Methane as a Greenhouse Gas
CCSP Research Highlight

NASA

Direct measurements of atmospheric methane, begun in 1983, show that methane levels have increased each year, but that the rate of growth has been slowing down, particularly in the early 1990s.   Methane is of concern to scientists because it is about 20 times more efficient than carbon dioxide at trapping heat in the Earth system. NASA scientists estimated global annual methane emissions for each source from 1980 onward using a combination of statistics, published methodologies, measurements, and models. NASA has also been collecting space-based measurements of methane in the upper troposphere and stratosphere since the 1991 launch of its Upper Atmosphere Research Satellite (UARS).

Our results indicate that over the last two decades anthropogenic sources of methane have either slightly increased or remained relatively constant. Emissions have remained quite stable for irrigated rice and increased slightly for ruminant animals and landfills. Fossil fuel emissions generally show modest increases throughout the period. Wetland emissions fluctuate from year to year, but do not exhibit a trend. Taken alone, these results indicate that no single source is responsible for the declining growth rate of atmospheric methane concentrations. Further research incorporating this information about emissions indicates that patterns in atmospheric methane concentrations can be explained by modest variations in source emissions together with small increases in the chemical sink.

Scientists believe wetlands produce about a fourth of the world's atmospheric methane. Tropical wetlands are responsible for approximately half of the wetland methane emissions. NASA scientists are using data from satellites in space to measure the amount of area in the tropics covered by wetland and combining that with existing knowledge about methane emissions to estimate how much methane is coming from tropical wetlands in the Amazon.

They found that natural wetlands in the Amazon basin contribute significantly to the total amount of global atmospheric methane (about 22 Tg of carbon per year), but not as much as previously reported. NASA satellite data analyses indicate that past studies overestimated the length of time that seasonal wetlands were flooded and therefore scientists previously overestimated methane annual emissions.

Recent (2005) studies suggest that methane’s overall impacts on climate may be twice as much as previous estimates because methane also contributes to the formation of low-level ozone, another significant greenhouse gas. In short, methane could account for as much as a third of the global warming experienced since the beginning of the Industrial Revolution.

For more information about the study of methane on Earth at NASA, please visit Methane: A Scientific Journey from Obscurity to Climate Super-Stardom, a research feature (dtd September 2004) by Gavin Schmidt (NASA Goddard Institute for Space Studies).