Borehole temperatures and tree rings: Seasonality and estimates of extratropical Northern Hemispheric warming

[1] 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., 2001) indicates considerably less warming over the same time period. The recognition that tree rings correlate most strongly with warm season temperatures (April–September), while boreholes reflect annual temperatures, offers an 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 (October–March) temperatures have warmed by 0.2 ± 0.1 C and 0.3 ± 0.3 C, respectively, between 1500 and 1856, prior to the start of the instrumental surface air temperature record.