borehole-based reconstructions also indicate an earlier persistent smaller warming of roughly 0.3°C from 1500 to 1850 (Pollack and Huang 2000, Huang et al. 2000) (Figure 8-1). Regional estimates for the warming of the last 150 years range from 2–4°C for northern Alaska (Lachenbruch and Marshall 1986, Pollack and Huang 2000) to 0.5°C for Australia (Pollack and Huang 2000). Estimates for the western and eastern sectors of North America are rather different, 0.4–0.6°C and 1.0–1.3°C, respectively (Pollack and Huang 2000).
The time resolution and length of borehole-based surface temperature reconstructions are severely limited by the physics of the heat transfer process (Clow 1992). A surface temperature signal is irrecoverably smeared as it is transferred to depth. The time resolution of the reconstruction thus decreases backward in time. For rock and permafrost boreholes, this resolution is a few decades at the start of the 20th century and a few centuries at 1500. Borehole temperatures thus only reveal long-term temperature averages and trends prior to the period of instrumental records; they tell us nothing about decadal variations or specific years, except for the most recent ones. For rock and permafrost boreholes, the thermal smearing is strong enough to prevent recovery of clear temperature signals prior to about 1500. The spatial distribution of borehole temperature records is also strongly weighted toward North America, Europe, South Africa, East Asia, and Australia (Pollack and Smerdon 2004), with almost no information from South America and North or Central Africa and spotty coverage in Asia. Furthermore, the usable boreholes are mostly a legacy of mining exploration, so the spatial coverage has not been chosen to optimize climate reconstruction.
Important quantitative uncertainties in borehole-based reconstructions could arise from two separate sources. First, borehole temperatures respond to the ground surface temperature and not the temperature in the overlying air (Gosnold et al. 1997, Smerdon et al. 2004, Pollack et al. 2005). It is possible that these two temperatures will vary differently over time, for example, due to changes in snow cover and soil moisture. Thus, a key question is whether long-term trends in air and ground temperature are similar. Although there are clear exceptions (Gosnold et al. 1997), the majority of evidence indicates that this similarity is generally strong: As a large-scale geographic average, measured ground temperatures match those predicted directly from air temperature changes (Harris and Chapman 2001), and air versus ground temperature trends are similar at some specific sites (Baker and Ruschy 1993, Majorowicz and Safanda 2005, Majorowicz et al. 2006).
The second important potential source of error in rock borehole-based temperature reconstructions is the downward percolation of groundwater, which can reduce the temperature at depth and be misinterpreted as a warming of the surface over time (Chisholm and Chapman 1992, Harris and Chapman 1995, Ferguson and Woodbury 2005, Majorowicz et al. 2006). This may introduce a “warming” bias to reconstructions based on continental boreholes, the magnitude of which has not been addressed systematically. However, the similarity between measured ground temperatures and ground temperatures calculated using air temperatures (Harris and Chapman 2001) suggests that the average bias must be small over the middle latitudes. In general, groundwater bias can only be a problem in humid climates and in rock that readily conducts groundwater (including all highly fractured rock and many sandstones and