Additions to Atmosphere

Because of the burning of fossil fuels, the clearing of forests, and other
such practices, the amount of CO2 in the atmosphere has been increasing
since the Industrial Revolution. Atmospheric concentrations have risen
from an estimated 260 to 300 parts per million (ppm) in preindustrial
times to more than 350 ppm today. This increase accounts for only half of
the estimated amount of carbon dioxide poured into the atmosphere. The
other 50 percent has probably been taken up by and stored in the oceans.
Although terrestrial vegetation may take up considerable quantities of
carbon, it is also an additional source of CO2.

Atmospheric CO2 acts as a shield over the earth. It is penetrated by
short-wave radiation from outer space but blocks the escape of long-wave
radiation. As increased quantities of CO2 are added to the atmosphere, the
shield thickens and more heat is retained, increasing global temperatures.
Although such increases have not yet been great enough to cancel out
natural climatic variability, projected increases in CO2 from the burning
of fossil fuels suggest that global temperatures could rise some 2° to 6°
C (about 4° to 11° F) by early in the 21st century. This increase would be
significant enough to alter global climates and thereby affect human
welfare.

Further information: http://en.wikipedia.org/wiki/Carbon_offset