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Methane ranks only behind water vapor and carbon dioxide among principal greenhouse gases, in terms of its abundance and global warming potential. Even though it is less abundant and has a shorter lifetime in the atmosphere than carbon dioxide, methane in the atmosphere accounts for nearly 20 percent of the heating effect of greenhouse gases compared with carbon dioxide's 50 percent. And similar to CO2, methane in the atmosphere emanates from both anthropogenic and natural sources.
Scientists have a decent understanding of the extent of anthropogenic methane emissions from sources such as landfills, livestock and fossil-fuel production, which altogether account for 60 percent of the gas's production. It has been tricky, however, to track its release from its major natural source—wetlands.
Wetlands are thought to be responsible for 70 percent of global atmospheric methane from natural sources—but not all wetlands are created equal. Water level, soil temperature, vegetation and topography all affect a wetland's methane production, complicating estimates of emissions from specific areas.
To try and clarify these estimates, a group of researchers recently created a model to predict wetland methane emissions from different geographic regions using the two most important properties affecting these discharges: water level and soil temperature. The team developed a model, using satellite and weather data, to explain the relationship between these two wetland properties and methane emissions. Then the researchers applied the model to estimate how much of the gas is emitted from wetlands areas, such as those in the tropics.
At the heart of the approach are data from NASA's twin GRACE satellites, which measure changes in Earth's gravity. "The gravity measurements are incredibly useful in trying to look at methane emissions," says Paul Palmer, a geoscientist at the University of Edinburgh in Scotland and one of the authors of a paper on the model, detailed in the January 15 Science.
Traditionally, gravity measurements have been used to determine variations in the extent of groundwater. In flooded areas, which include many tropical wetland areas such as in the Amazon and Congo, groundwater level can also be correlated with methane emissions. This is because methane-producing bacteria, known as methanogens, live in groundwater and can release methane directly into the atmosphere when water rises above soil.
In addition to the GRACE data as a measure of wetland water levels, Palmer and his colleagues used weather data from both the National Oceanic and Atmospheric Administration's (NOAA) National Centers for Environmental Prediction and the National Center for Atmospheric Research as a proxy for soil temperature—the other important wetland property. The modeling focused on determining the relationships among the weather information, water levels (based on GRACE data) and atmospheric methane levels during 10-day periods from 2003 to 2007 for areas measuring 350 square kilometers square. Information about the methane came from the European Space Agency's SCIAMACHY spectrographic satellite. Among other findings, the results showed that emissions increased by about 2 percent from 2003 to 2007, which Palmer says is important because tropical areas accounts for about 55 percent of worldwide wetland methane emissions.
The team's model also confirmed a relationship that other scientists had suspected: In tropical regions where the temperature is fairly constant, approximations of water level fluctuations are a better indicator than surface temperature of methane emissions. At higher latitudes more than 30 degrees above or below the equator, however, the surface temperature is a more important determinant of methane wetland emissions.
"The real contribution is [the researchers] could actually find a relationship between these parameters and methane [in the atmosphere]," says John Melack, an Earth scientist and remote sensing specialist at the University of California, Santa Barbara. Although Melack was not involved in the research, he said he gave the authors feedback when they were writing the paper.
The model could stand some improvements to make it more useful for inferring wetland methane emissions, says Elaine Matthews, a physical scientist at the NASA Goddard Institute for Space Studies in New York City. According to Matthews, the authors made a "huge leap" in using GRACE satellite data. This information is only useful in flooded areas but, as Matthews points out, only about two thirds of the world's wetlands are flooded at a given time. She suggests including data from a microwave-based satellite called the Special Sensor Microwave/Imager (SSM/I) developed at NASA Goddard in New York City , which detects water above the soil.
Although the model offers a new way to look at methane emissions from wetland areas, it does not necessarily point to solutions for regulating it—such as draining wetlands in flooded areas. "It's difficult to argue for that because there are groups promoting [wetland] preservation," says Changyong Cao, a satellite research scientist at NOAA's National Environmental Satellite, Data and Information Service. Instead, models that try to map methane emissions could help scientists in turn make better models for understanding climate change.
