of glacial periods, and that this state of affairs leads to low phytoplanktonic productivity, even in the presence of higher atmospheric CO2 concentrations. The evidence they acquired, however, suggests that marine N2 fixation may be much greater than what has generally been thought to be the case. In particular, they note that several Trichodesmium species of N2-fixing cyanobacteria have “a nearly ubiquitous distribution in the euphotic zone of tropical and subtropical seas and could play a major role in bringing new N to these oligotrophic systems.” And this feat, in their words, “could play a significant role in enhancing new production.”

The importance of these findings is perhaps best appreciated in light of the findings of Pahlow and Riebesell (2000), who in studying data obtained from 1173 stations in the Atlantic and Pacific Oceans, covering the years 1947 to 1994, detected changes in Northern Hemispheric deep-ocean Redfield ratios that are indicative of increasing nitrogen availability there, which increase was concomitant with an increase in export production that has resulted in ever-increasing oceanic carbon sequestration. These investigators further suggest that the growing supply of nitrogen has its origin in anthropogenic activities that release nitrous oxides to the air. In addition, the increased carbon export may be partly a consequence of the historical increase in the air’s CO2 concentration, which has been demonstrated to have the ability to enhance phytoplanktonic productivity (see Section 6.1.3.4. in this document), analogous to the way in which elevated concentrations of atmospheric CO2 enhance the productivity of terrestrial plants, including their ability to fix nitrogen.

Further elucidating the productivity-enhancing power of the ongoing rise in the air’s CO2 content is the study of Pasquer et al. (2005), who employed a complex model of growth regulation of diatoms, pico/nano phytoplankton, coccolithophorids and Phaeocystis spp. by light, temperature and nutrients (based on a comprehensive analysis of literature reviews focusing on these taxonomic groups) to calculate changes in the ocean uptake of carbon in response to a sustained increase in atmospheric CO2 concentration of 1.2 ppm per year for three marine ecosystems where biogeochemical time-series of the data required for model initialization and comparison of results were readily available. These systems were (1) the ice-free Southern Ocean Time Series station KERFIX (50°40’S, 68°E) for the period 1993-1994 (diatom-dominated), (2) the sea-ice associated Ross Sea domain (76°S, 180°W) of the Antarctic Environment and Southern Ocean Process Study AESOPS in 1996-1997 (Phaeocystis-dominated), and (3) the North Atlantic Bloom Experiment NABE (60°N, 20°W) in 1991 (coccolithophorids). Their results, in their words, “show that at all tested latitudes the prescribed increase of atmospheric CO2 enhances the carbon uptake by the ocean.” Indeed, we calculate from their graphical presentations that (1) at the NABE site a sustained atmospheric CO2 increase of 1.2 ppm per year over a period of eleven years increases the air-sea CO2 flux in the last year of that period by approximately 17 percent, (2) at the AESOPS site the same protocol applied over a period of six years increases the air-sea CO2 flux by about 45 percent, and (3) at the KERFIX site it increases the air-sea CO2 flux after nine years by about 78 percent. Although the results of this interesting study based on the complex SWAMCO model of Lancelot et al. (2000), as modified by Hannon et al. (2001), seem overly large, they highlight the likelihood that the ongoing rise in the air’s CO2 content may be having a significant positive impact on ocean productivity and the magnitude of the ocean carbon sink.

But what about increasing temperatures? Sarmiento et al. (2004) conducted a massive computational study that employed six coupled climate model simulations to determine the biological response of the global ocean to the climate warming they simulated from the beginning of the Industrial Revolution to the year 2050. Based on vertical velocity, maximum winter mixed-layer depth and sea-ice cover, they defined six biomes and calculated how their surface geographies would change in response to their calculated changes in global climate. Next, they used satellite ocean color and climatological observations to develop an empirical model for predicting surface chlorophyll concentrations from the final physical properties of the world’s oceans as derived from their global warming simulations, after which they used three primary production algorithms to estimate the response of oceanic primary production to climate warming based on their calculated chlorophyll concentrations. When all was said and done, the thirteen scientists from Australia, France, Germany, Russia, the United Kingdom and the United States arrived at a global warming-induced increase in global ocean primary production that ranged from 0.7 to 8.1 percent.

