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Earth's surface temperature: Past, present and future (pdf)/(jpg). This figure, from Chapman and Davis (2010), shows that views of temperature change in the next century are informed by temperature changes in the past. For illustrative and educational purposes, three sets of surface temperatures have been assembled: 1000-year reconstructions of past temperature change based on proxies (tree rings, corals, etc.), glacier lengths, and borehole temperatures; the instrumental record; and Intergovernmental Panel on Climate Change (IPCC) projections for temperature change from 2000 to 2100. Figure modified from National Research Council [2006] and IPCC [2007].

Geothermics of climate change (pdf)/(jpg). This figure, from Chapman and Davis (2010), shows: (a) Cumulative number of boreholes in the global climate borehole database with temperature observations to a given depth (shaded region). (b) Temperature versus depth at times following a step change (ΔT) in surface temperature. Solid dots mark depths where the signal is 5% of the surface change, a rough estimate of the noise level (red line) for a ΔT of 1°C. Climate change in the past 100 years is seen primarily in the uppermost 150 meters of a borehole; the past millennium is recorded down to depths of nearly 500 meters. (c) Average transient temperature from borehole observations (circles; see Harris and Chapman [2001] for details). Curves show various synthetic profiles derived from diffusing multiproxy temperature reconstructions (Figure 1) into the ground. The legend shows the initialization temperature to obtain the synthetic profiles. Those reconstructions with relatively warm ninth to thirteenth centuries (the Medieval Warm Period) produce a positive excursion in temperatures in the synthetic profile between 200- and 600-meter depth. A greater number of deep boreholes from geographically dispersed sites would assist in resolving whether this climatic warm period was a global or a regional phenomenon.

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