From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
What impact do global warming, the ongoing rise in the air’s carbon dioxide (CO2) content and a number of other contemporary environmental trends have on the atmosphere’s methane (CH4) concentration? The implications of this question are huge because methane is a more powerful greenhouse gas, molecule for molecule, than is carbon dioxide. Its atmospheric concentration is determined by the difference between how much CH4 goes into the air (emissions) and how much comes out of it (extractions) over the same time period. There are significant forces at play that will likely produce a large negative feedback toward the future warming potential of this powerful greenhouse gas, nearly all of which forces are ignored by the IPCC.
Early indications that atmospheric CO2 enrichment might significantly reduce methane emissions associated with the production of rice were provided by Schrope et al. (1999), who studied batches of rice growing in large vats filled with topsoil and placed within greenhouse tunnels maintained at atmospheric CO2 concentrations of 350 and 700 ppm, each of which tunnels was further subdivided into four sections that provided temperature treatments ranging from ambient to as much as 5°C above ambient. As would be expected, doubling the air’s CO2 content significantly enhanced rice biomass production in this system, increasing it by up to 35 percent above-ground and by up to 83 percent below-ground. However, in a truly unanticipated development, methane emissions from the rice grown at 700 ppm CO2 were found to be 10 to 45 times less than emissions from the plants grown at 350 ppm. As Schrope et al. describe it, “the results of this study did not support our hypothesis that an effect of both increased carbon dioxide and temperature would be an increase in methane emissions.” Indeed, they report that “both increased carbon dioxide and increased temperatures were observed to produce decreased methane emissions,” except for the first 2°C increase above ambient, which produced a slight increase in methane evolution from the plant-soil system.
In checking for potential problems with their experiment, Schrope et al. could find none. They thus stated that their results “unequivocally support the conclusion that, during this study, methane emissions from Oryza sativa [rice] plants grown under conditions of elevated CO2 were dramatically reduced relative to plants gown in comparable conditions under ambient levels of CO2,” and to be doubly sure of this fact, they went on to replicate their experiment in a second year of sampling and obtained essentially the same results. Four years later, however, a study of the same phenomenon by a different set of scientists yielded a different result in a different set of circumstances.
Inubushi et al. (2003) grew a different cultivar of rice in 1999 and 2000 in paddy culture at Shizukuishi, Iwate, Japan in a FACE study where the air’s CO2 concentration was increased 200 ppm above ambient. They found that the extra CO2 “significantly increased the CH4 [methane] emissions by 38 percent in 1999 and 51 percent in 2000,” which phenomenon they attributed to “accelerated CH4 production as a result of increased root exudates and root autolysis products and to the increased plant-mediated CH4 emission because of the higher rice tiller numbers under FACE conditions.” With such a dramatically different result from that of Schrope et al., many more studies likely will be required to determine which of these results is the more typical of rice culture around the world.
A somewhat related study was conducted by Kruger and Frenzel (2003), who note that “rice paddies contribute approximately 10-13 percent to the global CH4 emission (Neue, 1997; Crutzen and Lelieveld, 2001),” and that “during the next 30 years rice production has to be increased by at least 60 percent to meet the demands of the growing human population (Cassman et al., 1998).” Because of these facts they further note that “increasing amounts of fertilizer will have to be applied to maximize yields [and] there is ongoing discussion on the possible effects of fertilization on CH4 emissions.”
