Proc. Natl Acad. Sci. USA 105, 13252–13257 (2008)

A fresh analysis of climate indicators shows that the Northern Hemisphere is warmer now than it has been in at least 1,300 years.

Previous analyses of climatic history by Michael Mann of Pennsylvania State University in University Park and his colleagues produced a distinctive 'hockey stick' shape; but some of this analysis, and the tree-ring data it used, came under attack.

The latest work by Mann and his co-workers involves various climate proxies, including corals, ice cores, historical records and marine sediments. The authors show that current warming is anomalous even if all tree-ring data are eschewed.