Human activity is also adding CO2 to the
atmosphere through deforestation.  When
tropical forests are clear cut, the land is
typically converted to pasture.  The original
forest and its soil have a much higher carbon
content than the pastureland, so the process
of burning the forest must result in a
significant release of carbon to the
atmosphere.
Deforestation is thought to be releasing about
1.5 gigatons of carbon every year into the
atmosphere.