So what do real-world measurements of oceanic productivity reveal? Goes et al. (2005) analyzed seven years (1997-2004) of satellite-derived ocean color data pertaining to the Arabian Sea, as well as associated sea surface temperatures (SSTs) and winds. They report that for the region located between 52 to 57°E and 5 to 10°N, “the most conspicuous observation was the consistent year-by-year increase in phytoplankton biomass over the 7-year period.” This phenomenon was so dramatic that by the summer of 2003, in their words, “chlorophyll a concentrations were >350% higher than those observed in the summer of 1997.” They also report that the increase in chlorophyll a was “accompanied by an intensification of sea surface winds, in particular of the zonal (east-to-west) component,” noting that these “summer monsoon winds are a coupled atmosphere-land-ocean phenomenon, whose strength is significantly correlated with tropical SSTs and Eurasian snow cover anomalies on a year-to-year basis.” More specifically, they say that “reduced snow cover over Eurasia strengthens the spring and summer land-sea thermal contrast and is considered to be responsible for the stronger southwest monsoon winds.” In addition, they state that “the influence of southwest monsoon winds on phytoplankton in the Arabian Sea is not through their impact on coastal upwelling alone but also via the ability of zonal winds to laterally advect newly upwelled nutrient-rich waters to regions away from the upwelling zone.” They conclude that “escalation in the intensity of summer monsoon winds, accompanied by enhanced upwelling and an increase of more than 350 percent in average summertime phytoplankton biomass along the coast and over 300 percent offshore, raises the possibility that the current warming trend of the Eurasian landmass is making the Arabian Sea more productive.”

To the north and west on the other side of Eurasia, Marasovic et al. (2005) analyzed monthly observations of basic hydrographic, chemical and biological parameters, including primary production, that had been made since the 1960s at two oceanographic stations, one near the coast (Kastela Bay) and one in the open sea. They found that mean annual primary production in Kastela Bay averaged about 430 mg C m-2 d-1 over the period 1962-72, exceeded 600 mg C m-2 d-1 over the period 1972-82, and rose to over 700 mg C m-2 d-1 over the period 1982-96, accompanied by a similar upward trend in percent oxygen saturation of the surface water. The initial value of primary production in the open sea was much less (approximately 150 mg C m-2 d-1), but it began to follow the upward trend of the Kastela Bay data after about one decade. Marasovic et al. thus concluded that “even though all the relevant data indicate that the changes in Kastela Bay are closely related to an increase of anthropogenic nutrient loading, similar changes in the open sea suggest that primary production in the Bay might, at least partly, be due to global climatic changes,” which, in their words, are “occurring in the Mediterranean and Adriatic Sea open waters” and may be directly related to “global warming of air and ocean,” since “higher temperature positively affects photosynthetic processes.”

Raitsos et al. (2005) investigated the relationship between Sea-viewing Wide Field-of-view Sensor (SeaWiFS) chlorophyll-a measurements in the Central Northeast Atlantic and North Sea (1997-2002) and simultaneous measurements of the Phytoplankton Color Index (PCI) collected by the Continuous Plankton Recorder survey, which is an upper-layer plankton monitoring program that has operated in the North Sea and North Atlantic Ocean since 1931. By developing a relationship between the two data bases over their five years of overlap, they were able to produce a Chl-a history for the Central Northeast Atlantic and North Sea for the period 1948-2002. Of this record they say that “an increasing trend is apparent in mean Chl-a for the area of study over the period 1948-2002.” They also say “there is clear evidence for a stepwise increase after the mid-1980s, with a minimum of 1.3mg m-3 in 1950 and a peak annual mean of 2.1 mg m-3 in 1989 (62% increase).” Alternatively, it is possible that the data represent a more steady long-term upward trend upon which is superimposed a decadal-scale oscillation. In a final comment on their findings, they note that “changes through time in the PCI are significantly correlated with both sea surface temperature and Northern Hemisphere temperature,” citing Beaugrand and Reid (2003).

In a contemporaneous study, Antoine et al. (2005) applied revised data-processing algorithms to two ocean-sensing satellites, the Coastal Zone Color Scanner (CZCS) and SeaWiFS, over the periods 1979-1986 and 1998-2002, respectively, to provide an analysis of the decadal changes in global oceanic phytoplankton biomass. Results of the analysis showed “an overall increase of the world ocean average chlorophyll concentration by about 22%” over the two decades under study.

