The main greenhouse gas for the present climate is water vapour. In the lower atmosphere, however, water vapour levels are determined by the natural balance between evaporation and rainfall. Some GHGs are increasing as a direct result of man-made emissions. Deforestation may also be significant, but it is more difficult to quantify. Once in the atmosphere, CO2 is chemically stable and lasts for many decades.
Carbon dioxide is removed from the atmosphere by a complex network of natural sinks .
Most estimates suggest that about one-third of the CO2 being released at present is absorbed by the oceans . Another important and related sink is photosynthesis by vegetation on land and by plankton in the sea. Most of the CO2 absorbed by photosynthesis, however, is released again when plants and plankton decay or are eaten by animals. Only a small fraction is removed permanently. Now that atmospheric CO2 has risen well above its natural level, many aspects of this complex carbon cycle are changing.