Soot is the second largest man-made contributor to global warming and its influence on climate has been greatly underestimated, according to the first quantitative analysis of this issue, published in the Journal of Geophysical Research: Atmospheres.
The findings show that soot, also called black carbon, has a warming effect of about 1.1 watts per square meter, around two-thirds of the effect of the largest man-made contributor to global warming, carbon dioxide. This means that soot exerts about twice the impact on the Earth's climate as previously thought and pushes methane into third place as a human contributor to climate change.
Black carbon particles affect atmospheric temperatures in a variety of ways. The dark particles absorb incoming and scattered heat from the sun; they can promote the formation of clouds that can have either cooling or warming impact; and black carbon can fall on the surface of snow and ice, promoting warming and increasing melting. Many sources of black carbon also emit other particles whose effects counteract the soot, providing a cooling effect, the report says.
The report finds that black carbon is a significant cause of the rapid warming in the Northern Hemisphere at mid to high latitudes (see picture), including the northern United States, Canada, northern Europe and northern Asia. Its impacts can also be felt further south, inducing changes in rainfall patterns from the Asian monsoon. This demonstrates that curbing soot emissions could have significant impact on reducing regional climate change while having a positive impact on human health, the researchers say.
Cleaning up diesel engines and some wood and coal combustion could slow the warming immediately, according to researchers.
In December, the EPA toughened its air quality standards for soot, or fine particulate matter, released from automobile exhausts and power plants. The agency set the annual health standard at 12 micrograms per cubic meter, a 20 percent reduction from the previous rule.
The agency finalized the rules in response to a court-ordered deadline. The existing daily limit of 35 micrograms for fine particles as well as course particles, which includes dust from farms and other sources, remain unchanged.
Most of the US already meets the annual fine particulate health standard of 12 micrograms per cubic meter, the EPA said.
