Revisiting 2000 Years of Climate Change


Knowledge of climate change over past centuries provides the context of modern climate change; however, the lack of widespread instrumental climate records necessitates the use of proxy data such as tree-rings, corals, ice cores, and historical documentary records. A recent study in the September 8 issue of the Proceedings of National Academy of Sciences USA, co-authored by DOE-sponsored researcher Ray Bradley of the University of Massachusetts Amherst, examines reconstructions of surface temperature at hemispheric and global scale for the past 2000 years, using a greatly expanded set of proxy data. The 1998 ‘hockey stick’ reconstruction of Bradley et al. (referring to the shape of the time versus temperature curve) has been challenged by some in the climate change research community due to uncertainties in tree-ring data that were used. The new study is significant since it demonstrates that Northern Hemisphere surface temperature warming appears anomalous for at least the past 1300 years whether or not tree-ring data are used. The amplitude of warming during the Medieval Warm Period is greater than previously reported, albeit still not reaching recent levels.


Mann M.E., Z. Zhang, M.K. Hughes, R.S. Bradley, S.K. Miller, S. Rutherford and F. Ni. 2008. “Proxy-based reconstructions of hemispheric and global surface temperature  variations over the past two millennia,” Proc. Natl. Acad. Sci. USA 105, 13252.