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Questions About Purported Late 20th Century Warmth Raised by Tree-Ring Data
Briffa, K.R., Osborn, T.J. and Schweingruber, F.H. 2004. Large-scale temperature inferences from tree rings: a review. Global and Planetary Change 40: 11-26.

What was done
The authors review several recent analyses of maximum latewood density data obtained from a widespread network of tree-ring chronologies that span three to six centuries and were derived from nearly 400 locations.

What was learned
For the land area of the globe poleward of 20N latitude, the warmest period of the past six centuries occurred in the 1930s and early 1940s. Thereafter, however, the mean temperature of the region dropped dramatically. It recovered somewhat over the last two decades of the 20th century; but its final value was still below the mean value of the entire 1400s and portions of the 1500s.

Averaged over all land area poleward of 50N latitude, there was a marked divergence of reconstructed and instrumental temperatures subsequent to 1960, with measured temperatures rising and reconstructed temperatures falling, such that by the end of the record there was an approximate 1.5C difference between them. The authors attempted to relate this large temperature differential to an hypothesized decrease in tree growth caused by an hypothesized increase in ultraviolet radiation, which they hypothesized to have been caused by declining stratospheric ozone concentrations over this period. The results of this effort, however, proved "equivocal," as they describe them, leaving room for a possible urban heat island-induced warming in the instrumental temperature record to be the principal cause of the observed temperature divergence. The authors say these unsettled questions prevented them "from claiming unprecedented hemispheric warming during recent decades on the basis of these tree-ring density data alone."

What it means
The results of the authors' analyses fail to provide convincing evidence for the validity of the Northern Hemispheric temperature reconstructions of Mann et al. (1998, 1999), wherein the temperatures of the last two decades of the 20th century are depicted as being the warmest of the past millennium. They also fail to support the IPCC contention that the warming of high northern latitudes should be significantly greater than that of the entire Northern Hemisphere.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.

Reviewed 4 February 2004