Here’s a recently published paper on sea surface temperatures 3 million years ago when the Arctic was seasonally ice free: http://micropress.org/stratigraphy/papers/Stratigraphy_6_4_265-275.pdf The paper presents some interesting data. The most northern location sampled was found to be 18C warmer during the summer. That's huge! It was found just off the coast of Spitsbergen; a chain of island that was (until a few years ago) encrusted in ice most of the year. In contrast, temperatures at tropical sites were found to be similar to those of today. They also found evidence of significant warming off the coast of California and western South America compared to less warming further off shore. So, the global warming of that time was not at all uniform. Note; at the current rate of sea ice retreat and thinning, the Arctic may be seasonally ice free by 2040. As such, this study provides a possible analog for our future climate conditions. CO2 levels during that time period were close or just slightly higher than current levels.