If you dig deep enough into the Earth's climate change archives, you hear about the Palaeocene-Eocene Thermal Maximum, or PETM. And then you get scared.
This is a time period, about 56 million years ago, when something mysterious happened - there are many ideas as to what - that suddenly caused concentrations of carbon dioxide in the atmosphere to spike, far higher than they are right now. The planet proceeded to warm rapidly, at least in geologic terms, and major die-offs of some marine organisms followed due to strong acidification of the oceans.
The cause of the PETM has been widely debated. Some think it was an explosion of carbon from thawing Arctic permafrost. Some think there was a huge release of subsea methane that somehow made its way to the atmosphere - and that the series of events might have been kickstarted by major volcanic eruptions.
In any case, the result was a hothouse world from pole to pole, some 5 degrees Celsius warmer overall.