25 July 2005

Methane's Effect On Climate Change May Be Twice Previous Estimates

by Kate Melville

Establishing a suitable model of how the Earth's climate systems work is a necessary step in determining how much man contributes to climate change. But a recent study may prove that our current models of measuring climate change are dangerously inadequate. Despite the disparity of isolated climate studies, existing scientific consensus models on climate change decree that when it comes to gases that trap heat in our atmosphere, called greenhouse gases (GHGs), you need to look at those gas quantities in the atmosphere. Unfortunately, this assumption has led to an underestimation of methane levels, and how much methane contributes to climate change.

Well, that was the case, until Drew Shindell, a climatologist from NASA's Goddard Institute for Space Studies, told everyone that they should be looking at the GHGs when they are emitted at Earth's surface, and not at the GHGs themselves after they have been mixed into the atmosphere. "The gas molecules undergo chemical changes and once they do, looking at them after they've mixed and changed in the atmosphere doesn't give an accurate picture of their effect," claims Shindell. "For example, the amount of methane in the atmosphere is affected by pollutants that change methane's chemistry, and it doesn't reflect the effects of methane on other greenhouse gases," said Shindell, "so it's not directly related to emissions, which are what we set policies for."

Chemically reactive GHGs include methane and ozone (carbon dioxide, the most important GHG, is largely unreactive). Once methane and the molecules that create ozone are released into the air by both natural and human-induced sources, these gases mix and react together, which transforms their compositions. When gases are altered, their contribution to the greenhouse warming effect also shifts. So, the true effect of a single GHG emission on climate becomes very hard to single out.

The Intergovernmental Panel on Climate Change (IPCC) relies on reports from various climate experts in order to acquire a current assessment of climate change, but the reports rely on measurements of greenhouse gases as they exist in the atmosphere, after they may have mixed with other gases. If Shindell's analysis is correct, then it means that literally hundreds of climate experts' reports on global warming are wrong.

For example, one of the leading greenhouse gases, emitted from both man-made and natural sources, is methane. Methane is known as a 'well mixed' greenhouse gas because of its long lifetime of a decade or more, which allows it to disperse evenly around the atmosphere. The IPCC report, which calculates methane's affects once it exists in the atmosphere, states that methane increases in our atmosphere account for only about one sixth of the total effect of well-mixed greenhouse gases on warming.

However, Shindell's own computer modeling of methane, based on measurements of GHGs before they are mixed into the atmosphere, reveals that methane emissions may account for a third of the climate warming from well-mixed greenhouse gases between the 1750s and today.

Shindell's model provides a more accurate measure of GHGs and their effects because of its ability to single out the main offenders individually. Shindell and his colleagues found some striking differences in how much individual gases contribute overall to climate change. The problem with the IPCC reports is that they lump other warming effects like tropospheric ozone together, rather than acknowledging that tropospheric ozone is formed chemically from methane. By categorizing the climate effects according to emissions, Shindell and colleagues found the total effects of methane emissions are substantially larger.

One bonus of this perspective is that in order to manage greenhouse gases, policy decisions must focus on cutting emissions, because that's where humans have some control. "If we control methane, which the U.S. is already starting to do, then we are likely to mitigate global warming more than one would have thought, so that's a very positive outcome," Shindell said. "Control of methane emissions turns out to be a more powerful lever to control global warming than would be anticipated."

Sources of methane include natural sources like wetlands, gas hydrates in the ocean floor, permafrost, termites, oceans, freshwater bodies, and non-wetland soils. Fossil fuels, agricultural animals, landfills and rice paddies are the main human-related sources.

Source: Media release - NASA