Understanding Sensitivities in Paleoclimatic Reconstructions

Recent publications have seen the introduction of a number of new statistical methods for paleoclimatic reconstructions. When applied to archived multiproxy datasets, most of these methods produce reconstructed curves very similar to the “hockey stick” shape that was first observed by Mann, Bradley and Hughes. However, one recent reconstruction, by McShane and Wyner, produced a sharply different shape. Trying to understand the reasons for this leads to important insights for both statistical methodology and paleoclimatic datasets. The “divergence” phenomenon — that the relationship between temperature and some of the proxies may not be constant over time — has been extensively discussed in the paleoclimate literature, but mostly in the context of certain classes of northern hemisphere tree rings, which are not included among the proxies examined here. Closer scrutiny of the data suggests a new divergence phenomenon, associated with lake sediments. When these are removed from the data, the resulting reconstruction is much closer to the familiar hockey stick shape. This highlights the need both for careful scrutiny of the data, and for statistical methods that are robust against the divergence phenomenon. ∗Richard L. Smith is the Director of the Statistical and Applied Mathematical Sciences Institute (SAMSI), Research Triangle Park, N.C., and Mark L. Reed III Distinguished Professor in the Department of Statistics and Operations Research, University of North Carolina, Chapel Hill. SAMSI is supported by the National Science Foundation, grant DMS 0635449. Contact email address: [email protected]. The author would like to thank Dr. Michael Mann for answering a number of queries about the data and for some very helpful discussion of the divergence phenomenon.