To help promote that discussion, Kruger and Frenzel investigated the effects of N-fertilizer (urea) on CH4 emission, production, and oxidation in rice culture in laboratory, microcosm and field experiments they conducted at the Italian Rice Research Institute in northern Italy. They report that in some prior studies “N-fertilisation stimulated CH4 emissions (Cicerone and Shetter, 1981; Banik et al., 1996; Singh et al., 1996),” while “methanogenesis and CH4 emission was found to be inhibited in others (Cai et al., 1997; Schutz et al., 1989; Lindau et al., 1990),” similar to the polarized findings of Schrope et al. and Inubushi et al. with respect to the effects of elevated CO2 on methane emissions. In the mean, therefore, there may well be little to no change in overall CH4 emissions from rice fields in response to both elevated CO2 and increased N-fertilization. With respect to their own study, for example, Kruger and Frenzel say that “combining our field, microcosm and laboratory experiments we conclude that any agricultural praxis improving the N-supply to the rice plants will also be favourable for the CH4 oxidising bacteria,” noting that “N-fertilisation had only a transient influence and was counter-balanced in the field by an elevated CH4 production.” The implication of these findings is well articulated in the concluding sentence of their paper: “neither positive nor negative consequences for the overall global warming potential could be found.”
Another agricultural source of methane is the fermentation of feed in the rumen of cattle and sheep. Fievez et al. (2003) studied the effects of various types and levels of fish-oil feed additives on this process by means of both in vitro and in vivo experiments with sheep, observing a maximal 80 percent decline in the ruminants’ production of methane when using fish-oil additives containing n-3-eicosapentanoic acid. With respect to cattle, Boadi et al. (2004) report that existing mitigation strategies for reducing CH4 emissions from dairy cows include the addition of ionophores and fats to their food, as well as the use of high-quality forages and grains in their diet, while newer mitigation strategies include “the addition of probiotics, acetogens, bacteriocins, archaeal viruses, organic acids, [and] plant extracts (e.g., essential oils) to the diet, as well as immunization, and genetic selection of cows.” To this end, they provide a table of 20 such strategies, where the average maximum potential CH4 reduction that may result from the implementation of each strategy is 30 percent or more.
With as many as 20 different mitigation strategies from which to choose, each one of which (on average) has the potential to reduce CH4 emissions from dairy cows by as much as a third, it would appear there is a tremendous potential to dramatically curtail the amount of CH4 released to the atmosphere by these ruminants and, by implication, the host of other ruminants that mankind raises and uses for various purposes around the world. Such high-efficiency approaches to reducing the strength of the atmosphere’s greenhouse effect, while not reducing the biological benefits of elevated atmospheric CO2 concentrations in the process, should be at the top of any program designed to achieve that difficult (but still highly questionable) objective.
In view of these several observations, we can be cautiously optimistic about our agricultural intervention capabilities and their capacity to help stem the tide of earth’s historically rising atmospheric methane concentration, which could take a huge bite out of methane-induced global warming. But do methane emissions from natural vegetation respond in a similar way?
We have already discussed the results of Davidson et al. (2004) in our Nitrous Oxide section, which results suggest that a global warming-induced drying of the Amazon Basin would initiate a strong negative feedback to warming via (1) large drying-induced reductions in the evolution of N2O and CH4 from its soils and (2) a huge drying-induced increase in the consumption of CH4 by its soils. In a contemporaneous study, Strack et al. (2004) also reported that climate models predict increases in evapotranspiration that could lead to drying in a warming world and a subsequent lowering of water tables in high northern latitudes. This prediction cries out for an analysis of how lowered water tables will affect peatland emissions of CH4.
In a theoretical study of the subject, Roulet et al. (1992) calculated that for a decline of 14 cm in the water tables of northern Canadian peatlands, due to climate-model-derived increases in temperature (3°C) and precipitation (1mm/day) predicted for a doubling of the air’s CO2 content, CH4 emissions would decline by 74-81 percent. In an attempt to obtain some experimental data on the subject, at various times over the period 2001-2003 Strack et al. measured CH4 fluxes to the atmosphere at different locations that varied in depth-to-water table within natural portions of a poor fen in central Quebec, Canada, as well as within control portions of the fen that had been drained eight years earlier.
At the conclusion of their study, the Canadian scientists reported that “methane emissions and storage were lower in the drained fen.” The greatest reductions (up to 97 percent) were measured at the higher locations, while at the lower locations there was little change in CH4 flux. Averaged over all locations, they determined that the “growing season CH4 emissions at the drained site were 55 percent lower than the control site,” indicative of the fact that the biosphere appears to be organized to resist warming influences that could push it into a thermal regime that might otherwise prove detrimental to its health.
