The end of the Ordovician geological period (444 million years ago) poses a nice puzzle for climatologists, and for related reasons has become a favorite of the more scientifically minded among the AGW denialist crowd. It was a mainly hot time for the planet, with temperature mostly around 40-50 C (104-122 F) with occasional excursions to 60 C (140 F). This was apparently due to the very high CO2 content of the atmosphere, roughly 14 to 18 times early modern values.
The puzzle is that the period concluded with two sharp episodes of severe glaciation, and one of the most severe mass extinctions known. The glaciation was followed by more hot weather.
It was a pretty different world - the day was only about 21.5 hours long, there were few plants and no vertebrates on land, and probably most significantly, the continents were arranged very differently.
There were only four continents (Baltica, Siberia, Laurentia, and Gondwana, by far the biggest, incorporating modern Antarctica, India, parts of Africa and South America and more). The Northern hemisphere was nearly all water, and it was a very watery world, with shallow seas covering much of what is now dry land. Gondwana reached the South Pole just about the time the glaciation started.
"Well that explains the big chill," one might say. Not so fast though. There remain a couple of problems. First, the glaciations were geologically brief - 1 million years or less - and careful simulations show that even with the landmasses mostly around the pole it's hard to get that much chill with 14X (14 times modern) CO2. 1 million years is just an eyeblink in terms of continental movement, so how could this arriving at the pole account for the brief but intense chill?
The detailed answer is not known (at least to me) but the most plausible explanations involve our old friends CO2 and feedback. The time scale is right, at any rate. An oversimplified scenario might go like this: change in ocean circulation, driven by continental position; provokes cooling which leads to lower CO2; which leads to more cooling, glaciation; drastic sea level changes; causing changes in carbonate and silicate weathering; causing another CO2 spike which melts everything and leaves behind plenty of CO2.
As usual, corrections by those who understand more are greatly appreciated. Other comments and questions are also valued. Ignorant rantings by those who know less will be tolerated if at least slightly polite.