The study suggests current levels of carbon dioxide, which are on par with what they were 3.5 million years ago, may not be low enough to stop the polar ice caps from melting, and could still result in a 2 C increase in the Arctic. An increase this small may mean Arctic temperatures 16 C warmer than today by 2100.
"Our data from the early Pliocene, when carbon dioxide levels remained close to modern levels for thousands of years, may indicate how warm the planet will eventually become if carbon dioxide levels are stabilized at the current value of 400 parts per million," said UCLA assistant professor Aradhna Tripati in a press release.

http://ca.news.yahoo.com/blogs/dailybrew/arctic-seashells-lead-discovery-global-warming-20110412-083313-649.html