Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data

Palaeoclimate: tracking the trend 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. A number of reconstructions of millennial-scale climate variability have been carried out in order to understand patterns of natural climate variability, on decade to century timescales, and the role of anthropogenic forcing 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 . These reconstructions have mainly used tree-ring data and other data sets of annual to decadal resolution. Lake and ocean sediments have a lower time resolution, but provide climate information at multicentennial timescales that may not be captured by tree-ring data 9 , 10 . Here we reconstruct Northern Hemisphere temperatures for the past 2,000 years by combining low-resolution proxies with tree-ring data, using a wavelet transform technique 11 to achieve timescale-dependent processing of the data. Our reconstruction shows larger multicentennial variability than most previous multi-proxy reconstructions 1 , 2 , 3 , 4 , 7 , but agrees well with temperatures reconstructed from borehole measurements 12 and with temperatures obtained with a general circulation model 13 , 14 . According to our reconstruction, high temperatures—similar to those observed in the twentieth century before 1990—occurred around ad 1000 to 1100, and minimum temperatures that are about 0.7 K below the average of 1961–90 occurred around ad 1600. This large natural variability in the past suggests an important role of natural multicentennial variability that is likely to continue.