In this file photo, a line of Holstein dairy cows feed through a fence at a farm outside Jerome, Idaho. Belching from the nation's 170 million cattle, sheep and pigs produces about one-quarter of the methane released in the U.S. each year, according to the Environmental Protection Agency.
Cutting short-lived emissions such as soot spewed from trucks and methane belched from cattle will do little as a short-term fix for global warming, a new study says.
Previous modeling work indicated that such cuts could shave about 1 degree Fahrenheit from human-caused warming by 2050, enough to buy the world time to wrench the energy economy away from oil, coal and natural gas — major sources of the long-term heat-trapping gas carbon dioxide.
The new research is based on a type of model that its users said more accurately represents the real world and the evolution of human societies over time. It finds the climate benefit of soot and methane cuts is only about 0.3 degree Fahrenheit by midcentury.
While such cuts would bring "substantial health benefits … we found the near-term climate benefit to be smaller" than the earlier work, Steven Smith, a climate researcher with the Department of Energy's Pacific Northwest National Laboratory and the study's lead author, told NBC News.
"The real take home is that if we want to stabilize climate, then the main focus has to be on overall greenhouse gases and carbon dioxide in particular," added Smith, who works at the lab's Joint Global Change Research Institute at the University of Maryland in College Park.
The new findings were published Monday in the Proceedings of the National Academy of Sciences. They are based on results from the research institute's Global Change Assessment Model, which assumes soot and methane emissions will decline as world wealth increases and technology advances.
For example, people who spend several hours a day now collecting firewood for cooking will switch to cleaner-burning stoves as their incomes rise, Smith explained. As well, the model assumes emissions standards for cars and trucks will improve around the world, as they have in the U.S. and Europe.
"Against those background changes, we measure the effect of phasing in very stringent emission controls" for things such as diesel burning trucks as well as programs to encourage widespread adoption of clean burning cook stoves, Smith said.
In this scenario, the effect on climate change is "not surprisingly" less than in models that don't assume changes in wealth and technology will automatically lead to reduced soot and methane emissions, Drew Shindell, a climate scientist with the NASA Goddard Institute for Space Studies in New York who led much of the earlier work, explained to NBC News in an email.
"Most pollution is cleaned up by default," he said, but noted that cleaning up emissions doesn't happen automatically. Rather, he said, it requires "dedicated work" on the part of policymakers.
"So I think it's more important to look at the benefits such dedicated work would achieve than to look at what small addition one could make after already assuming wealth and idealized economic behavior has already dealt with the majority of the problem," Shindell said.
The models used in the various studies differ in other ways as well, such as how they account for the effect of ocean warming on the global climate and the atmospheric distribution of soot and methane emissions.
At the end of the day, Shindell said, the overall picture emerging from the models is fairly consistent: "Stringent reductions of carbon dioxide, methane and black carbon are needed and useful for averting severe climate change and its impacts."
There is, however, disagreement over which models better inform policymakers on how to put the world on the track of emissions reductions. Figuring out the best strategy, Smith said, will come as the science progresses.
"People use different tools and they get different answers," he said. "And then we get together to figure out why."
John Roach is a contributing writer for NBC News. To learn more about him, visit his website.
First published August 12 2013, 12:03 PM