Dropping down to the Southern Ocean, Hirawake et al. (2005) analyzed chlorophyll a data obtained from Japanese Antarctic Research Expedition cruises made by the Fuji and Shirase ice-breakers between Tokyo and Antarctica from 15 November to 28 December of nearly every year between 1965 and 2002 in a study of interannual variations of phytoplankton biomass, calculating results for the equatorial region between 10°N and 10°S, the Subtropical Front (STF) region between 35°S and 45°S, and the Polar Front (PF) region between 45°S and 55°S. They report that an increase in chl a was “recognized in the waters around the STF and the PF, especially after 1980 around the PF in particular,” and that “in the period between 1994 and 1998, the chl a in the three regions exhibited rapid gain simultaneously.” They also say “there were significant correlations between chl a and year through all of the period of observation around the STF and PF, and the rates of increase are 0.005 and 0.012 mg chl a m-3 y-1, respectively.” In addition, they report that the satellite data of Gregg and Conkright (2002) “almost coincide with our results.” In commenting on these findings, the Japanese scientists say that “simply considering the significant increase in the chl a in the Southern Ocean, a rise in the primary production as a result of the phytoplankton increase in this area is also expected.”

Also working in the Southern Hemisphere, Sepulveda et al. (2005) presented “the first reconstruction of changes in surface primary production during the last century from the Puyuhuapi fjord in southern Chile, using a variety of parameters (diatoms, biogenic silica, total organic carbon, chlorins, and proteins) as productivity proxies.” Noting that the fjord is located in “a still-pristine area,” they say it is “suitable to study changes in past export production originating from changes in both the paleo-Patagonian ice caps and the globally important Southern Ocean.”

The analysis revealed that the productivity of the Puyuhuapi fjord “was characterized by a constant increase from the late 19th century to the early 1980s, then decreased until the late 1990s, and then rose again to present-day values.” For the first of these periods (1890-1980), they additionally report that “all proxies were highly correlated (r > 0.8, p < 0.05),” and that “all proxies reveal an increase in accumulation rates.” From 1980 to the present, however, the pattern differed among the various proxies; and the researchers say that “considering that the top 5 cm of the sediment column (~10 years) are diagenetically active, and that bioturbation by benthic organisms may have modified and mixed the sedimentary signal, paleo-interpretation of the period 1980-2001 must be taken with caution.” Consequently, there is substantial solid evidence that, for the first 90 years of the 111-year record, surface primary production in the Puyuhuapi fjord rose dramatically, while with lesser confidence it appears to have leveled out over the past two decades. In spite of claims that the “unprecedented” increases in mean global air temperature and CO2 concentration experienced since the inception of the Industrial Revolution have been bad for the biosphere, Sepulveda et al. presented yet another case of an ecosystem apparently thriving in such conditions.

Still, claims of impending ocean productivity declines have not ceased, and some commentators single out the study of Behrenfeld et al. (2006) in support of their claims. Working with NASA’s Sea-viewing Wide Field-of-view Sensor (Sea WiFS), the team of 10 U.S. scientists calculated monthly changes in net primary production (NPP) from similar changes in upper-ocean chlorophyll concentrations detected from space over the past decade. (See Figure 7.9.5.) They report that this period was dominated by an initial NPP increase of 1,930 teragrams of carbon per year (Tg C yr-1), which they attributed to the significant cooling of “the 1997 to 1999 El Niño to La Niña transition,” and they note that this increase was “followed by a prolonged decrease averaging 190 Tg C yr-1,” which they attributed to subsequent warming.

The means by which changing temperatures were claimed by the researchers to have driven the two sequential linear-fit trends in NPP is based on their presumption that a warming climate increases the density contrast between warmer surface waters and cooler underlying nutrient-rich waters, so that the enhanced stratification that occurs with warming “suppresses nutrient exchange through vertical mixing,” which decreases NPP by reducing the supply of nutrients to the surface waters where photosynthesizing phytoplankton predominantly live. By contrast, the ten scientists suggest that “surface cooling favors elevated vertical exchange,” which increases NPP by enhancing the supply of nutrients to the ocean’s surface waters, which are more frequented by phytoplankton than are under-lying waters, due to light requirements for photosynthesis. Figure 7.9.5. Monthly anomalies of global NPP (green line) plus similar results for the permanently stratified ocean regions of the world (grey circles and black line), adapted from Behrenfeld et al. (2006).


