Elevated Temperature

From ClimateWiki

Respiration

In a recent News & Views item published in Nature Geoscience, Agren (2010) states “it is often assumed that warming will stimulate carbon dioxide release from soils,” but he notes “soil-warming experiments suggest that warming-induced increases in soil carbon dioxide release are transitory, and that emissions return to pre-warming levels after a period of five to ten years,” citing Kirschbaum (2004) and Eliasson et al. (2005). In much the same vein, Bradford et al. (2010) write that in actual field studies “elevated soil respiration rates under experimental warming are relatively short-lived,” citing Jarvis and Linder (2000), Oechel et al. (2000), Luo et al. (2001), Rustad et al. (2001), and Melillo et al. (2002).

Similarly, Bronson and Gower (2010) state “the boreal forest historically has been considered a carbon sink,” but “autotrophic respiration is [supposedly] more sensitive than photosynthesis to increases in temperature (Ryan, 1991; Amthor, 1994),” and therefore, in response to global warming, “most models predict autotrophic respiration will increase at a greater rate than photosynthesis, which infers decreased carbon use efficiency and net primary production (Ryan, 1995; VEMAP Members, 1995; Ryan et al., 1996),” which implies “a substantial increase in temperature could turn the boreal forest into a carbon source (Goulden et al., 1998).” That positive feedback phenomenon could lead to an intensification of the warming of the globe—if the assumptions are correct, which was tested by the next study.

Working about 20 km south of Thompson, Manitoba, Canada (55°53’N, 98°20’W), within large enclosed greenhouse chambers containing black spruce trees (Picea mariana (Mill.) B.S.P.) and the majority of their fine roots, along with soil-heating cables used to warm air and soil temperatures about 5°C over ambient control temperatures, Bronson and Gower (2010) measured light-saturated net photosynthesis, foliage respiration, and stem respiration in heated and control forest plots during the 2005, 2006, and 2007 growing seasons. Throughout the entire study, “both the older foliage, which developed before the experiment, and the new foliage, developed during the experiment, had similar rates of light-saturated net photosynthesis, foliage respiration and stem respiration across all treatments.” This, Bronson and Gover write, “underscores the ability of black spruce to maintain homeostasis in a 5°C warmer environment.” In addition, while noting many climate change models predict a doubling of respiration for every 10°C increase in temperature, Bronson and Gower state in the concluding sentence of their paper, and in no uncertain terms, that “the results from this and other whole-ecosystem warming experiments do not support this model assumption.”

In another study, Mahecha et al. (2010) write, “the respiratory release of carbon dioxide from the land surface is a major flux in the global carbon cycle,” and “understanding the sensitivity of respiratory processes to temperature is central for quantifying the climate-carbon cycle feedback.” They set out to do that via a complex set of operations that “approximated the sensitivity of terrestrial ecosystem respiration to air temperature (Q10) across 60 FLUXNET sites with the use of a methodology that circumvents confounding effects.” The international team of 14 researchers—from Belgium, Canada, Germany, Italy, Norway, Portugal, Switzerland, and the United States—reports their results “may partly explain recent findings indicating a less pronounced climate-carbon cycle sensitivity (Frank et al., 2010) than assumed by current climate-carbon cycle model parameterizations.” In fact, “contrary to previous findings,” they state their results “suggest that Q10 is independent of mean annual temperature, does not differ among biomes, and is confined to values around 1.4 ± 0.1”

Perhaps the most significant consequence of this new assessment was articulated by Reich (2010) in a Perspective article in Science that discusses the Mahecha et al. findings; he writes that their new work “reduces fears that respiration fluxes may increase strongly with temperature, accelerating climate change.” This concern longer appears to have much support within the community of global-change researchers.

Two years earlier, Burton et al. (2008) also had cited the theory that “increases in terrestrial ecosystem respiration as temperatures warm could create a positive feedback that causes atmospheric CO2 concentration, and subsequently global temperature, to increase more rapidly,” but they suggested “if plant tissue respiration acclimates to temperature over time, this feedback loop will be weakened, reducing the potential temperature increase.” And when Burton et al. demonstrated that plant tissue respiration does indeed so acclimate, Bradford et al. (2008) explored the same subject as it pertains to soil microbial respiration, acquiring pertinent data obtained as late as 15 years after the start of a soil-warming experiment established in 1991 on an even-aged mixed-deciduous tract of trees in the Harvard Forest (Massachusetts, USA), where heating cables were used to elevate soil temperatures to 5°C above ambient temperatures measured in non-heated control plots. They discovered—as has been found by many others in long-term field experiments—that “elevated respiration rates under soil warming return to pre-warming values within a few years,” citing the corroborative findings of Jarvis and Linder (2000), Oechel et al. (2000), Luo et al. (2001), Rustad et al. (2001), Melillo et al. (2002), and Eliasson et al. (2005).

In light of this wealth of empirical observations, the nine researchers state, in the concluding sentence of the abstract of their paper, “stimulatory effects of global temperature rise on soil respiration rates may be lower than currently predicted,” and in the final sentence of the body of their paper they reiterate, in slightly different language, “the changes in the biomass and physiology of soil microbial communities that we observed may decrease the expected strength of climate warming on soil respiration rates,” a state of affairs that ends up “reducing the potential temperature increase.”

In a temperate steppe grassland located in Duolun County, Inner Mongolia, China (42°02’N, 116°17’E, 1,324 meters above sea level), Wan et al. (2009) suspended infrared radiators 2.25 meters above the ground over 24 plots divided into four temperature treatments: (1) control, (2) day (06:00–18:00, local time) warming, (3) night (18:00–06:00) warming, and (4) diurnal (24–hour) warming,. They then measured diurnal cycles of net ecosystem gas exchange and daytime ecosystem respiration twice a month over the growing seasons (May–October) of 2006, 2007, and 2008.