In another experimental study, Garnet et al. (2005) grew seedlings of three emergent aquatic macrophytes (Orontium aquaticum L., Peltandra virginica L., and Juncus effusus L.) plus one coniferous tree (Taxodium distichum L.), all of which are native to eastern North America, in a five-to-one mixture of well-fertilized mineral soil and peat moss in pots submerged in water in tubs located within controlled environment chambers for a period of eight weeks. Concomitantly, they measured the amount of CH4 emitted by the plant foliage, along with net CO2 assimilation rate and stomatal conductance, which were made to vary by changing the CO2 concentration of the air surrounding the plants and the density of the photosynthetic photon flux impinging on them.
Methane emissions from the four wetland species increased linearly with increases in both stomatal conductance and net CO2 assimilation rate; but the researchers found that changes in stomatal conductance affected foliage methane flux “three times more than equivalent changes in net CO2 assimilation,” making stomatal conductance the more significant of the two CH4 emission-controllers. In addition, they note that evidence of stomatal control of CH4 emission has also been reported for Typha latifolia (Knapp and Yavitt, 1995) and Carex (Morrissey et al., 1993), two other important wetland plants. Hence, since atmospheric CO2 enrichment leads to approximately equivalent—but oppositely directed—changes in foliar net CO2 assimilation (which is increased) and stomatal conductance (which is reduced) in most herbaceous plants (which are the type that comprise most wetlands), it can be appreciated that the ongoing rise in the air’s CO2 content should be acting to reduce methane emissions from earth’s wetland vegetation, because of the three-times-greater negative CH4 emission impact of the decrease in stomatal conductance compared to the positive CH4 emission impact of the equivalent increase in net CO2 assimilation.
According to Prinn et al. (1992), one of the major means by which methane is removed from the atmosphere is via oxidation by methanotrophic bacteria in the aerobic zones of soils, the magnitude of which phenomenon is believed to be equivalent to the annual input of methane to the atmosphere (Watson et al., 1992). This soil sink for methane appears to be ubiquitous, as methane uptake has been observed in soils of tundra (Whalen and Reeburgh, 1990), boreal forests (Whalen et al., 1992), temperate forests (Steudler et al., 1989; Yavitt et al., 1990), grasslands (Mosier et al., 1997), arable lands (Jensen and Olsen, 1998), tropical forests (Keller, 1986; Singh et al., 1997), and deserts (Striegl et al., 1992), with forest soils—especially boreal and temperate forest upland soils (Whalen and Reeburgh, 1996)—appearing to be the most efficient in this regard (Le Mer and Roger, 2001).
In an attempt to learn more about this subject, Tamai et al. (2003) studied methane uptake rates by the soils of three Japanese cypress plantations composed of 30- to 40-year-old trees. Through all seasons of the year, they found that methane was absorbed by the soils of all three sites, being positively correlated with temperature, as has also been observed in several other studies (Peterjohn et al., 1994; Dobbie and Smith, 1996); Prieme and Christensen, 1997; Saari et al., 1998). Methane absorption was additionally—and even more strongly—positively correlated with the C/N ratio of the cypress plantations’ soil organic matter. Based on these results, it can be appreciated that any global warming, CO2-induced or natural, would produce two biologically mediated negative feedbacks to counter the increase in temperature: (1) a warming-induced increase in methane uptake from the atmosphere that is experienced by essentially all soils, and (2) an increase in soil methane uptake from the atmosphere that is produced by the increase in plant-litter C/N ratio that typically results from atmospheric CO2 enrichment.