It is informative to note, however, that from approximately the middle of 2001 to the end of the data series in early 2006 (which interval accounts for more than half of the data record), there has been, if anything, a slight increase in global NPP. (See again Figure 7.9.5.) Does this observation mean there has been little to no net global warming since mid-2001? Or does it mean the global ocean’s mean surface temperature actually cooled a bit over the last five years? Neither alternative is what one would expect if global warming were a real problem. On the other hand, the relationship between global warming and oceanic productivity may not be nearly as strong as what Behrenfeld et al. have suggested; and they themselves say “modeling studies suggest that shifts in ecosystem structure from climate variations may be as [important as] or more important than the alterations in bulk integrated properties reported here,” noting that some “susceptible ecosystem characteristics” that might be so shifted include “taxonomic composition, physiological status, and light absorption by colored dissolved organic material.” It is possible that given enough time, the types of phenomena Behrenfeld et al. describe as possibly resulting in important “shifts in ecosystem structure” could compensate for or even overwhelm what might initially appear to be negative warming-induced consequences.

Another reason for not concluding too much from the oceanic NPP data set of Behrenfeld et al. is that it may be of too short a duration to reveal what might be occurring on a much longer timescale throughout the world’s oceans, or that its position in time may be such that it does not allow the detection of greater short-term changes of the opposite sign that may have occurred a few years earlier or that might occur in the near future.

Consider, for example, the fact that the central regions of the world’s major oceans were long thought to be essentially vast biological deserts (Ryther, 1969), but that several studies of primary photosynthetic production conducted in those regions over the 1980s (Shulenberger and Reid, 1981; Jenkins, 1982; Jenkins and Goldman, 1985; Reid and Shulenberger, 1986; Marra and Heinemann, 1987; Laws et al., 1987; Venrick et al., 1987; Packard et al., 1988) yielded results that suggested marine productivity at that time was twice or more as great as it likely was for a long time prior to 1969, causing many of that day to speculate that “the ocean’s deserts are blooming” (Kerr, 1986).

Of even greater interest, perhaps, is the fact that over this particular period of time (1970-1988), the data repository of Jones et al. (1999) indicates the earth experienced a (linear-regression-derived) global warming of 0.333°C, while the data base of the Global Historical Climatology Network indicates the planet experienced a similarly calculated global warming of 0.397°C. The mean of these two values (0.365°C) is nearly twice as great as the warming that occurred over the post-1999 period studied by Behrenfeld et al.; yet this earlier much larger warming (which according to the ten researchers’ way of thinking should have produced a major decline in ocean productivity) was concomitant with a huge increase in ocean productivity. Consequently, it would appear that just the opposite of what Behrenfeld et al. suggest about global warming and ocean productivity is likely to be the more correct of the two opposing cause-and-effect relationships.

Moving closer to the present, Levitan et al. (2007) published a study of major significance that addresses the future of oceanic productivity under rising atmospheric CO2 concentrations. In their paper the authors note that “among the principal players contributing to global aquatic primary production, the nitrogen (N)-fixing organisms (diazotrophs) are important providers of new N to the oligotrophic areas of the oceans,” and they cite several studies which demonstrate that “cyanobacterial (photo-trophic) diazotrophs in particular fuel primary production and phytoplankton blooms which sustain oceanic food-webs and major economies and impact global carbon (C) and N cycling.” These facts compelled them to examine how the ongoing rise in the air’s CO2 content might impact these relationships. They began by exploring the response of the cyanobacterial diazotroph Trichodesmium to changes in the atmosphere’s CO2 concentration, choosing this particular diazotroph because it dominates the world’s tropical and sub-tropical oceans in this regard, contributing over 50 percent of total marine N fixation.

The eight Israeli and Czech researchers grew Trichodesmium IMS101 stock cultures in YBCII medium (Chen et al., 1996) at 25°C and a 12-hour:12-hour light/dark cycle (with the light portion of the cycle in the range of 80-100 µmol photons m-2 s-1) in equilibrium with air of three different CO2 concentrations (250, 400 and 900 ppm, representing low, ambient and high concentrations, respectively), which was accomplished by continuously bubbling air of the three CO2 concentrations through the appropriate culture vessels throughout various experimental runs, each of which lasted a little over three weeks, during which time they periodically monitored a number of diazotrophic physiological processes and properties.

So what did the scientists learn? Levitan et al. report that Trichodesmium in the high CO2 treatment “displayed enhanced N fixation, longer trichomes, higher growth rates and biomass yields.” In fact, they write that in the high CO2 treatment there was “a three- to four-fold increase in N fixation and a doubling of growth rates and biomass,” and that the cultures in the low CO2 treatment reached a stationary growth phase after only five days, “while both ambient and high CO2 cultures exhibited exponential growth until day 15 before declining.”

