From Global Warming Art
This image compares the instrumental temperature record of Brohan et al. 2006 to the borehole temperature reconstruction of Huang et al. 2000. For simplicity, the two have been aligned to have the same mean over the period of overlap.
Rock has a very low thermal conductivity which means that it can take centuries for rocks underground to become aware of changes in surface temperatures. By taking very careful measurements of the temperature of rock in boreholes tens and hundreds of meters underground, it is possible to detect shifts in the long-term mean surface temperature at that location. This is underlying goal of borehole temperature reconstruction. However, as thermal diffusion is such a slow process, short term changes are averaged out and this technique only provides information about changes in the long-term average temperature. Here the borehole temperature reconstruction assigns a single rate of temperature change for each of the last five centuries and thus does not capture short-term variability.
Unlike most other methods for studying paleoclimate, borehole thermometry is a direct measurement of temperature and doesn't need to be calibrated against the instrumental record. Hence, borehole thermometry provides and independent record of paleoclimate against which other paleoclimate techniques can be validated.
It is worth noting that that the Huang et al. reconstruction does not capture the full rate of temperature change during the twentieth century. There are several factors that may be responsible for this. Foremost among these, is that borehole measurements from the upper few meters (corresponding to the most recent decades) are often discarded to avoid contamination from soil and surface processes. In addition, a significant fraction of the boreholes were measured pre-1990 and hence would not capture any acceleration in temperature change during the end of the twentieth century.
This image was created by Robert A. Rohde from published data.
- [abstract] [ [ Brohan, P., J.J. Kennedy, I. Haris, S.F.B. Tett and P.D. Jones (2006). "Uncertainty estimates in regional and global observed temperature changes: a new dataset from 1850". J. Geophysical Research 111: D12106.
- [abstract] [ Huang, Shaopeng, Henry N. Pollack and Po-Yu Shen (2000). "Temperature trends over the past five centuries reconstructed from borehole temperatures". Nature 403: 756-758.
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