Soot is heating up the planet, experts warn

U. LEEDS (UK) — Soot’s role in global warming may be underestimated, but a major effort in reduction could potentially gain us a few decades of relief.

The direct warming effect of black carbon, the term used by scientists to describe soot, could be about twice the previous estimates, according to a study published in the Journal of Geophysical Research Atmospheres.

Black carbon is believed to have a warming effect of about 1.1 Watts per square meter (W/m2), approximately two thirds the warming effect of carbon dioxide—the largest man-made contributor to global warming—and greater than that of methane.

The figures indicate there may be a greater potential to curb warming by reducing soot emissions than previously thought.

Professor Piers Forster from the School of Earth and Environment at the University of Leeds, who co-led the study, says: “There are exciting opportunities to cool climate by cutting soot emissions, but it is not straightforward.

“Reducing emissions from diesel engines and domestic wood and coal fires is a no-brainer, as there are tandem health and climate benefits.

“If we did everything we could to reduce these emissions, we could buy ourselves up to half a degree less warming—or a couple of decades of respite,” Forster says.

David Fahey from the US National Oceanic and Atmospheric Administration (NOAA), says: “This study confirms and goes beyond other research that suggested black carbon has a strong warming effect on climate, just ahead of methane.” Previous studies had also indicated underestimates of some regions’ soot emissions.

Sources of soot

Huge quantities of man-made soot enter the atmosphere every year, with about 7.5 million tons emitted in 2000 alone. These emissions come from a variety of sources.

The largest global cause is the burning of forest and savannah grasslands, but diesel engines account for about 70 percent of emissions from Europe, North America, and Latin America.

Residential fires contribute 60 to 80 percent of Asian and African emissions, and coal fires are also a significant source of soot in China, some Eastern European countries, and the former USSR.

The small, dark particles that make up black carbon directly heat the atmosphere by absorbing incoming and scattered heat from the sun. They can promote the formation of clouds that can have either cooling or warming effects. Black carbon can also fall on the surface of snow and ice, promoting warming and increasing melting by reducing light reflection.

“Mitigation is a complex issue because soot is typically emitted with other particles and gases that probably cool the climate,” Forster says. “For instance, organic matter in the atmosphere produced by open vegetation burning likely has a cooling effect. Therefore the net effect of eliminating that source might not give us the desired cooling.”

Immediate effects

There are some targets for reduction that scientists believe will have a clear benefit.

“One great candidate is soot from diesel engines. It may also be possible to look at wood and coal burning in some kinds of industry and in small household burners. In these cases, soot makes up a large fraction of their emissions, so removing these sources would likely cool the climate,” Forster says.

Reducing black carbon emissions has a major advantage: its effect would be seen almost immediately because black carbon’s influence is a continuous short-term process. While carbon dioxide remains in the atmosphere for relatively long periods, existing soot emissions are washed out of the atmosphere in a few weeks.

However, black carbon reduction could only be part of a solution. Forster says: “Soot mitigation is an immediate effect but helps for a short time only. We will always need to mitigate CO2 to achieve a long-term cooling.”

The report finds black carbon is a significant cause of the rapid warming in the Northern Hemisphere at mid to high latitudes, including the northern United States, Canada, northern Europe, and northern Asia. Its effect can also be felt further south, causing changes in rainfall patterns from the Asian Monsoon. Curbing black carbon emissions could significantly reduce regional climate change, while also having a positive impact on human health.

The International Global Atmospheric Chemistry Project (IGAC) commissioned the study.

Source: University of Leeds