I remember watching a documentary which talked about feedback loops in the climate. It might have been in the context of history. If the ocean warms up enough, it causes the CO2 in the ocean itself (either in the deposits or the gas saturated in the water) to get released into the atmosphere. Maybe I'm not using the right technical words, but there is a certain amount of CO2 that sits within the ocean that is separate from the atmosphere. In any case, after a certain point it becomes a feedback loop: warmer climate -> more CO2 joins the atmosphere -> warmer climate.