News Archive

posted 05/13/03

Researchers Find Soot Has Impact on Global Climate

Soot Particle Under a Microscope Image credit: D.M. Smith, University of Denver

A team of researchers, led by Columbia University and NASA scientists, found airborne, microscopic, black-carbon (soot) particles are even more plentiful around the world, and contribute more to climate change, than was previously assumed by the Intergovernmental Panel of Climate Change (IPCC).

The researchers concluded that if these soot particles are not reduced, at least as rapidly as light-colored pollutants, the world could warm more quickly.

The findings appear in the latest issue of the Proceedings of the National Academy of Sciences. It is authored by Makiko Sato, James Hansen and others from NASA’s Goddard Institute for Space Studies (GISS) and the Earth Institute at Columbia University, New York; Oleg Dubovik, Brent Holben and Mian Chin of NASA’s Goddard Space Flight Center, Greenbelt, Md.; and Tica Novakov, Lawrence Berkeley National Laboratory, Berkeley, Calif.

Sato, Hansen and colleagues used global atmospheric measurements taken by the Aerosol Robotic Network (AERONET). AERONET is a global network of more than 100 sun photometers that measure the amount of sunlight absorbed by aerosols (fine particles in the air) at wavelengths from ultraviolet to infrared. The scientists compared the AERONET data with Chin’s global-aerosol computer model and GISS climate model, both of which included sources of soot aerosols consistent with the estimates of the IPCC.

outdoor biomass burning

Outdoor biomass burning, including forest fires and the burning of fields in the tropics, is a large source of soot. Image credit: NASA/GSFC

The researchers found the amount of sunlight absorbed by soot was two-to-four times larger than previously assumed. This larger absorption is due in part to the way the tiny carbon particles are incorporated inside other larger particles: absorption is increased by light rays bouncing around inside the larger particle.

According to the researchers, the larger absorption is attributable also to previous underestimates of the amount of soot in the atmosphere. The net result is soot contributes about twice as much to warming the world as had been estimated by the IPCC.

Black carbon or soot is generated from traffic, industrial pollution, outdoor fires and household burning of coal and biomass fuels. Soot is a product of incomplete combustion, especially of diesel fuels, biofuels, coal and outdoor biomass burning. Emissions are large in areas where cooking and heating are done with wood, field residue, cow dung and coal, at a low temperature that does not allow for complete combustion. The resulting soot particles absorb sunlight, just as dark pavement becomes hotter than light pavement.

Both soot and the light-colored tiny particles, most of which are sulfates, pose problems for air quality around the world. Efforts are beginning to reduce the sulfate aerosols to address air quality issues.

“There is a pitfall, however, in reducing sulfate emissions without simultaneously reducing black carbon emissions,” Hansen said. Since soot is black, it absorbs heat and causes warming. Sulfate aerosols are white, reflect sunlight, and cause cooling. At present, the warming and cooling effects of the dark and light particles partially balance.

Soot particles are measured in micrograms (um), and are smaller than the diameter of a human hair. Image credit: NASA

This research continues observations of global climate change. It was funded by NASA’s Earth Science Enterprise. The Enterprise is dedicated to understanding the Earth as an integrated system and applying Earth System Science to improve prediction of climate, weather, and natural hazards using the unique vantage point of space.

The Goddard Institute for Space Studies at the Earth Institute is the only urban laboratory of the National Aeronautic and Space Administration (NASA). The Goddard Institute is a climate research center that models and monitors earth systems. In addition to research, it plays an important teaching function, running educational programs at more than twenty universities, schools and organizations in the New York metropolitan area.

The Earth Institute at Columbia University is among the world’s leading academic centers for the integrated study of Earth, its environment, and society. The Earth Institute builds upon excellence in the core disciplines—earth sciences, biological sciences, engineering sciences, social sciences and health sciences—and stresses cross-disciplinary approaches to complex problems. Through its research, training and global partnerships, it mobilizes science and technology to advance sustainable development, while placing special emphasis on the needs of the world’s poor.