The researchers found “nocturnal warming increased leaf respiration of two dominant grass species by 36.3%, enhanced consumption [drawdown] of carbohydrates in the leaves (72.2% and 60.5% for sugar and starch, respectively), and consequently stimulated plant photosynthesis by 19.8% in the subsequent days.” In addition, they state “the enhancement of plant photosynthesis overcompensated the increased carbon loss via plant respiration under nocturnal warming and shifted the steppe ecosystem from a minor carbon source (1.87 g C/m2/year) to a carbon sink (21.72 g C/m2/year) across the three growing seasons.” In addition, the four Chinese researchers note the local climate in their study area “experienced asymmetrical diurnal warming (0.57, 0.45 and 0.30°C increases in daily minimum, mean and maximum temperatures per decade, respectively) over the past half century (1953–2005),” and “similar diurnal scenarios of climate warming have been widely reported at the regional and global scales,” citing the studies of Karl et al. (1991), Easterling et al. (1997), Stone and Weaver (2002), Vose et al. (2005), Lobell et al. (2007), and Zhou et al. (2007). Consequently, and in light of the many well-documented “greater increases in daily minimum than maximum temperature” that have been observed throughout the world, Wan et al. conclude, “plant photosynthetic overcompensation may partially serve as a negative feedback mechanism for [the] terrestrial biosphere to climate warming,” where “the photosynthetic overcompensation induced by nocturnal warming can ... regulate terrestrial carbon sequestration and negatively feed back to climate change.”

Agricultural Crops

In a controversial study published in the Proceedings of the National Academy of Sciences (USA), Schlenker and Roberts (2009) compared U.S. county-level yields of corn, soybeans, and cotton for the years 1950–2005 with fine-scale weather datasets that incorporated the entire distribution of temperatures that occurred within each day and across all days of the crops’ growing seasons, in order to determine yield responses to the range of temperatures experienced by the crops. They then used the yield vs. temperature relationships they had thereby derived to estimate yield changes expected throughout the remainder of the twenty-first century, based on temperatures predicted to occur by the Hadley III climate model.

The first stage of the scientists’ research indicated yields had historically increased as temperatures rose to an optimum value of 29°C for corn, 30°C for soybeans, and 32°C for cotton. At temperatures above these optimum values, crop yields declined, and they did so with slopes that were significantly steeper than the upward slopes that had preceded them. Then, in the second stage of their research, Schlenker and Roberts found, “holding current growing regions fixed, area-weighted average yields are predicted to decrease by 30–46% before the end of the century under the slowest warming scenario and decrease by 63–82% under the most rapid warming scenario under the Hadley III model.”

This was not good news, but on the upside, it was much too bad to be true. About six weeks later, the Proceedings published a letter by Meerburg et al. (2009) that provided a new perspective on the issue.

The seven Dutch scientists began their critique of Schlenker and Roberts’ study by noting that yields of the crops in question will continue to increase in years to come, because of “the development and adoption of new technologies and improved farm management,” citing Ewert et al. (2005), who found that continuing advances in technology historically have been the most important driver of productivity change, outweighing the negative effects of detrimental climate change. And in further illustration of this phenomenon, Meerburg et al. report that between 1961 and 2007, “average US corn yields increased by 240%, from 3.9 tons per hectare per year to 9.4 tons per hectare per year (FAO, 2009),” while noting some researchers have predicted “advances in agronomics, breeding, and biotechnology will lead to an average corn yield in the US of just over 20 tons per hectare per year in 2030,” citing Duvick (2005).

Meerburg et al. also make note of the fact that farmers in Brazil successfully increased the productivity of soybeans, maize, and cotton during the past decade even though the cumulative number of days of exposure to temperatures above the three crops’ optimum values “is far greater than in the US.” In the Brazilian state of Mato Grosso, for example, “maximum average day temperature exceeds 35°C for 118 days per year, of which 75 days are in the average soybean-growing season.” Nevertheless, they report, in 2008 average production of soybeans was about 3.1 tons per hectare per year in Mexico, while the average yield in the US was 2.8 tons per hectare per year. Similarly, they note the mean cotton yield in Brazil in 2006/2007 was 1.4 tons per hectare per year, while in the U.S. it was only 0.9 tons per hectare per year.

The seven scientists thus conclude “temperatures higher than currently experienced in the US do not necessarily need to coincide with lower crop yields and that already existing technology and future advances (new varieties, optimized farm management, biotechnology, etc.) can overrule the negative effect of increasing temperatures on yield,” as has in fact been observed in the historical crop yield data of the United States.

A final flaw in the analysis of Schlenker and Roberts (2009) is their acknowledged “inability to account for CO2 concentrations,” the increasing levels of which, in their own words, “might spur plant growth and yields,” such that “yield declines stemming from warmer temperatures therefore may be offset by CO2 fertilization.” This has been found to be the case by many different studies, as we recount in Section 5.5 of this report.

In light of Schlenker and Roberts’ stated admissions, therefore, as well as the facts cited by Meerburg et al.—which should have been known by the two U.S. researchers as well as the communicator of their paper to the Proceedings of the National Academy of Sciences and the editorial staff of the journal—it is clear their paper never should have been published, especially with a title that proclaims as fact that “nonlinear temperature effects indicate severe damages to U.S. crop yields under climate change.”

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