Isotopic constraints on the rise in atmospheric methane.

Abstract:

The rise in atmospheric methane that began in 2007 and continued strongly through 2014 has been accompanied by a global decrease in methane's stable carbon isotopic ratio. The δ¹³_CH4 isotopic shifts and loci of growth observed globally imply that a major driver of recent growth in atmospheric methane has been the response of microbial methane sources, particularly tropical wetlands, to meteorological changes over the past 8 years.

In the southern tropics, summer (Dec-March) wetland or ruminant emissions can lead to isotopically depleted excursions, while winter (June-Sept) biomass burning of C4 grasslands will produce isotopically relatively enriched methane. At Ascension (8^oS) in marine boundary air a trend (>0.2‰) to more ¹³C-depleted values began in 2009, becoming more marked with excursions to much more negative values in early 2011 and 2012. Values have since recovered slightly but Ascension δ¹³_CH4 values in early 2015 remained markedly more negative than in 2007-8. At Cape Point (34^oS), the methane record (2011-2014) also shows a similar isotopic shift.

To identify the causes of the large changes in the global methane budget, we perform a budget analysis of methane mole fraction and δ¹³_CH4 data from NOAA and RHUL sites. The increase in tropical methane from 2011 fits a shift to more isotopically negative (lighter) δ¹³_CH4 values, most likely a consequence of the exceptional water transfer to wetland by rainfall in the major La Niña event. Strong tropical wetland emissions may have continued in warm subsequent years. High precipitation and floods east of the Andes, and exceptional global warmth (emissions are exponentially temperature-dependent provided enough water is present) may have combined to give strong emission in 2014. In contrast to the methane increase in the 1980s, which was probably mainly driven by rising anthropogenic emissions, the recent isotopic shift suggests the present growth is more likely to have been a consequence of meteorological events, especially in the tropics.