Hockey Stick Controversy - Scientific Debates, RegEM Climate Field Reconstructions

Scientific Debates, RegEM Climate Field Reconstructions

The Huang, Pollack & Shen 2000 borehole temperature reconstruction covering the past five centuries supported the conclusion that 20th century warming was exceptional

In a perspective commenting on MBH99, Wallace Smith Broecker argued that the Medieval Warm Period (MWP) was global. He attributed recent warming to a roughly 1500-year cycle which he suggested related to episodic changes in the Atlantic's conveyor circulation.

A March 2002 tree ring reconstruction by Jan Esper et al. noted the debate, and Broecker's criticism that MBH99 did not show a clear MWP. They concluded that the MWP was likely to have been widespread in the extratropical northern hemisphere, and seemed to have approached late 20th century temperatures at times. In an interview, Mann said the study did not contradict MBH as it dealt only with extratropical land areas, and stopped before the late 20th century. He reported that Edward R. Cook, a co-author on the paper, had confirmed agreement with these points, and a later paper by Cook, Esper and D'Arrigo reconsidered the earlier paper's conclusions along these lines.

The original MBH98 and MBH99 papers used a principal component analysis step, but from 2001 Mann stopped using this method and introduced a multivariate Climate Field Reconstruction (CFR) technique based on the regularized expectation–maximization (RegEM) method which did not require the PCA step. In May 2002 Mann and Scott Rutherford discussed this in a paper on testing methods of climate reconstruction. By adding artificial noise to actual temperature records or to model simulations they produced synthetic datasets which they called "pseudoproxies". When the reconstruction procedure was used with these pseudoproxies, the result was then compared with the original record or simulation to see how closely it had been reconstructed. They discussed the issue that regression methods of reconstruction tended to underestimate the amplitude of variation.

Lonnie Thompson published a paper on "Tropical Glacier and Ice Core Evidence of Climate Change" in January 2003, featuring Figure 7 showing graphs based on ice cores closely resembling a graph based on the MBH99 reconstruction combined with thermometer readings from Jones et al. 1999.