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SCIENTISTS PINPOINT CAUSE OF SLOWING METHANE EMISSIONS
Trend May Revert to 'Typical' Rate of Increase, With Climate Impacts

NOAA image of global average and growth rate of atmospheric methane.Sept. 28, 2006 Scientists have been trying to figure out for years why a buildup of atmospheric methane, the second most abundant greenhouse gas, slowed dramatically in the last decade after levels had nearly tripled since preindustrial times. Now, an international team of scientists, including two at the NOAA Earth System Research Lab in Boulder, Colo., attributes the 15-year lull to a temporary decline in industrial emissions during the 1990s, along with a slowdown in wetland emissions during prolonged droughts. (Click NOAA image for larger view of global average and growth rate of atmospheric methane. Click here for high resolution version. Please credit “NOAA.”)

The authors warn that methane levels could resume a more typical rate of increase soon "with significant impact on climate." The analyses were largely based on 20 years of NOAA global methane data. The findings appear in the Sept. 28 issue of the journal Nature.

"If methane's growth rate resumes its historic pace, its future contribution to global warming could be significant," says Ed Dlugokencky, an author of the Nature paper and a methane expert with the NOAA Earth System Research Lab in Boulder. Philippe Bousquet and Philippe Ciais of France's National Council for Scientific Research and the University of Versailles are the paper's lead authors.

NOAA image of global distribution of atmospheric methane.In recent decades, extracting and distributing fossil fuels have been a major source of atmospheric methane in northern latitudes. A slowdown in the production of coal, oil and natural gas following the breakup of the Soviet Union in 1991 reduced methane emissions during the 1990s, along with an ongoing drought that shrunk tropical wetlands, according to the study. Spurred on by China's economic development, methane emissions have risen again since 1999, but this recent increase has been masked by a simultaneous dip in northern wetland emissions caused by dryer conditions there since 1999. (Click NOAA image for larger view of global distribution of atmospheric methane. Click here for high resolution version. Please credit “NOAA.”)

In the 1980s, methane increased at a fairly steady rate of about 12 parts per billion (ppb) per year. That rate dropped to 4 ppb during the following decade, with much larger fluctuations. Carbon dioxide molecules in the atmosphere outnumber those of methane 200 times. This is partly attributed to the fact that methane's average lifetime in the atmosphere is 8.5 years, compared to carbon dioxide, which is well over 100 years. However, methane is 20 times more efficient as a heat-trapping greenhouse gas.

"While many efforts have made progress in explaining short-term influences on methane emissions over the past 15 years, this is a comprehensive study that helps explain why emissions failed to resume their historic pace over the entire period," says author John B. Miller. Miller is a scientist at NOAA ESRL through the Cooperative Institute for Research in the Environmental Sciences, a joint NOAA-University of Colorado institute. Shorter-term influences on changes in atmospheric methane levels are wildfires, the amount of methane-destroying hydroxyl radical in the atmosphere and interannual wind patterns.

The team analyzed methane data from an international network of 68 monitoring stations (including 50 NOAA sites) around the world to quantify changes in emissions. To trace sources, they also examined isotopic data for tell-tale "fingerprints" and inverted results from transport and chemistry models to work backwards through the gas's atmospheric transport and evolution.

"Better knowledge of the current CH4 [methane] budget helps to reduce uncertainties in future projections of climate change and of tropospheric ozone evolution, and to design effective mitigation strategies," report the authors.

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. However, a recent study showed that plants create methane in oxygen-rich environments as well. The gas also escapes during fossil fuel extraction and distribution and is emitted during wildfires.

In 2007 NOAA, an agency of the U.S. Commerce Department, celebrates 200 years of science and service to the nation. Starting with the establishment of the U.S. Coast and Geodetic Survey in 1807 by Thomas Jefferson much of America's scientific heritage is rooted in NOAA. The agency is dedicated to enhancing economic security and national safety through the prediction and research of weather and climate-related events and information service delivery for transportation, and by providing environmental stewardship of the nation's coastal and marine resources. Through the emerging Global Earth Observation System of Systems (GEOSS), NOAA is working with its federal partners, more than 60 countries and the European Commission to develop a global monitoring network that is as integrated as the planet it observes, predicts and protects.

Relevant Web Sites
NOAA Carbon Cycle Greenhouse Gases Group (CCGG)

NOAA Carbon Cycle Greenhouse Gases Figures

NOAA Earth System Research Lab

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