David Greenwood, whose explanation of paleoclimate I linked to earlier, has a new article published in Geology, which can be read for free.

Significantly warmer Arctic surface temperatures during the Pliocene indicated by multiple independent proxies
Ballantyne et al.
http://geology.gsapubs.org/content/38/7/603.abstract

He says this of it:

“The research reports on our reconstruction of climate for Ellesmere Island in the Canadian high Arctic where we show that temperatures 4 to 5 million years ago were ~19C higher than today, at a time when atmospheric CO2 levels were very close to those today…
[...]
The implication of our research is that we may already have passed a tipping point for major increases in Arctic temperatures due to increasing levels of atmospheric CO2.”

Here are some recent papers coming up with similar findings in terms of temperature and CO2 levels.

Palaeoclimate: Global warmth with little extra CO2. Birgit Schneider & Ralph Schneider (2010).
(3 – 4C once thought to be associated with CO2 at 500 – 600 ppmv therein put at 385 ppmv)

Coupling of CO2 and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years. Tripati et al (December 2009)

Earth system sensitivity inferred from Pliocene modelling and data. Lunt et al (December 2009)
Here’s a chart

High Earth-system climate sensitivity determined from Pliocene carbon dioxide concentrations. Pagani et al (December 2009)

Atmospheric CO2 concentrations during ancient greenhouse climates were similar to those predicted for A.D. 2100. Breecker et al (October 2009)