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

Credit: TOM COHILL

A new global temperature reconstruction, undertaken as a follow-up to the infamous 1998 'hockey stick' curve, confirms that the past two decades are the warmest in recent history. The original graph was a focal point of the 2001 Intergovernmental Panel on Climate Change assessment report and became a symbol of fierce debates over the evidence for global warming.

Michael Mann at Pennsylvania State University and colleagues used seasonally to decadally resolved climate records from corals, cave deposits, sediments and tree rings from around the world to reconstruct global climate variability over the past 2,000 years. The team created two curves, with and without tree-ring records — data whose validity had previously been questioned — to place recent temperature observations in a historical context. Without inclusion of the tree-ring measurements, the data showed that recent warming is greater than at any point in at least the past 1,300 years; inclusion of tree-ring data extended this period to at least 1700 years.

Both curves show that global surface temperature increased during the Medieval Warm Period, an episode that lasted from around AD 800 to AD 1300, but indicate that average temperatures during this period were below those of the early twenty-first century.