The planet has a natural "carbon cycle", where carbon is stored in carbon sinks, such as trees, and the largest carbon sink, the ocean floor.
Over millenia, these sinks can't take in any more carbon dioxide, and it is released into the atmosphere, causing global warming (this explains the
periods of global warming in earths distant past).
Man hasn't helped the situation, by artificially putting CO2 into the atmosphere, while chopping down all the trees we can find. The remaining carbon
sinks are having trouble coping with all the CO2 we keep putting into the environment.
The scariest thing is, once the ocean floor cannot cope anymore, warming will get quicker, then the carbon dioxide, as well as methane stored in the
sea bed, will be released, making it look as if the sea is "boiling" (its not, its the gas being released, along with colouring coming from all the
organic material from the sea bed coming to the surface as well).
Once this happens, we have had it. There will be so much CO2 & methane in the atmospehere, the planet won't cope, and temperatues will rise alot more
than they are already - most likely an extermination level event.
This was all in a documentary I saw on the BBC, part of the Horizon series:
The Day The Earth Nearly Died