Potential biases in inferring Holocene temperature trends from long-term borehole information

We use a set of global coupled ocean-atmosphere general circulation (OAGCM) experiments for timeslices over the Holocene from 9K Yr BP onwards to examine differences in Northern Hemisphere Ground Surface Temperature (GST) and Surface Air Temperature (SAT) trends. The model results are driven mainly by the orbitallyforced changes in seasonal insolation, and in particular, the increase in northern hemisphere seasonality in the earlyto-mid Holocene. The model reproduces qualitatively presumed past trends in NH temperatures, though it may underestimate their magnitude. For this period, we see on average a significant increase in GST relative to SAT as a result of a competition between the effects of changing seasonal insolation, and the varying extent of insulating seasonal snow cover. The model shows a mid-Holocene peak in annual mean terrestrial Northern Hemisphere GST, but not in annual Surface Air Temperatures (SAT). We conclude that the factors influencing long-term GST trends are potentially quite complex, and that considerable care must be taken in interpreting SAT changes from the GST evidence when there is the possibility of substantial seasonal variation in warmth and snow cover.