Another study that deals with this topic is that of Menyailo and Hungate (2003), who assessed the influence of six boreal forest species—spruce, birch, Scots pine, aspen, larch, and Arolla pine—on soil CH4 consumption in the Siberian artificial afforestation experiment, in which the six common boreal tree species had been grown under common garden conditions for the past 30 years under the watchful eye of the staff of the Laboratory of Soil Science of the Institute of Forest, Siberian Branch of the Russian Academy of Sciences (Menyailo et al., 2002). They determined, in their words, that “soils under hardwood species (aspen and birch) consumed CH4 at higher rates than soils under coniferous species and grassland.” Under low soil moisture conditions, for example, the soils under the two hardwood species consumed 35 percent more CH4 than the soils under the four conifers; under high soil moisture conditions they consumed 65 percent more. As for the implications of these findings, Pastor and Post (1988) have suggested, in the words of Menyailo and Hungate, that “changes in temperature and precipitation resulting from increasing atmospheric CO2 concentrations will cause a northward migration of the hardwood-conifer forest border in North America.” Consequently, if such a shifting of species does indeed occur, it will likely lead to an increase in methane consumption by soils and a reduction in methane-induced global warming potential, thereby providing yet another biologically mediated negative feedback factor that has yet to be incorporated into models of global climate change.
Last, we note that increases in the air’s CO2 concentration will likely lead to a net reduction in vegetative isoprene emissions, which, as explained in Section 7.7.1. under the heading of Isoprene, should also lead to a significant removal of methane from the atmosphere. Hence, as the air’s CO2 content—and possibly its temperature—continues to rise, we can expect to see a significant increase in the rate of methane removal from earth’s atmosphere, which should help to reduce the potential for further global warming.
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/m/methaneextract.php, http://www.co2science .org/subject/m/methaneag.php, and http://www.co2 science.org/subject/m/methagnatural.php.
In Section 2.6.1, we reported on several real-world phenomena that can act to reduce or extract methane (CH4) from the atmosphere, most of which feedbacks are enhanced as the air’s CO2 concentration rises. That those feedbacks may already be operating and having a significant impact on global methane concentrations is illustrated in a discussion of observed atmospheric methane trends.
We begin with Figure 126.96.36.199, the graph of real-world data from Simpson et al. (2002), which clearly shows a linear-trend decline in CH4 growth rates since the mid-1980s. The authors contended it was “premature to believe” the rate of growth was falling, even though their own data bore witness against them. Figure 188.8.131.52. Global tropospheric methane (CH4) growth rate vs. time. Adapted from Simpson et al. (2002).
The first of the 1990s’ large CH4 spikes is widely recognized as having been caused by the eruption of Mt. Pinatubo in June 1991 (Bekki et al., 1994; Dlugokencky et al., 1996; Lowe et al., 1997), while the last and most dramatic of the spikes has been linked to the remarkably strong El Niño of 1997-98 (Dlugokencky et al., 2001). As noted earlier, Dlugokencky et al. (1998), Francey et al. (1999), and Lassey et al. (2000) have all suggested that the annual rate-of-rise of the atmosphere’s CH4 concentration is indeed declining and leading to a cessation of growth in the atmospheric burden of methane.
Dlugokencky et al. (2003) revisited the subject with an additional two years’ of data. Based on measurements from 43 globally distributed remote boundary-layer sites that were obtained by means of the methods of Dlugokencky et al. (1994), they defined an evenly spaced matrix of surface CH4 mole fractions as a function of time and latitude, from which they calculated global CH4 concentration averages for the years 1984-2002. We have extracted the results from their graphical presentation and re-plotted them as shown in Figure 184.108.40.206.
With respect to these data, Dlugokencky et al. note that the globally averaged atmospheric methane concentration “was constant at ~1751 ppb from 1999 through 2002,” which suggests, in their words, that “during this 4-year period the global methane budget has been at steady state.” They caution, however, that “our understanding is still not sufficient to tell if the prolonged pause in CH4 increase is temporary or permanent.” We agree. However, we feel confident in suggesting that if the recent pause in CH4 increase is indeed temporary, it will likely be followed by a decrease in CH4 concentration, since that would be the next logical step in the observed progression from significant, to much smaller, to no yearly CH4 increase.
