Environmental Stresses and Plant Methane Emissions
Wang, Z.-P., Gulledge, J., Zheng, J.-Q., Liu, W., Li, L.-H. and Han, X.-G. 2009. Physical injury stimulates aerobic methane emissions from terrestrial plants. Biogeosciences 6: 615-621.
In discussing their findings, and those of previous studies that suggest, in their words, "that a variety of environmental stresses stimulate CH4 emission from a wide variety of plant species," Wang et al. concluded that "global change processes, including climate change, depletion of stratospheric ozone, increasing ground-level ozone, spread of plant pests, and land-use changes, could cause more stress in plants on a global scale, potentially stimulating more CH4 emission globally," while further concluding that "the role of stress in plant CH4 production in the global CH4 cycle could be important in a changing world."
Several things "could" be important in this regard, but the ongoing rise in the air's CO2 content is hard at work countering stress-induced CH4 emissions. Environmental stresses of all types do indeed generate highly-reactive oxygenated compounds that damage plants, but atmospheric CO2 enrichment typically boosts the production of antioxidant enzymes that scavenge and detoxify those highly-reactive oxygenated compounds. Thus, it can be appreciated that the historical rise in the air's CO2 content should have gradually been alleviating the level of stress experienced by Earth's plants; and this phenomenon should have been gradually reducing the rate at which the planet's vegetation releases CH4 to the atmosphere. In addition, it should have been doing it at an accelerating rate commensurate with the accelerating rate of the upward trend in the air's CO2 content.
Wang et al.'s way of thinking therefore suggests that the air's CH4 concentration should be rising ever faster, as "global change processes" lead to more plant stress, more ROS production in plants, and more CH4 emissions from Earth's vegetation, whereas a conflicting hypothesis suggests that the air's CH4 concentration should be rising ever slower, as higher concentrations of atmospheric CO2 lead to less plant stress, more antioxidants that scavenge and detoxify ROS in plants, and less CH4 emissions from Earth's vegetation.
So which view is winning? A quick glance at the atmosphere's recent methane history - shown below - provides the answer.
Figure 1. Trace gas mole fractions of methane (CH4) as measured at Mauna Loa, Hawaii. Adapted from Schnell and Dlugokencky (2008).
As can be seen from this figure, the rate of increase in atmospheric methane abundance has steadily declined since the late 1980s, with near-zero increase from 1999 through the end of the record. Is the ongoing rise in the air's CO2 content responsible for knocking its biggest greenhouse-gas competitor (other than water vapor) entirely out of the picture with respect to future global warming? Or, will further increases in CO2 emissions actually cause the atmosphere's methane concentration to decline and thereby begin to counteract its (CO2's) own warming effect. Only time will tell.
Schnell, R.C. and Dlugokencky, E. 2008. Methane. In: Levinson, D.H. and Lawrimore, J.H., Eds. State of the Climate in 2007. Special Supplement to the Bulletin of the American Meteorological Society 89: S27.