EUGENE R. WAHL1 and CASPAR M. AMMANN2
1Environmental Studies and Geology Division, Alfred University, Alfred, New York, U.S.A.
May 10, 2005 In review: Climatic Change
The Mann et al. (1998) Northern Hemisphere annual temperature reconstruction over 1400-1980 is examined in light of recent criticisms concerning the nature and processing of climate proxy-data used in the reconstruction. A systematic sequence of analyses is presented to examine issues concerning the proxy evidence, utilizing both indirect analysis via exclusion of proxies and processing steps subject to criticism, and direct analysis of principle component (PC) processing methods in question. Altogether new reconstructions over 1400-1980 are developed in both the indirect and direct analyses, which demonstrate that the Mann et al. reconstruction of 1998 is highly robust against the proxy-based criticisms addressed.
In particular, reconstructed temperatures are demonstrated to be virtually unaffected by the use or non-use of PCs to summarize proxy evidence from the data-rich North American region (both in terms of the time period used to "center" the proxy data before PC calculation and the way the PC calculations 'are done), as long as the full information in the proxy data is represented by the PC time series. Clear convergence of the resulting climate reconstructions is a strong indicator for this criterion. Also, recent "corrections" to the Mann et al. reconstruction that suggest 15th century temperatures could have been as high as those of the late-20th century are shown to be without statistical and climatological merit.
Our examination does suggest that a slight modification to the original Mann et al. reconstruction is appropriate for the early 15th century (~ +0.05°), which leaves entirely unaltered the primary conclusion of Mann et al. (as well as many other reconstructions) that both the 20th century upward trend and high late-20th century hemispheric surface temperatures are anomalous over at least the last 600 years. Our new reconstructions are also used to evaluate the separate criticism of reduced downward magnitude in the Mann et al. reconstructions over significant portions of 1400-1900, suggesting that from the perspective of the proxy data themselves, such losses may be smaller than those reported in other recent work.
Last modified: Tue May 10 23:05:58 MDT 2005