Are Recent Increases in Atmospheric Methane Related to Arctic Climate Change?
Atmospheric methane (CH4), a strong greenhouse gas, affects background air quality because it is a precursor for O3 production. Measurements of atmospheric CH4 from air samples collected weekly at 46 remote surface sites show that after a decade of near-zero growth, globally averaged atmospheric methane increased during 2007 and 2008. During 2007, CH4 increased by 7.7±0.2 ppb. CH4 mole fractions averaged over polar northern latitudes and the southern hemisphere increased more than other zonally averaged regions. In 2008, globally averaged CH4 increased by 6.9±0.2 ppb; the largest increase was in the tropics, while polar northern latitudes did not increase. During the first half of 2009, globally averaged atmospheric CH4 was ~7 ppb greater than it was in 2008, suggesting that the increase will continue in 2009. There is the potential for increased CH4 emissions from strong positive climate feedbacks in the Arctic where there are huge stores of carbon in permafrost and hydrates, so the causes of these recent increases must be understood.
The sources typically responsible for interannual variability in CH4 growth rate are wetlands and biomass burning. Satellite and in situ CO observations suggest only a minor contribution to increased CH4 from biomass burning. The most likely drivers of the CH4 anomalies observed during 2007 and 2008 are anomalously high temperatures in the Arctic and greater than average precipitation in the tropics. In the Arctic, the unusual warmth observed during 2007 has been rare, but the occurrence of such conditions is predicted by climate models to become more frequent in the future, with a potentially large positive feedback on climate from the resulting carbon emissions. A return to zero CH4 growth rate in the Arctic during 2008 suggests the 2007 anomaly was part of natural variability, and not yet a sign that feedbacks in the Arctic have emerged as a significant term in the global CH4 budget.