In discussing possible explanations for what they observed, the researchers suggest that “enhanced N fixation and growth in the high CO2 cultures occurs due to reallocation of energy and resources from carbon concentrating mechanisms required under low and ambient CO2.” Consequently, they conclude, in their words, that “in oceanic regions, where light and nutrients such as P and Fe are not limiting, we expect the projected concentrations of CO2 to increase N fixation and growth of Trichodesmium,” and that “other diazotrophs may be similarly affected, thereby enhancing inputs of new N and increasing primary productivity in the oceans.” And to emphasize these points, they write in the concluding sentence of their paper that “Trichodesmium’s dramatic response to elevated CO2 may consolidate its dominance in subtropical and tropical regions and its role in C and N cycling, fueling subsequent primary production, phytoplankton blooms, and sustaining oceanic food-webs.”

Arrigo et al. (2008) introduce their work by writing that “between the late 1970s and the early part of the 21st century, the extent of Arctic Ocean sea ice cover has declined during all months of the year, with the largest declines reported in the boreal summer months, particularly in September (8.6 ± 2.9% per decade),” citing the work of Serreze et al. (2007). In an effort to “quantify the change in marine primary productivity in Arctic waters resulting from recent losses of sea ice cover,” the authors “implemented a primary productivity algorithm that accounts for variability in sea ice extent, sea surface temperature, sea level winds, downwelling spectral irradiance, and surface chlorophyll a concentrations,” and that “was parameterized and validated specifically for use in the Arctic (Pabi et al., 2008) and utilizes forcing variables derived either from satellite data or NCEP reanalysis fields.”

Arrigo et al. determined that “annual primary production in the Arctic increased yearly by an average of 27.5 Tg C per year since 2003 and by 35 Tg C per year between 2006 and 2007,” 30 percent of which total increase was attributable to decreased minimum summer ice extent and 70 percent of which was due to a longer phytoplankton growing season. Arrigo et al. thus conclude that if the trends they discovered continue, “additional loss of ice during Arctic spring could boost productivity >3-fold above 1998-2002 levels.” Hence, they additionally state that if the 26 percent increase in annual net CO2 fixation in the Arctic Ocean between 2003 and 2007 is maintained, “this would represent a weak negative feedback on climate change.”

On the other side of the globe and working in the Southern Ocean, Smith and Comiso (2008) employed phytoplankton pigment assessments, surface temperature estimates, modeled irradiance, and observed sea ice concentrations—all of which parameters were derived from satellite data—and incorporated them into a vertically integrated production model to estimate primary productivity trends according to the technique of Behrenfeld et al. (2002). Of this effort, the two authors say that “the resultant assessment of Southern Ocean productivity is the most exhaustive ever compiled and provides an improvement in the quantitative role of carbon fixation in Antarctic waters.” So what did they find? During the nine years (1997-2006) analyzed in the study, “productivity in the entire Southern Ocean showed a substantial and significant increase,” which increase can be calculated from the graphical representation of their results as ~17 percent per decade. In commenting on their findings, the two researchers note that “the highly significant increase in the productivity of the entire Southern Ocean over the past decade implies that long-term changes in Antarctic food webs and biogeochemical cycles are presently occurring,” which changes we might add are positive.

In light of these several real-world observations, we find no indications of a widespread decline in oceanic productivity over the twentieth century in response to increases in air temperature and CO2 concentration. In fact, we see evidence that just the opposite is occurring, that environmental changes are occurring that are proving to be beneficial.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/o/oceanproductivity.php.


Global

How have earth’s terrestrial plants responded—on average and in their entirety—to the atmospheric temperature and CO2 increases of the past quarter-century? In this subsection we report the results of studies that have looked at either the world as a whole or groups of more than two continents at the same time.

In one of the earlier studies of the subject, Joos and Bruno (1998) used ice core and direct observations of atmospheric CO2 and 13C to reconstruct the histories of terrestrial and oceanic uptake of anthropogenic carbon over the past two centuries. This project revealed, in their words, that “the biosphere acted on average as a source [of CO2] during the last century and the first decades of this century … Then, the biosphere turned into a [CO2] sink,” which implies a significant increase in global vegetative productivity over the last half of the twentieth century.

More recently, Cao et al. (2004) derived net primary production (NPP) values at 8-km and 10-day resolutions for the period 1981-2000 using variables based almost entirely on satellite observations, as described in the Global Production Efficiency Model (GLO-PEM), which consists, in their words, “of linked components that describe the processes of canopy radiation absorption, utilization, autotrophic respiration, and the regulation of these processes by environmental factors (Prince and Goward, 1995; Goetz et al., 2000).” They learned that over the last two decades of the twentieth century, when temperatures were rising, “there was an increasing trend toward enhanced terrestrial NPP,” which they say was “caused mainly by increases in atmospheric carbon dioxide and precipitation.”

