The National Academy of Sciences published in June 2008 a peer-reviewed study in which a team of climatologists and earth scientists reconstructed average global temperatures over the past 2 millennia using records in tree rings, coral growth rings, ice cores, and other proxies for temperature. The paper concludes that the sharp rise from the late 19th Century to now, in both northern and southern hemispheres, is unprecedented in the interval studied. A good summary of the paper, with its startling graphs, is at Climate Progress, which also links to the NAS paper. The work confirms and extends earlier work based only on tree rings and is further compelling evidence of anthropogenic global warming over the last century.