In recent years, scientists and policymakers have focused on controlling climate pollutants other than carbon dioxide as a potential way to curb global warming in the short term.

Curbing emissions of methane and soot, also called black carbon, could limit short-term global warming, the idea goes, because these substances have a strong effect on global temperatures in the short term.

Plus, at least politically, such short-lived climate pollutants are often seen as easier to control than carbon dioxide. Last year, a coalition of countries including the United States formed the Climate and Clean Air Coalition, dedicated to reducing these types of emissions (ClimateWire Feb. 16, 2012).

A study published online yesterday in the Proceedings of the National Academy of Sciences, though, says the climate benefit of curbing methane and black carbon may not be quite as large as some previous studies have estimated.

The authors of the new paper, Steven Smith and Andrew Mizrahi, say that even if the maximum reductions on these two pollutants were put into place, there would be only a "modest" reduction in average global warming of 0.16 degree Celsius (0.28 degree Fahrenheit) by 2050.

Previous estimates have put the reduction closer to 0.5 C, or 0.9 F.

Reducing methane and black carbon emissions "is likely to have a climate benefit. It's just that this climate benefit is smaller than we previously thought," said Smith, a senior scientist at the Joint Global Change Research Institute, a joint venture of the University of Maryland and the Pacific Northwest National Laboratory.

Questioning assumptions of models
As a result, the paper authors write, a comprehensive strategy that includes reducing emissions from carbon dioxide and other long-lived greenhouse gases as well as short-lived pollutants should be the main focus of policy efforts to stabilize the climate.

"We hope that this scientific information will help inform the discussion," Smith said.

The researcher pointed out that there are still strong reasons to reduce emissions of such short-lived pollutants, because they can have serious negative health consequences.

"We are talking about people that are dying prematurely and having less quality of life because of air pollution worldwide from particulates and also ground-level ozone," Smith said.

There are a few reasons Smith's calculation of the benefits of curbing such pollutants differs from some earlier ones.

The researchers used a model that assumed existing trends in pollution control would happen regardless of any additional policies. So as incomes rise and cookstoves get cleaner and put out less black carbon, for example, the base-line model assumed this trend would continue until the end of the century.

They also took into account the fact that generally, as incomes go up, air pollution decreases. This has happened in the United States and Europe, and their model assumed it will happen elsewhere as well.

The model also assumed that methane emissions will be reduced in the future, because capturing leaking methane, which can be used as a fuel, is an economically sensible thing to do.

"You have this declining base line, so we did everything relative to that," Smith said.

Behavior may not follow economics
That approach makes sense in some ways, said Drew Shindell, a climate scientist at NASA's Goddard Institute for Space Studies, but the assumptions that underlie it may not play out that way in the future.

Shindell was the lead author of a 2012 paper in Science that estimated around a 0.5 C reduction by 2050 if short-lived climate pollutants were curbed. Shindell's paper had a different outcome because his model did not automatically assume that future emissions will continue to go down, he said. So in a way, the two studies ask different questions.

"Their question is really, assuming a world where everything economically attractive has already happened, then what extra benefit can you get by targeting these [short-lived] pollutants?" Shindell said. "That's a really important difference."

To Shindell, it is important that policymakers understand that policies need to be put into place to see such emissions reductions -- that they are not automatically going to happen.

Another difference between the model used in Smith's study and the ones Shindell has used is that it doesn't include the additional warming effects of black carbon based on where it falls.

"If soot gets carried over the Himalayas, it has a much bigger physical impact," Shindell said.

Smith said he hopes to look at the regional impacts of black carbon in the future.

"We know that black carbon has a disproportionate effect in the Arctic in part because if you deposit black soot on snow or ice, that's extra warming that doesn't occur here," he said.

Correction: An earlier version of the story incorrectly described the researchers' base-line model regarding cookstoves.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500