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Effects of Elevated CO2 on Methane Emissions from Terrestrial Plants Growing Under Aerobic Conditions
Qaderi, M.M. and Reid, D.M. 2011. Stressed crops emit more methane despite the mitigating effects of elevated carbon dioxide. Functional Plant Biology 38: 97-105.

The authors report that the release of aerobic methane (CH4) by vegetation has been indirectly confirmed by the field studies of Braga do Carmo et al. (2006), Crutzen et al. (2006) and Sanhueza and Donoso (2006), as well as by the satellite studies of Frankenberg et al. (2005, 2008). In addition, they note that CH4 emissions from plants can be stimulated by higher air temperatures (Vigano et al., 2008; Qaderi and Reid, 2009) and water stress (Qaderi and Reid, 2009). And since "methane is the second most important long-lived greenhouse gas after carbon dioxide and is thought to be ~25 times more potent than CO2 in its ability to act as a greenhouse gas," as they describe it, they decided to see what effect the ongoing rise in the air's CO2 content might possibly have on this phenomenon.

What was done
Qaderi and Reid "examined the combined effects of temperature, carbon dioxide and watering regime on CH4 emissions from six commonly cultivated crop species: faba bean, sunflower, pea, canola, barley and wheat" in an experiment where "plants were grown from seeds in controlled-environment growth chambers under two temperature regimes (24°C day/20°C night and 30°C day/26°C night), two CO2 concentrations (380 and 760 ppm) and two watering regimes (well watered and water stressed)," where the "plants were first grown under 24/20°C for one week from sowing, and then placed under experimental conditions for a further week," after which "plant growth, gas exchange and CH4 emission rates were determined."

What was learned
First of all, the two researchers say they found "no detectable CH4 from [a] control treatment (without plant tissue), indicating that CH4 from the experimental treatments was emitted only from plant tissues." Second, they found that the plants grown under higher temperature and water stress emitted more CH4 than those grown under lower temperature and no water stress. And third, they found that "elevated CO2 had the opposite effect," so that it "partially reverses" the effects of the other two factors.

What it means
Qaderi and Reid conclude that "although rising atmospheric CO2 reduces plant CH4 emissions, it may not fully reverse the effects of temperature and drought," which they assume will increase in tandem with the ongoing rise in the air's CO2 content. Nevertheless, this result is still a positive finding. In addition, it may well be much more positive than they make it out to be, especially if temperatures and drought do not increase with the passage of time and continued increases in the air's CO2 content, which many -- such as us -- believe to be a real possibility, in that (1) droughts have not been shown to be more prevalent worldwide in warmer as opposed to colder periods of earth's history (see Drought under D in our Subject Index), and (2) due to the natural oscillatory behavior of earth's surface air temperature on millennial timescales -- which over the past two millennia has successively brought us the last phase of the Roman Warm Period, the Dark Ages Cold Period, the Medieval Warm Period, the Little Ice Age, and the initial phase of the Current Warm Period -- it will likely not warm much more than it has already warmed before the globe's mean surface air temperature plateaus out and ultimately begins a slow decline to a cooler state, aided by the ever-increasing CO2-induced reduction in aerobic plant CH4 emissions.

Braga do Carmo, J., Keller, M., Dezincourt Dias, J., Barbosa de Camargo, P. and Crill, P. 2006. A source of methane from upland forests in the Brazilian Amazon. Geophysical Research Letters 33: 10.1029/2005GL025436.

Crutzen, P.J., Sanhueza, E. and Brenninkmeijer, C.A.M. 2006. Methane production from mixed tropical savanna and forest vegetation in Venezuela. Atmospheric Chemistry and Physics Discussions 6: 3093-0397.

Frankenberg, C., Meirink, J.F., van Weele, M., Platt, U. and Wagner, T. 2005. Assessing methane emissions from global space-borne observations. Science 308: 1010-1014.

Frankenberg, C., Bergamaschi, P., Butz, A., Houweling, S., Meirink, J.F., Notholt, J., Petersen, A.K., Schrijver, H., Warneke, T. and Aben, I. 2008. Tropical methane emissions: a revised view from SCIAMACHY onboard ENVISAT. Geophysical Research Letters 35: 10.1029/goo8GL034300.

Qaderi, M.M. and Reid, D.M. 2009. Methane emissions from six crop species exposed to three components of global climate change: temperature, ultraviolet-B radiation and water stress. Physiologia Plantarum 137: 139-147.

Sanhueza, E. and Donoso, L. 2006. Methane emission from tropical savanna Trachypogon sp. grasses. Atmospheric Chemistry and Physics 6: 5315-5319.

Vigano, I., van Weelden, H., Holzinger, R., Keppler, F. and Rockmann, T. 2008. Effect of UV radiation and temperature on the emission of methane from plant biomass and structural components. Biogeosciences Discussions 5: 243-270.

Reviewed 1 June 2011