If today's Earth becomes warmer, how will it compare to the Earth of the Cretaceous period?
During the cretaceous the earth was on average 4 degrees warmer, but a 4 degree average results in nearly no change at the equator but a significantly higher rise at the poles, 8 to 15 degrees warmer. Antarctica was not covered in an icecap in the cretaceous period, although smaller ones may have formed periodically. Northern antarctica was largely covered in wet forest. Ecologically you would see wet temperate forest similar to those in australia and south america.   

Note however warming was not the the only factor different positions of continents leads to a significantly different ocean circulation. Higher sea levels turn lowlands into seas changing rainfall patterns significantly. So the cretaceous is not a great predictor of the near future climate. 

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