From Dot Earth Blog - NYTimes.com

[JAMES C. ZACHOS ] It appears that for all periods in the past when greenhouse gas levels were higher, tropical temperatures were higher as well.

Note that over the past 600 million years, CO2 levels have usually been much higher than they are today

Late Carboniferous to Early Permian time (315 mya -- 270 mya) is the only time period in the last 600 million years when both atmospheric CO2 and temperatures were as low as they are today (Quaternary Period ).