Environmental Stresses and Plant Methane Emissions

Concluding from a review of the scientific literature that "aerobic CH₄ [methane] emissions from plants may be affected by O₂ stress or any other stress leading to ROS [reactive oxygen species] production," authors Wang et al. (2009) sought to determine whether physical injury would also affect CH₄ emissions from plants. Their work revealed that "physical injury (cutting) stimulated CH₄ emissions from fresh twigs of Artemisia species under aerobic conditions," and that "more cutting resulted in more CH₄ emissions," as did hypoxia in both cut and uncut Artemisia frigida twigs.

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.

Additional Reference
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.