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Climate Regime Shifts of the Past Four Centuries
D'Arrigo, R., Wilson, R., Deser, C., Wiles, G., Cook, E., Villalba, R., Tudhope, A., Cole, J. and Linsley, B. 2005. Tropical-North Pacific climate linkages over the past four centuries. Journal of Climate 18: 5253-5265.

What was done
The authors developed a tree-ring-based reconstruction of the December-May North Pacific Index (NPI) - which is a measure of the atmospheric circulation related to the Aleutian low pressure cell - for the period 1600-1983, based on data derived from 18 tree-ring chronologies (selected from a total of 67 candidate chronologies) obtained from sites surrounding the North Pacific rim that calibrated "significantly at or above the 90% significance level" against winter/spring monthly values of the NPI derived from 20th-century instrumental data. In addition, they employed an intervention analysis to the NPI reconstruction "to identify significant shifts in the series."

What was learned
D'Arrigo et al. report that "the NPI reconstruction successfully tracks the known regime shifts (1924/25, 1946/47, and 1976/77) seen in the instrumental NPI during the twentieth century." They also note that "prior to the instrumental period there are decadal-scale variations that may also represent regime shifts," noting that "significant 'shifts' (at the 90% confidence limit) are identified in 1627, 1695, 1762, 1806, 1833, 1853, and 1891."

What it means
The nine researchers conclude that their analysis "suggests that the 1976 transition was not unique in terms of magnitude." In addition, the recurring nature of the climate regime shifts suggests that they are natural non-anthropogenic-forced phenomena that have nothing to do with the historical increase in the air's CO2 content. This conclusion is particularly noteworthy in light of the fact that the study of Seidel and Lanzante (2004) suggests, in their words, that "it is reasonable to consider most of the warming during 1958-2001 to have occurred at the time of the climate 'regime shift,' modeled here at the start of 1977." Consequently, the complementary findings of these two studies do much to relieve anthropogenic CO2 emissions of responsibility for the global warming of the last fifty or more years.

Seidel, D.J. and Lanzante, J.R. 2004. An assessment of three alternatives to linear trends for characterizing global atmospheric temperature changes. Journal of Geophysical Research 109: 10.1029/2003JD004414.

Reviewed 2 July 2008