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

Abstract

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 technique¹¹ 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 measurements¹² 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.