A year later, Cao et al. (2005) used the CEVSA (Carbon Exchanges in the Vegetation-Soil-Atmosphere system) model (Cao and Woodward, 1998; Cao et al., 2002), forced by observed variations in climate and atmospheric CO2, to quantify changes in NPP, soil heterotrophic respiration (HR) and net ecosystem production (NEP) from 1981 to 1998. As an independent check on the NPP estimate of CEVSA, they also estimated 10-day NPP from 1981-2000 with the GLO-PEM model that uses data almost entirely from remote sensing, including both the normalized difference vegetation index (NDVI) and meteorological variables (Prince and Goward, 1995; Cao et al., 2004). This protocol revealed, in Cao et al.’s words, that “global terrestrial temperature increased by 0.21°C from the 1980s to the 1990s, and this alone increased HR more than NPP and hence reduced global annual NEP.” However, they found that “combined changes in temperature and precipitation increased global NEP significantly,” and that “increases in atmospheric CO2 produced further increases in NPP and NEP.” They also discovered that “the CO2 fertilization effect [was] particularly strong in the tropics, compensating for the negative effect of warming on NPP.” Enlarging on this point, they write that “the response of photosynthetic biochemical reactions to increases in atmospheric CO2 is greater in warmer conditions, so the CO2 fertilization effect will increase with warming in cool regions and be high in warm environments.” The end result of the application of these models and measurements was their finding that global NEP increased “from 0.25 Pg C yr-1 in the 1980s to 1.36 Pg C yr-1 in the 1990s.”

Commenting on their findings, Cao et al. note that “the NEP that was induced by CO2 fertilization and climatic variation accounted for 30 percent of the total terrestrial carbon sink implied by the atmospheric carbon budget (Schimel et al., 2001), and the fraction changed from 13 percent in the 1980s to 49 percent in the 1990s,” which indicates the growing importance of the CO2 fertilization effect. Also, they say “the increase in the terrestrial carbon sink from the 1980s to the 1990s was a continuation of the trend since the middle of the twentieth century, rather than merely a consequence of short-term climate variability,” which suggests that as long as the air’s CO2 content continues its upward course, so too will its stimulation of the terrestrial biosphere likely continue its upward course.

Using a newly developed satellite-based vegetation index (Version 3 Pathfinder NDVI) in conjunction with a gridded global climate dataset (global monthly mean temperature and precipitation at 0.5° resolution from New et al., 2000), Xiao and Moody (2005) analyzed trends in global vegetative activity from 1982 to 1998. The greening trends they found exhibited substantial latitudinal and longitudinal variability, with the most intense greening of the globe located in high northern latitudes, portions of the tropics, southeastern North America and eastern China. Temperature was found to correlate strongly with greening trends in Europe, eastern Eurasia and tropical Africa. Precipitation, on the other hand, was not found to be a significant driver of increases in greenness, except for isolated and spatially fragmented regions. Some decreases in greenness were also observed, mainly in the Southern Hemisphere in southern Africa, southern South America and central Australia, which trends were associated with concomitant increases in temperature and decreases in precipitation. There were also large regions of the globe that showed no trend in greenness over the 17-year period, as well as large areas that underwent strong greening that showed no association with trends of either temperature or precipitation. These greening trends, as they concluded, must have been the result of other factors, such as “CO2 fertilization, reforestation, forest regrowth, woody plant proliferation and trends in agricultural practices,” about which others will have more to say as we continue.

Working with satellite observations of vegetative activity over the period 1982 to 1999, Nemani et al. (2003) discovered that the productivity of earth’s terrestrial vegetation rose significantly over this period. More specifically, they determined that terrestrial net primary production (NPP) increased by 6.17 percent, or 3.42 PgC, over the 18 years between 1982 and 1999. What is more, they observed net positive responses over all latitude bands studied: 4.2 percent (47.5-22.5°S), 7.4 percent (22.5°S-22.5°N), 3.7 percent (22.5-47.5°N), and 6.6 percent (47.5-90.0°N).

