tropics

14th January 2010.

Words by Anna Pigott.

During 2007 and 2008 concentrations of atmospheric methane increased after a decade of near-zero growth.

In 2007, globally averaged CH4 increased by ~8.3 ppb, with the greatest increase recorded over polar northern latitudes and in the Southern Hemisphere. In 2008, CH4 in the atmosphere increased by ~4.4 ppb, but this time with no increase observed in the northern latitudes.

The team behind this study, lead by Ed Dlugokencky at NOAA, used atmospheric observation techniques to investigate the reasons behind the increases. They discovered two likely contributory factors: first, 2007 and 2008 had the third largest and largest (respectively) positive rainfall anomalies for the Tropics since 1986, brought on by La Niña conditions. This had the effect of increasing tropical wetland area, so allowing microbes inhabiting them to produce more methane. Second, the large increase in CH4 in polar northern latitudes in 2007 coincided with the warmest year recorded for those regions during the entire study period. Higher temperatures encourage a process that converts trapped permafrost carbon to methane, and may also encourage the release of methane from hydrates.

There has been much concern and speculation over the global effects of methane release from Arctic environments of late. The authors write that the return to zero growth in methane from northern latitudes observed in 2008 “suggests we have not yet activated strong climate feedbacks from permafrost and hydrates”, though they point out that, with between 50% and 90% of near-surface permafrost projected to melt by 2100, this is a serious possibility. In addition, the study highlights the relatively small contribution (~20%) that the burning of biomass had to methane levels during 2007 (the year of greatest increase) when compared to the effects of increased rainfall and temperature.

Reference: E. J. Dlugokencky, et al. (2009), Observational constraints on recent increases in the atmospheric CH burden, Geophys. Res. Lett., 36, L18803, doi:10.1029/2009GL039780.