New research by a team at the NASA Goddard Institute for Space Studies (GISS) in New York suggests that gas-aerosol interactions can amplify the global warming impact of some greenhouse gases. In particular, the study led by Drew Shindell found that methane emissions have a larger warming impact due to those interactions than accounted for in current carbon-trading schemes or in the Kyoto Protocol.
Among other conclusions, they found that the 100-year global warming potential (GWP) of methane is ~10% greater (~20 to 40%, including aerosol indirect effects AIE) than earlier estimates that neglected interactions between oxidants and aerosols. Calculations for the shorter 20-year GWP, including aerosol responses, yielded values of 79 and 105 for methane, including direct and direct+indirect radiative effects of aerosols, respectively. The UNIPCC AR4 estimates the 100-year GWP for methane at 25, with a value of 72 for the 20-year GWP.
As a result of their findings, published in the 30 October issue of the journal Science, the authors argue that assessments of multigas mitigation policies, as well as any separate efforts to mitigate warming from short-lived pollutants, should include gas-aerosol interactions.
Despite their limitations, GWPs are widely used for comparison among long-lived gases, forming the basis for worldwide political agreements on climate and carbon trading. Because the latter was a $126 billion/year market in 2008, even small differences in GWPs can have large economic consequences. Our results suggest that gas-aerosol interactions play an important role in methane’s GWP, and hence our larger value would allow better optimization of climate change mitigation policies. Methane’s GWP may also change with time as air quality regulations alter the background state of tropospheric chemistry. Finally, our results demonstrate that improving our knowledge of aerosol-climate interactions is important not only for better understanding the aerosol contribution to past and future climate change, but even for correctly evaluating the effects of long-lived greenhouse gas emissions from methane-oxidant-aerosol interactions.
—Shindell et al.
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, including reducing the production of particles called aerosols that can influence both the climate and the air quality. These two gases, as well as others, are part of a complicated cascade of chemical reactions that features competition with aerosols for highly reactive molecules that cleanse the air of pollutants.
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.
|“We’ve known for years that methane and carbon monoxide have a warming effect, but our new findings suggest these gases have a significantly more powerful warming impact than previously thought.”
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.
In the chemical soup of the lower atmosphere, however, sulfur dioxide isn’t the only substance interacting with hydroxyl. Similar reactions influence the creation of nitrate aerosols. And hydroxyls drive long chains of reactions involving other common gases, including ozone.
Methane and carbon monoxide use up hydroxyl that would otherwise produce sulfate, thereby reducing the concentration of sulfate aerosols. It’s a seemingly minor change, but it makes a difference to the climate.
More methane means less hydroxyl, less sulfate, and more warming.
The team’s modeling experiment, one of the first to rigorously quantify the impact of gas-aerosol interactions on both climate and air quality, showed that increases in global methane emissions have caused a 26% decrease in hydroxyl and an 11% decrease in the number concentration of sulfate particles. Reducing sulfate unmasks methane’s warming by 20 to 40% over current estimates, but also helps reduce negative health effects from sulfate aerosols.
|“The bottom line is that the chemistry of the atmosphere can get hideously complicated. Sorting out what affects climate and what affects air quality isn’t simple, but we’re making progress.”
In comparison, the model calculated that global carbon monoxide emissions have caused a 13% reduction in hydroxyl and 9% 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.
Although our calculations are more complete than previous studies, additional processes should be included as they become better understood. These include mixing between aerosol types, formation of secondary organic aerosols, which are sensitive to both organic aerosol emissions and oxidant levels, and interactions between pollutants and ecosystems. The latter includes suppression of CO2 uptake by increased surface ozone concentrations, aerosols enhancing the ratio of diffuse to direct radiation reaching the biosphere leading to increased CO2 uptake (at least for some plant types when aerosol loading is not so large as to dramatically reduce total surface irradiance), and the effects of nitrogen and sulfur deposition on ecosystems. These effects may be important but are highly uncertain at present.
Ecosystem-chemistry interactions add both positive and negative forcing terms to the GWP of NOx (NOx leads to increased ozone, causing increased CO2, but also leads to increased aerosol, causing decreased CO2), adding to an already complex set of multiple, sometimes opposing, forcings. For CO and methane, however, increased emissions lead to increased CO2 from both the ozone-ecosystem interactions and the aerosol-ecosystem interactions, so would simply increase their positive GWPs still further. Hence, the uncertainty in quantifying these processes implies only that the larger estimates of CO and methane GWPs presented here may still be too low.
—Shindell et al.
Abundance-based vs. Emissions-based modeling. 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,” explained 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 parse out whether pollutants have human or natural origins.
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.
The GISS team used the emissions-based approach as the groundwork for their 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 policymakers. Such treaties underestimate the contributions of methane and carbon monoxide to global warming, Shindell said.Implications. 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.
Our calculations suggest that all the non-carbon dioxide greenhouse gases together have a net impact that rivals the warming caused by 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.
Shindell, D.T., G. Faluvegi, D.M. Koch, G.A. Schmidt, N. Unger, and S.E. Bauer (2009) Improved Attribution of Climate Forcing to Emissions. Science 326, 716 - 718 doi: 10.1126/science.1174760
Arneth, A., N. Unger, M. Kulmala, and M.O. Andreae (2009) Perspectives: Clean the air, heat the planet? Science, 326, 672-673, doi: 10.1126/science.1181568