Black carbon -- emitted from diesel engines, burning wood and other sources -- acts as an absorbing aerosol, a particle that absorbs the sun's heating rays, researchers at the Carnegie Institution in California said.
The researchers ran simulations of adding a theoretical 1 million tons of black carbon uniformly around the globe at different altitudes in the atmosphere, a Carnegie release said Thursday. They found the addition of black carbon near the land and ocean surface caused the surface to heat, but surface warming decreased as the altitude of the soot increased.
The simulated addition of black carbon to the stratosphere caused the opposite effect, cooling the land and oceans even though the soot caused Earth as a whole to absorb more energy from the sun.
This is because when black carbon is high in the atmosphere it can lose its energy to space while helping to shade the land and ocean surface, the researchers said.
"We showed that black carbon near Earth's surface has the greatest effect on global warming," Carnegie researcher Ken Caldiera said.
"Unfortunately, this is exactly where we are putting most of the black carbon that we add to the atmosphere. This black carbon also often causes health problems, so cleaning up these emissions would help both the environment and human health."