Some countries have been trying to reduce greenhouse gases emissions over the last decade, but a perfect storm of methane emissions may undo all the good work. According to a new study, the environmental threat posed by China's booming economy has been partially masked by a decline in natural methane emissions from wetlands. Soon, however, the drought that has reduced wetland emissions will end, pumping additional greenhouse gases into the atmosphere as China's own emissions continue to rise.
Methane accounts for about 20% of the 20th century enhanced greenhouse effect. Carbon dioxide makes up about 60%, and other gases account for the remainder. Whereas carbon dioxide emissions have marched steadily upward, methane emissions have varied considerably over the last 25 years. Different explanations have been offered for these fluxes, including emissions from fires, wetlands, melting permafrost, and plants (ScienceNOW, 28 August).
The team--led by climate scientist Philippe Bousquet of Laboratoire des Sciences du Climat et de l'Environnement in Gif-sur-Yvette, France, and colleagues--used a technique known as inverse modeling to figure out why the methane flux has varied. It's a bit like the game show Jeopardy, in which you have an answer but must find the question that best fits it. Methane from each source--such as fires, wetlands, and so on--has a distinctive isotopic signature, which allows researchers to identify the source of the methane with more precision.
The team found that anthropogenic methane emissions began to increase in 1999, especially in northern Asia, probably a result of the fierce pace of growth of China's economy. Meanwhile, natural methane emissions from wetlands around the world declined by 5 to 20 teragrams between 1999 and 2003, somewhat offsetting China's contribution. The results suggest that when the global drying trend is reversed, atmospheric methane levels will increase again, exacerbating global warming.
The biggest concern is what might happen in the Arctic, says co-author Ed Dlugokencky, a research chemist at the National Oceanic and Atmospheric Administration in Boulder, Colorado. As permafrost gets warmer, it could become wetlands. This would release large amounts of additional CO2 and methane and further drive warming trends, the team reports tomorrow in Nature.
"The comparison with model emissions from wetlands driven by climate and biomass-burning models looks good," says David Archer, a geochemist at the University of Chicago in Illinois. "The conclusions about a possible continued rise in atmospheric methane concentration seem sensible and justified."