Atmospheric Methane Levels are Increasing, but Why?

methane_eoe_atmosphere

by Emil Morhardt

Methane (CH4) is a much stronger—30 times as strong—greenhouse gas than CO2 in the short term, but has a much shorter atmospheric lifetime, being oxidized to CO2 by another atmospheric chemical entity, the hydroxyl radical (OH-). But either the production of CH4 has been increasing, or that of OH- decreasing, because since 2007 atmospheric levels of CH4 have increased by 3% after years of being flat. Why?

Is it the large amounts of CH4 now known to have been released by fracking? This has seemed a likely candidate, but Paul Voosen (2016) writing about a session in the American Geophysical Union annual meeting in San Francisco in December, indicates that the data are not compelling.

Is it that more cows are belching CH4? Probably not because cow numbers have been relatively flat since 2007, whereas they were increasing before when CH4 levels were relatively constant.

Is it rising rice cultivation? Apparently not likely.

A new candidate is that heavy tropical rains from 2008 to 2014, likely a result of global warming and climate change, have increased tropical wetlands, known habitat for methanogenic bacteria, a major source of CH4. If this pans out, and the results are shown to be accurate, then we have another example of positive feedback likely accelerating the rate of global warming. The fact that that the percentage of carbon-12 to carbon-13 in atmospheric methane is increasing points toward the production of new methane rather than the release of fossil methane from oil and gas wells. But this same change in CH4 composition might also indicate a decline in OH- production. How would that occur? Perhaps clean air regulations have reduced the production of nitrous oxide sufficiently to slow OH- production.

If this all seems confusing, it isn’t surprising. It is difficult enough to piece together these subtle atmospheric changes when they are documented by detailed scientific research. So far, most of the data leading to the possibility discussed in Voosen’s piece are unpublished, and, according to some of the scientists involved, highly anecdotal. We shall probably know more as 2017 proceeds.

Voosen, P., 2016. Scientists flag new causes for surge in methane levels. Science 354, 1513.

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