NASA - National Aeronautics and Space Administration

Even on a cold winter day, standing inside a greenhouse can be downright balmy if the sun is shining outside. The glass lets the sun's warming rays in, but doesn't let as much of that warmth escape outdoors. Our Earth is much like that greenhouse, where a mixture of gases in our atmosphere acts together like a pane of glass, letting the sun's rays in, and without letting as much warmth escape out to space.

Image to right -- Rice Paddies in China: This image shows Chinese farmers transplanting rice in paddy fields in Yunnan Province, China, July 1999. Fossil fuels, cattle, landfills and rice paddies are the main human-related sources. Previous studies have shown that new rice harvesting techniques can significantly reduce methane emissions and increase yields. Click on image to enlarge Credit: Changsheng Li

Singling out how much each greenhouse gas (GHG) contributes overall to climate warming can be a tricky task. When it comes to measuring the impacts of greenhouse gases on our climate, scientists typically look at how much of each gas exists in the atmosphere.

  However, Drew Shindell, a climatologist at NASA's Goddard Institute for Space Studies, New York, NY, believes we need to look at the GHGs when they are emitted at Earth's surface, instead of looking 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," Shindell said. "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."

Once GHGs like methane and the molecules that create ozone are released into the air, 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.

 
Image to left -- Inland Wetlands: Sources of methane include natural sources like wetlands, gas hydrates in the ocean floor, permafrost, termites, oceans, freshwater bodies, and non-wetland soils. Click on image to enlarge. Credit: U.S. EPA, Leo Kenney, Region 1

The leading greenhouse gases include carbon dioxide, methane, nitrous oxide, and halocarbons. These gases are called ‘well mixed’ greenhouse gases because of their long lifetimes of a decade or more, which allows them to disperse evenly around the atmosphere. They are emitted from both man-made and natural sources. Ozone in the lower atmosphere, called tropospheric ozone or smog, also has greenhouse warming effects. In the upper atmosphere, ozone protects life on Earth from the sun’s harmful ultraviolet rays.

Some of the major investigations into the state of our warming planet come from a series of reports from the Intergovernmental Panel on Climate Change (IPCC) Assessment. These reports involved the work of hundreds of climate experts. The reports rely on measurements of greenhouse gases as they exist in the atmosphere, after they may have mixed with other gases.

  Shindell finds there are advantages to measuring emissions of greenhouse gases and isolating their impacts, as opposed to analyzing them after they have mixed in the atmosphere. His study on the subject was just published in the journal Geophysical Research Letters.

   According to new calculations, methane's effect on warming the world's climate may be double what is currently thought. The new interpretations reveal methane emissions may account for a whopping third of the climate warming from well-mixed greenhouse gases between the 1750s and today. The IPCC report states that methane increases in our atmosphere account for only about one sixth of the total effect of well-mixed greenhouse gases on warming.

Part of the reason the new calculations give a larger effect is that they include the effect methane has on air pollution. A major component of air pollution is near-surface-level or tropospheric ozone, which is not directly emitted, but is instead formed chemically from methane other hydrocarbons, carbon monoxide and nitrogen oxides. The IPCC report includes the effects of tropospheric ozone increases on climate, but it is not attributed to particular sources. By categorizing the climate effects according to emissions, Shindell and colleagues found the total effects of methane emissions are substantially larger. In other words, the true source of some of the warming that is normally attributed to smog is really due to methane that leads to increased smog.

"If we control methane, which is viable, then we are likely to soften global warming more than one would have thought, so that's a very positive outcome," Shindell said. 

  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, cattle, landfills and rice paddies are the main human-related sources.

   Krishna Ramanujan
Goddard Space Flight Center