Tree-rings and Borehole Temperatures: Complementary Estimates of Extratropical Northern Hemispheric Warming

by Robert Harris
COAS, Oregon State University

ABSTRACT
Tree-ring records of climate change are an important component of multiproxy records used to reconstruct climate. They provide powerful tools for temperature reconstruction because they are widespread, well dated, capture high-frequency events and extend estimates of climatic variability well beyond the instrumental period. Unfortunately tree-rings predominantly reflect warm season temperatures and, because of detrending practices, it is difficult to obtain low-frequency variations in temperature. Borehole temperatures provide a good complement to tree-rings because they are exceptional at capturing low-frequency events, are sensitive to the annual cycle of temperature change and cover similar geographic regions. Additionally because borehole temperature reconstructions are based on the physics of heat diffusion, an empirical calibration to temperature is not needed as with tree-rings. Borehole temperatures, however, do not have high time resolution.

We construct an extratropical reduced temperature-depth profile for land areas north of 20° N latitude from the global borehole temperature database compiled for climate reconstruction. The mean reduced temperature profile compares well with a time series constructed from an initial baseline temperature (0.6° ± 0.1° C) and the last 140 years of gridded annual surface air temperature data diffused into the ground. This analysis yields a root mean square misfit of only 15 mK and indicates warming of 1.1° C over the past 500 years. In contrast a tree-ring analysis from the same area [Briffa et al., JGR, 2001] indicates considerably less warming over the same time period. The recognition that tree-rings correlate most strongly with warm season temperatures (April – Sept.), while boreholes reflect annual temperatures, offers a plausible explanation for the discrepancy in warming estimates. This analysis yields a reconstruction of surface temperature over the past 500 years that is consistent with both the borehole and tree-ring analysis, and also provides an estimate of long-term cold season temperature. We estimate that continental extratropical northern hemisphere annual and cold season (March – Oct.) temperatures have warmed by 0.2° ± 0.1° C and 0.4° ± 0.3° C, respectively, between 1500 and 1856 prior to the start of instrumental surface air temperature record.