Methane is causing more climate change than thought

Published Monday, 2nd November 2009

Methane and carbon monoxide are worse climate change culprits than we think, according to new research from NASA’s Goddard Institute for Space Studies (GISS). In fact, they and other gases besides carbon dioxide could rival the effects of carbon dioxide itself.

“Our calculations suggest that all the non-carbon dioxide greenhouse gases together have a net impact that rivals the warming caused by carbon dioxide,” says Drew Shindell, a GISS climate scientist.

Shindell and his research team reached that conclusion after running an experiment that’s one of the first to rigorously model the impact of gas-aerosol interactions on both climate and air quality,

“We’ve known for years that methane and carbon monoxide have a warming effect,” Shindell says. “But our new findings suggest these gases have a significantly more powerful warming impact than previously thought.”

When vehicles, factories, landfills and livestock emit methane and carbon monoxide into the atmosphere, they are doing more than just increasing their atmospheric concentrations. The release of these gases also have indirect effects on a variety of other atmospheric constituents and can thus reduce the production of particles known as aerosols.

Aerosols can have either a warming or cooling effect, depending on their composition, but the two aerosol types that Shindell modeled — sulfates and nitrates — scatter incoming light and affect clouds in ways that cool Earth. They are also related to the formation of acid rain and can cause respiratory distress and other health problems for those who breathe them.

Human activity is a major source of sulfate aerosols, but smokestacks don’t emit sulfate particles directly. Rather, coal power production and other industrial processes release sulfur dioxide — the same gas that billows from volcanoes — that later reacts with atmospheric molecules called hydroxyl radicals to produce sulfates as a byproduct. Hydroxyl is so reactive scientists consider it an atmospheric “detergent” or “scrubber” because it cleanses the atmosphere of many types of pollution.

However, methane and carbon monoxide use also react with hydroxyl, eliminating the amount that’s left to react with sulfur dioxide. The result is reduced concentrations of sulfate aerosols … and a reduction in the amount of cooling those aerosols provide.

“More methane means less hydroxyl, less sulfate, and more warming,” Shindell said.

Using their model experiment, Shindell’s team found that increases in global methane emissions have caused a 26 per cent decrease in hydroxyl and an 11 per cent decrease in the number concentration of sulfate particles. Reducing sulfate unmasks methane’s warming by 20 to 40 per cent over current estimates, although it also helps to reduce negative health effects from sulfate aerosols.

In comparison, the model calculated that global carbon monoxide emissions have caused a 13 per cent reduction in hydroxyl and 9 per cent reduction in sulfate aerosols.

Nitrogen oxides — pollutants produced largely by power plants, trucks and cars — led to overall cooling when their effects on aerosol particles are included, said Nadine Unger, another coauthor on the paper and a climate scientist at GISS. That’s noteworthy because nitrogen oxides have primarily been associated with ozone formation and warming in the past.

To determine the climate impact of particular greenhouse gases, scientists have traditionally relied on surface stations and satellites to measure the concentration of each gas in the air. Then, they have extrapolated such measurements to arrive at a global estimate.

The drawback to that “abundance-based approach,” said Gavin Schmidt, another GISS climate scientist and coauthor of the study, is that it doesn’t account for the constant interactions that occur between various atmospheric constituents. Nor is it easy to determine whether pollutants have human or natural origins.

Natural sources of methane include wetlands, termites, decomposing organic materials in ocean and fresh water, and a type of ice called methane hydrate. Man-made methane sources include livestock, rice paddies, biomass burning, landfills, coal mining, and gas production.

“You get a much more accurate picture of how human emissions are impacting the climate — and how policy-makers might effectively counteract climate change — if you look at what’s emitted at the surface rather than what ends up in the atmosphere,” said Shindell, who used this “emissions-based” approach as the groundwork for this modeling project.

However, the abundance-based approach serves as the foundation of key international climate treaties, such as the Kyoto Protocol or the carbon dioxide cap-and-trade plans being discussed among policy-makers. Such treaties underestimate the contributions of methane and carbon monoxide to global warming, Shindell said.

According to Shindell, the new findings underscore the importance of devising multi-pronged strategies to address climate change rather than focusing exclusively on carbon dioxide.

In particular, the study reinforces the idea that proposals to reduce methane may be an easier place for policy-makers to start climate change agreements.

“Since we already know how to capture methane from animals, landfills and sewage treatment plants at fairly low cost, targeting methane makes sense,” said Michael MacCracken, chief scientist for the Climate Institute in Washington, DC.

This research also provides regulators insight into how certain pollution mitigation strategies might simultaneously affect climate and air quality. Reductions of carbon monoxide, for example, would have positive effects for both climate and the public’s health, while reducing nitrogen oxide could have a positive impact on health but a negative impact on the climate.

“The bottom line is that the chemistry of the atmosphere can get hideously complicated,” said Schmidt. “Sorting out what affects climate and what affects air quality isn’t simple, but we’re making progress.”

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