Black carbon ranks as second-biggest human cause of global warming

January 15, 2013

Soot ranks as the second-largest human contributor to climate change, exerting twice as much of an impact as previously  thought, according to an analysis released Tuesday.

The four-year, 232-page study of black carbon, published in the Journal of Geophysical Research: Atmospheres, shows that short-lived pollution known as soot, such as emissions from diesel engines and wood-fired stoves, has about two-thirds the climate impact of carbon dioxide. The analysis has pushed methane, which comes from landfills and other forces, into third place as a human contributor to global warming.

Black carbon, or soot, accelerates warming because the fine particles absorb heat when they are in the air and when they darken snow and ice. Although some lighter-colored fine particles can have a cooling effect because they block sunlight, other black carbon sources have a warming effect because they absorb it. They also accelerate glacier melting and can disrupt regional weather patterns.

The findings came out on the same day that the National Oceanic and Atmospheric Administration announced that last year’s global average temperature ranked as the 10th-warmest on record, and NASA found it was the ninth-warmest. The two agencies analyze temperature data differently, but both found that with the exception of 1998, the nine warmest years since 1880 have all occurred since 2000.

Piers Forster, one of the soot study’s authors and a professor at the University of Leeds School of Earth and Environment, said in a statement that reducing black carbon can help address rising temperatures.

“There are exciting opportunities to cool climate by reducing soot emissions but it is not straightforward,” Forster said, adding that cutting emissions from diesel engines and domestic wood and coal fires is “a no-
brainer” because it would improve public health and the climate. Fine particles cause heart and respiratory problems, leading to premature death, as well as asthma and other illnesses.

These emissions cuts would produce an immediate cooling effect, the authors estimated, which would avoid a nearly 1-degree Fahrenheit temperature rise in the near term.

“You save lives, and you produce really fast cooling,” said Durwood Zaelke, president of the Institute for Governance and Sustainable Development.

Veerabhadran Ramanathan, an atmospheric scientist at the Scripps Institution of Oceanography at the University of California at San Diego, identified black carbon in 2008 as the second-
biggest human contributor to climate change. But many researchers questioned his analysis because it was based on observations rather than computer modeling.

“We seem to have put one major debate behind us, that black carbon is a significant contributor to the planet’s heat,” Ramanathan said in a phone interview.

Although the United States has made major strides in curbing soot — Ramanathan and his colleagues recently found that black carbon concentrations in California have fallen by 50 percent in 25 years, largely because of stricter diesel emissions rules — Southeast Asia and China still suffer major pollution from diesel engines and wood- and coal-burning combustion.

Just this week, Beijing experienced a string of hazardous air days, driven, in part, by soot emissions.

The new study’s authors emphasized that in the long term, major cuts in carbon dioxide would be required to avert dangerous climate impacts.

“Mitigating black carbon is good for curbing short-term climate change, but to really solve the long-term climate problem, carbon dioxide emissions must also be reduced,” said one of the paper’s lead authors, Tami Bond, a University of Illinois at Urbana-Champaign atmospheric scientist.

In describing last year’s temperature record, James E. Hansen, who directs NASA’s Goddard Institute for Space Studies, cautioned that just because 2012’s average was in keeping with recent years does not mean global warming has stalled.

“On the decadal time scale, it’s going to get warmer, because we know the planet is out of energy balance,” Hansen told reporters. “This standstill, I think, is a temporary one.”

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