Borehole Climate Reconstructions: Spatial Structure and Hemispheric Averages

Ground surface temperature (GST) reconstructions determined from terrestrial borehole temperature profiles, when averaged over the northern hemisphere, estimate a surface warming of approximately 1 K during the interval AD 1500-2000. Proxy-based estimates suggest less warming during the same interval. Borehole-based reconstructions have recently been the target of two criticisms. The first emerges from the application of a method for optimal detection of the surface air temperature (SAT) signal in the GST reconstructions, whereas the second focuses on the need for spatial gridding and area-weighting of the ensemble of borehole-based GST reconstructions. The optimal detection method has purported to show only a weak correlation between 20th century trends in the GST and SAT. We examine this issue with an assessment of the sub-hemispheric spatial correlation of GST and SAT trends at various spatial scales. We show that in the five-degree grid employed for optimal detection, the majority of grid-element means are weakly determined from three or fewer boreholes, a number that is insufficient to suppress site-specific noise. Thus most mean GST reconstructions in five-degree grid elements are deprived of signal enhancement through ensemble averaging, and are therefore obscured by unsuppressed noise. Agreement between GST and SAT trends in individual five-degree grid elements increases substantially with the number of boreholes occupying the element. Spatial correlation between SAT and GST improves significantly at larger grid sizes in which grid-element means are determined from many more GST reconstructions. We demonstrate the robustness of GST warming estimates by showing that over a wide range of grid-element area and occupancy weighting schemes, the hemispheric five-century GST change falls in the range of 0.89-1.06 K, a range which includes the estimate of ca.1 K warming determined by simple averaging of the ensemble of borehole-based reconstructions.