Have you ever seen a factory smoke stack hard at work? Or perhaps the exhaust pipe of a truck as it idles while waiting at a traffic light? If you have, you’ve probably observed thick, black smoke pouring out. This smoke is black carbon—commonly referred to as "soot"—which is comprised of microscopic particles released, but not broken down entirely, when burning fossil fuels. In most developed countries, diesel fuels are the main source of black carbon; elsewhere, the main sources are from the burning of wood, animal dung, vegetable oil, and other biomass fuels.

Scientists have begun to recognize soot as having the potential to cause changes in climate. Soot absorbs sunlight and, therefore, heats the surrounding air, also reducing the amount of sunlight that reaches the ground, resulting in a cooler surface. The heated air can create an unstable atmosphere resulting in rising air, forming clouds and bringing rainfall to areas heavily concentrated with soot. The rising air is often balanced by an increase in sinking air in neighboring regions, which tends to prevent cloud formation and rainfall. An example of this rising and sinking air can be found in southern and northern China. Southern China has experienced rising air forcing increased rainfall and flooding due to soot and other pollutants, while in northern China sinking air has increased the occurrence of dust storms and has resulted in droughts.

Soot has also been found to cause climate changes in areas of higher latitude where ice and snow are more common. Typically, ice and snow reflect sunlight rather than absorb it due to having a white background, also known as an albedo effect. When snow and ice are covered in soot, the soot absorbs the sunlight warming the ice and snow and causing a faster than normal melt. As more melting occurs, the warming effect increases as the soot becomes more concentrated on the snow surface and the land surface is exposed. A study by the Goddard Institute and Columbia University’s Earth Institute found that the effect that soot has on snow albedo may be contributing to the trend of early spring in the Northern Hemisphere. Earlier springs may be a further cause of the thinning of Arctic sea ice, and the melting of glaciers and permafrost.

Depending on the meteorology of an area, soot pollution is generally not restricted to an area surrounding the source due to the long-range transport of pollutants. In areas of South Asia, pollution lofts into the upper atmosphere where it has been shown to be transported to even the furthest reaches of the North Pole. Studies have revealed that South Asia has become the largest contributor of black carbon emissions, and is also believed to be the greatest contributor of soot deposited within Greenland.

Studies continue to indicate that soot is a likely factor in climate change. In 2003, a computer simulation done by NASA suggested that black soot may be responsible for up to 25 percent of the observed global warming over the past century. Another NASA experiment found that the amount of sunlight absorbed by soot was two-to-four times larger than previously assumed. However, the extent that black soot has an effect on climate change will continue to be debated since it has only recently become a factor included in studies of global warming.


BLACK CARBON - Digging A Little Deeper

The Environmental Literacy Council