Canie-the projection that CO2 in 100 years will be like it was 50 million years ago is a worst case. In that case, climate effects could be advanced millions of years-not 100, not 1000. But, sea level will not rise 100s of feet in 100 years-you just can't melt the East Antarctic Ice sheet that fast. As far as poles melting in the past-over the last few hundred thousand years sea level probably has never been higher than 6 or 10 m higher than present-so ice volume has only been a little smaller during interglacial times (smallest at 120,000 and 400,000 years). The East Antarctic Ice Sheet has probably been present in something like its present form for 15 million years-with glaciers in Antarctica back beyond 34 million years. Northern hemisphere ice sheets have been important maybe only in the last 2 or 3 million years (~~2 million is start of Pleistocene). My saying "be afraid...." was an overstatement perhaps, but global warming needs to be taken seriously.

As far as "how things happened in the past..." we may know more than you think...
I've also read that some areas may cool-for example, if the Gulf Stream is disrupted, northern Europe may be cooled by about as much as CO2 warms it-so it may be a wash there. If the sea ice disappears in summer from the Artic ocean, there will be more precipitation around it. While there would be more snow part of the year, if it is much warmer, it may not keep...So, I don't think the consensus is that warming will initiate an ice age-more likely it will just get warmer.

One thing I did not talk about is methane hydrate. there is a massive amount of methane locked up near the seafloor in a "frozen" form. If seawater warms in places where frozen methane is barely stable, it could be released-and methane is a much more powerful greenhouse gas than CO2. Chris