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

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 - 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 - 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 0.7K 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.