Unusually high temperatures in the Arctic and heavy rains in the tropics likely drove a global increase in atmospheric methane in 2007 and 2008 after a decade of near-zero growth, according to a new study. Methane is the second most abundant greenhouse gas after carbon dioxide, albeit a distant second.
NOAA scientists and their colleagues analyzed measurements from 1983 to 2008 from air samples collected weekly at 46 surface locations around the world. Their findings will appear in the September 28 print edition of the American Geophysical Union's Geophysical Research Letters and are available online now. "At least three factors likely contributed to the methane increase," said Ed Dlugokencky, a methane expert at NOAA's Earth System Research Laboratory in Boulder, Colo. "It was very warm in the Arctic, there was some tropical forest burning, and there was increased rain in Indonesia and the Amazon."
In the tropics, the scientists note, the increased rainfall resulted in longer periods of rainfall and larger wetland areas, allowing microbes to produce more methane. Starting in mid-2007, scientists noticed La Niña conditions beginning, waning and then intensifying in early 2008. This kind of climate condition typically brings wetter-than-normal conditions in some tropical regions and cooler sea surface temperatures in the central and eastern tropical Pacific Ocean. It can persist for as long as two years. In the United States, La Niña often signals drier-than-normal conditions in the Southwest and Central Plains regions, and wetter fall and winter seasons in the Pacific Northwest.
Observations from satellites and ground sites suggest that biomass burning - the burning of plant and other organic material that releases carbon dioxide and methane - contributed about 20 percent, of the total methane released into the atmosphere in 2007. However, during the scientists' 2007 measurement of methane for northern wetland regions, including the Arctic, temperatures for the year were the warmest on record. This temperature increase coincided with the large jump in the amount of methane measured in that area.
Dlugokencky and his colleagues from the United States and Brazil note that while climate change can trigger a process which converts trapped carbon in permafrost to methane, as well as release methane embedded in Arctic hydrates - a compound formed with water - their observations "are not consistent with sustained changes there yet." Methane is typically created in oxygen-deprived environments, such as flooded wetlands, peat bogs, rice paddies, landfills, termite colonies, and the digestive tracts of cows and other ruminant animals. The gas also escapes during fossil fuel extraction and distribution and is emitted during fires.
Authors of the study are Dlugokencky, L. Bruhwiler, P.C. Novelli, S. A. Montzka, K. A. Masarie, P. M. Lang, A.M. Crotwell, and J.B. Miller of NOAA's Earth System Research Laboratory, Boulder, Colo.; J.W.C. White of the Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colo.; L. K. Emmons of the National Center for Atmospheric Research, Boulder, Colo.; and L.V. Gatti of the Laboratorio de Quimica Atmosferica, Instituto de Pesquisas Energéticas e Nucleares, São Paulo, Brazil. Crotwell and Miller are also at the Cooperative Institute for Research in Environmental Sciences in Boulder, Colo. The paper is available online.