BioEd Online

The contribution of ordinary soot to global warming is much higher than previously thought, according to a comprehensive new assessment, which ranks so-called “black carbon” second only to carbon dioxide in terms of its warming impact on the current climate.

Released online Tuesday by the Journal of Geophysical Research, the four-year study roughly doubles most previous estimates of the warming that occurs when carbon particles absorb solar radiation, heating the atmosphere as well as melting snow and ice. Black carbon’s impact on climate is larger than that of methane and roughly two-thirds that of carbon dioxide, the principle greenhouse gas, according to the study.

Although many scientists suspected that global climate models underestimated the role of black carbon, the magnitude of the impact nonetheless surprised many team members, says David Fahey, an atmospheric scientist at the National Oceanic and Atmospheric Administration in Boulder, Colorado, and one of the lead authors.

Major sources of black carbon in industrialized countries are diesel emissions and agricultural fires, while in the developing world they include sources such as biomass burning for cooking and heat. “This study suggests we should be putting even more effort into reducing black carbon pollution,” says Durwood Zaelke, who heads the Institute for Governance and Sustainable Development in Washington DC. Although carbon dioxide dominates the long-term effect, understanding the timescale is critical, Zaelke says. “Reducing black carbon gives you immediate cooling.”

A study published in Science last year estimated that aggressive action on black carbon and methane could cut the rate of warming in half over the next 40 years, an appealing idea given the lack of progress within the United Nations climate negotiations, which often get hung up on carbon dioxide reductions. The Climate & Clean Air Coalition to Reduce Short-Lived Climate Pollutants formed last year and now includes 25 countries.

The present analysis includes data from a ground-based aerosol sensor network run by NASA as well as satellite observations and global emissions inventories. Team members then used detailed atmospheric models to analyse the movement and evolution of aerosol particles. The results align fairly well with the work of Veerabhadran Ramanathan, a climate scientist at the Scripps Institution of Oceanography in La Jolla, California, who has long emphasized black carbon’s impacts on climate, regional weather and human health. Ramanathan says that the study seems to confirm the bigger impact of black carbon on the temperature of the atmosphere, but does not yet answer questions about the overall effect of aerosols, which include climate-cooling particles such as sulphates. On this point, Ramanathan says, “there are no new insights from this study.”

Fahey also acknowledges the remaining uncertainties in the study. Although black carbon contributes to warming, he says, the impact of aerosol emissions could end up being either significantly positive or negative. “It’s not over yet.”