The eight researchers mention a number of likely contributing factors to these significant NPP increases: nitrogen deposition and forest regrowth in northern mid and high latitudes, wetter rainfall regimes in water-limited regions of Australia, Africa, and the Indian subcontinent, increased solar radiation reception over radiation-limited parts of Western Europe and the equatorial tropics, warming in many parts of the world, and the aerial fertilization effect of rising atmospheric CO2 concentrations everywhere.

With respect to the latter factor, Nemani et al. say “an increase in NPP of only 0.2% per 1-ppm increase in CO2 could explain all of the estimated global NPP increase of 6.17% over 18 years and is within the range of experimental evidence.” However, they report that terrestrial NPP increased by more than 1 percent per year in Amazonia alone, noting that “this result cannot be explained solely by CO2 fertilization.”

We tend to agree with Nemani et al. on this point, but also note that the aerial fertilization effect of atmospheric CO2 enrichment is most pronounced at higher temperatures, rising from next to nothing at a mean temperature of 10°C to a 0.33 percent NPP increase per 1-ppm increase in CO2 at a mean temperature of 36°C for a mixture of plants comprised predominantly of herbaceous species (Idso and Idso, 1994). For woody plants, we could possibly expect this number to be two (Idso, 1999) or even three (Saxe et al., 1998; Idso and Kimball, 2001; Leavitt et al., 2003) times larger, yielding a 0.7 percent to 1 percent NPP increase per 1-ppm increase in atmospheric CO2, which would represent the lion’s share of the growth stimulation observed by Nemani et al. in tropical Amazonia.

The message of Nemani et al.’s study is that satellite-derived observations indicate the planet’s terrestrial vegetation significantly increased its productivity over the last two decades of the twentieth century, in the face of a host of both real and imagined environmental stresses, chief among the latter of which was what the IPCC claims to be unprecedented CO2-induced global warming.

Perhaps the most striking evidence for the significant twentieth century growth enhancement of earth’s forests by the historical increase in the air’s CO2 concentration was provided by the study of Phillips and Gentry (1994). Noting that turnover rates of mature tropical forests correlate well with measures of net productivity (Weaver and Murphy, 1990), the two scientists assessed the turnover rates of 40 tropical forests from around the world in order to test the hypothesis that global forest productivity was increasing in situ. In doing so, they found that the turnover rates of these highly productive forests had indeed been rising ever higher since at least 1960, with an apparent pan-tropical acceleration since 1980. In discussing what might be causing this phenomenon, they stated that “the accelerating increase in turnover coincides with an accelerating buildup of CO2,” and as Pimm and Sugden (1994) stated in a companion article, it was “the consistency and simultaneity of the changes on several continents that lead Phillips and Gentry to their conclusion that enhanced productivity induced by increased CO2 is the most plausible candidate for the cause of the increased turnover.”

Four years later, a group of eleven researchers headed by Phillips (Phillips et al., 1998), working with data on tree basal area (a surrogate for tropical forest biomass) for the period 1958-1996, which they obtained from several hundred plots of mature tropical trees scattered about the world, found that average forest biomass for the tropics as a whole had increased substantially. In fact, they calculated that the increase amounted to approximately 40 percent of the missing terrestrial carbon sink of the entire globe. They suggested that “intact forests may be helping to buffer the rate of increase in atmospheric CO2, thereby reducing the impacts of global climate change,” as Idso (1991a,b) had earlier suggested, and they identified the aerial fertilization effect of the ongoing rise in the air’s CO2 content as one of the factors responsible for this phenomenon. Other contemporary studies also supported their findings (Grace et al., 1995; Malhi et al., 1998), verifying the fact that neotropical forests were indeed accumulating ever more carbon. Phillips et al. (2002) continued to state that this phenomenon was occurring “possibly in response to the increasing atmospheric concentrations of carbon dioxide (Prentice et al., 2001; Malhi and Grace, 2000).”

As time progressed, however, it became less popular (i.e., more “politically incorrect”) to report positive biological consequences of the ongoing rise in the air’s CO2 concentration. The conclusions of Phillips and others began to be repeatedly challenged (Sheil, 1995; Sheil and May, 1996; Condit, 1997; Clark, 2002; Clark et al., 2003). In response to those challenges, CO2 Science published an editorial rebuttal (see http://www.co2science.org/articles/V6/ N25/EDIT.php), after which Phillips, joined by 17 other researchers (Lewis et al., 2005b), including one who had earlier criticized his and his colleagues’ conclusions, published a new analysis that vindicated Phillips et al.’s earlier thoughts on the subject.

