Past climate comes into focus but warm forecast stays put

Abstract

A reassessment of past climate records suggests greater fluctuation than was previously thought. A 2,000-year reconstruction of annual temperature for the Northern Hemisphere breaks new ground in the way it combines data from climate proxies with different inherent time scales — such as lake and ocean sediment and tree-ring data — to give full weight to each proxy at its optimum resolution. This technique, using wavelet transformation, makes the most of the available palaeoclimate data. The resulting reconstruction supports the case that multicentennial natural variability has been larger than is commonly thought, and that considerable natural climate variation can be expected in future. High temperatures occurred during the tenth century and notable ‘lows’ around 1600. But post-1990 temperatures stand out still as higher than at any time in the previous 2,000 years.