Carbon dioxide (CO2) is the most important greenhouse gas
produced by human activities, primarily through the combustion
of fossil fuels. It accounts for the largest proportion of the
'trace gases' and is currently responsible for 60% of the
'enhanced greenhouse effect'. It is also thought that it's been
in the atmosphere for over 4 billion of the Earth's 4.6 billion
year geological history and in much larger proportions (up to
80%) than today. Most of the carbon dioxide was removed from the
atmosphere as early organisms evolved photosynthesis. This
locked away carbon dioxide as carbonate minerals, oil shale and
coal, and petroleum in the Earth's crust when the organisms
died. This left 0.03% in the atmosphere today.  The natural
carbon dioxide cycle - Atmospheric carbon dioxide comes from a
number of natural sources, mainly the decay of plants, volcanic
eruptions and as a waste product of animal respiration. - It is
removed from the atmosphere by photosynthesis in plants and by
dissolving in water, especiall