One of the primary concerns of the critics of Phillips et al.’s work was that their meta-analyses included sites with a wide range of tree census intervals (2-38 years), which they claimed could be confounding or “perhaps even driving conclusions from comparative studies,” as Lewis et al. (2005b) describe it. However, in Lewis et al.’s detailed study of this potential problem, which they concluded was indeed real, they found that re-analysis of Phillips et al.’s published results “shows that the pan-tropical increase in stem turnover rates over the late twentieth century cannot be attributed to combining data with differing census intervals.” Or as they state more obtusely in another place, “the conclusion that turnover rates have increased in tropical forests over the late twentieth century is robust to the charge that this is an artifact due to the combination of data that vary in census interval (cf. Sheil, 1995).”

Lewis et al. (2005b) additionally noted that “Sheil’s (1995) original critique of the evidence for increasing turnover over the late twentieth century also suggests that the apparent increase could be explained by a single event, the 1982-83 El Niño Southern Oscillation (ENSO), as many of the recent data spanned this event.” However, as they continued, “recent analyses from Amazonia have shown that growth, recruitment and mortality rates have simultaneously increased within the same plots over the 1980s and 1990s, as has net above-ground biomass, both in areas largely unaffected, and in those strongly affected, by ENSO events (Baker et al., 2004; Lewis et al., 2004a; Phillips et al., 2004).”

In a satellite study of the world’s tropical forests, Ichii et al. (2005) “simulated and analyzed 1982-1999 Amazonian, African, and Asian carbon fluxes using the Biome-BGC prognostic carbon cycle model driven by National Centers for Environmental Prediction reanalysis daily climate data,” after which they “calculated trends in gross primary productivity (GPP) and net primary productivity (NPP).” This work revealed that solar radiation variability was the primary factor responsible for interannual variations in GPP, followed by temperature and precipitation variability, while in terms of GPP trends, Ichii et al. report that “recent changes in atmospheric CO2 and climate promoted terrestrial GPP increases with a significant linear trend in all three tropical regions.” In the Amazonian region, the rate of GPP increase was 0.67 PgC year-1 decade-1, while in Africa and Asia it was about 0.3 PgC year-1 decade-1. Likewise, they report that “CO2 fertilization effects strongly increased recent NPP trends in regional totals.”

In a review of these several global forest studies, as well as many others (which led to their citing 186 scientific journal articles), Boisvenue and Running (2006) examined reams of “documented evidence of the impacts of climate change trends on forest productivity since the middle of the twentieth century.” In doing so, they found that “globally, based on both satellite and ground-based data, climatic changes seemed to have a generally positive impact on forest productivity when water was not limiting,” which was most of the time, because they report that “less than 7% of forests are in strongly water-limited systems.”

Last, Young and Harris (2005) analyzed, for the majority of earth’s land surface, a near 20-year time series (1982-1999) of NDVI data, based on measurements obtained from the Advanced Very High Resolution Radiometer (AVHRR) carried aboard U.S. National Oceanic and Atmospheric Administration satellites. In doing so, they employed two different datasets derived from the sensor: the Pathfinder AVHRR Land (PAL) data set and the Global Inventory Modeling and Mapping Studies (GIMMS) dataset. Based on their analysis of the PAL data, the two researchers determined that “globally more than 30% of land pixels increased in annual average NDVI greater than 4% and more than 16% persistently increased greater than 4%,” while “during the same period less than 2% of land pixels declined in NDVI and less than 1% persistently declined.” With respect to the GIMMS dataset, they report that “even more areas were found to be persistently increasing (greater than 20%) and persistently decreasing (more than 3%).” All in all, they report that “between 1982 and 1999 the general trend of vegetation change throughout the world has been one of increasing photosynthesis.”

As for what has been responsible for the worldwide increase in photosynthesis—which is the ultimate food source for nearly all of the biosphere—the researchers mention global warming (perhaps it’s not so bad after all), as well as “associated precipitation change and increases in atmospheric carbon dioxide,” citing Myneni et al. (1997) and Ichii et al. (2002). In addition, they say that “many of the areas of decreasing NDVI are the result of human activity,” primarily deforestation (Skole and Tucker, 1993; Steininger et al., 2001) and urbanization (Seto et al. (2000)).

In conclusion, the results of these many studies demonstrate there has been an increase in plant growth rates throughout the world since the inception of the Industrial Revolution, and that this phenomenon has been gradually accelerating over the years, in concert with the historical increases in the air’s CO2 content and its temperature.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/g/greeningearth.php.


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