Khalil et al. (2007) essentially “put the nails in the coffin” of the idea that rising atmospheric CH4 concentrations pose any further global warming threat at all. In their study, the three Oregon (USA) researchers combined two huge atmospheric methane datasets to produce the unified dataset depicted in Figure 220.127.116.11.
Figure 18.104.22.168. Global methane (CH4) concentration. Adapted from Khalil et al. (2007).
In viewing this graph, to which we have added the smooth line, it is clear that the rate of methane increase in the atmosphere has dropped dramatically over time. As Khalil et al. describe it, “the trend has been decreasing for the last two decades until the present when it has reached near zero,” and they go on to say that “it is questionable whether human activities can cause methane concentrations to increase greatly in the future.” Figure 22.214.171.124. Global tropospheric methane (CH4) concentration vs. time. Adapted from Dlugokencky et al. (2003).
One year later, Schnell and Dlugokencky (2008) provided an update through 2007 of atmospheric methane concentrations as determined from weekly discrete samples collected on a regular basis since 1983 at the NOAA/ESRL Mauna Loa Observatory. Our adaptation of the graphical rendition of the data provided by the authors is presented in Figure 126.96.36.199.
Figure 188.8.131.52. Trace gas mole fractions of methane (CH4) as measured at Mauna Loa, Hawaii. Adapted from Schnell and Dlugokencky (2008).
In commenting on the data contained in the figure above, Schnell and Dlugokencky state that “atmospheric CH4 has remained nearly constant since the late 1990s.” This is a most important finding, because, as they also note, “methane’s contribution to anthropogenic radiative forcing, including direct and indirect effects, is about 0.7 Wm-2, about half that of CO2.” In addition, they say that “the increase in methane since the preindustrial era is responsible for approximately one-half the estimated increase in background tropospheric O3 during that time.”
Most recently, Rigby et al. (2008) analyzed methane data obtained from the Advanced Global Atmospheric Gases Experiment (AGAGE) and the Australian Commonwealth Scientific and Industrial Research Organization (CSIRO) over the period January 1997 to April 2008. The results of their analysis indicated that methane concentrations “show renewed growth from the end of 2006 or beginning of 2007 until the most recent measurements,” with the record-long range of methane growth rates mostly hovering about zero, but sometimes dropping five parts per billion (ppb) per year into the negative range, while rising near the end of the record to mean positive values of 8 and 12 ppb per year for the two measurement networks.
Although some people might be alarmed by these findings, as well as by the US, UK, and Australian researchers’ concluding statement that the methane growth rate during 2007 “was significantly elevated at all AGAGE and CSIRO sites simultaneously for the first time in almost a decade,” there is also reassurance in the recent findings. We note, for example, that near the end of 1998 and the beginning of 1999, both networks measured even larger methane growth rate increases of approximately 13 ppb per year, before dropping back to zero at the beginning of the new millennium. And we note that the most current displayed data from the two networks indicate the beginning of what could well be another downward trend.
Additional reassurance in this regard comes from the work of Simpson et al. (2002), the findings of whom we reproduced previously in Figure 184.108.40.206. As can be seen there, even greater methane growth rates than those observed by Rigby et al. occurred in still earlier years. Hence, these periodic one-year-long upward spikes in methane growth rate must be the result of some normal phenomenon, the identity of which has yet to be determined.
In light of these finding, it can be appreciated that over the past decade there have been essentially no increases in methane emissions to the atmosphere, and that the leveling out of the atmosphere’s methane concentration—the exact causes of which, in the words of Schnell and Dlugokencky, “are still unclear”—has resulted in a one-third reduction in the combined radiative forcing that would otherwise have been produced by a continuation of the prior rates-of-rise of the concentrations of the two atmospheric greenhouse gases.
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/m/methaneatmos.php.
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