A four year study published in the Journal of Geophysical Research: Atmospheres determined that soot, or black carbon, is the second
most damaging greenhouse agent in the atmosphere after carbon dioxide.
The black color absorbs heat,
contributing to an increase in temperature where large quantities of soot are
found in the atmosphere.
Black carbon is
particularly damaging in frozen regions. When it falls on snow and ice,
it increases the amount of light and heat that they absorb, quickening the pace at
which they melt. All of this contributes
to global warming.
The largest impact of
soot is on human respiratory health. A UNEP report suggests that
controlling carbon emissions could save 2.4 million lives a year.
The good news is that black carbon is easier
to eliminate as an environmental hazard than CO2. Unlike carbon dioxide, once you stop soot
from entering the atmosphere, it cleans itself up in a matter of weeks.
Also see this news article regarding black carbon in the Economist.