Fig. 17. Repeated measured temperature logs (together with the reduced temperatures) performed in the Hearst hole, NE Ontario, Canada (personal communication by W.D. Gosnold and A.M. Jessop, see text).

diagrams below present the temperature log not only on the measured, but also on a reduced scale obtained by subtracting from the measured temperatures, a temperature value = gradient X depth (see also Eq. (2.5), Section 2.2). This representation enhances the nonlinearities. The shape of the reduced temperature-depth profiles is more complex than that occurring in the case of the single warming event (Figure 13). The waves of the opposite sign in the reduced temperature profiles hint the presence of the recent warming that may be amplified by the environmental effect of the forest clearing occurred approximately 100 years ago (Wang et al., 1992), subsequent cooling, warming, and cooling again. Examples of the GST history reconstruction for this hole are presented in Section 2.4.3 (Chapter 2).

The surface temperature history can be inferred directly from the borehole temperature logs. Earlier the subsurface anomalies were found by forward calculation using appropriate physical models with given surface temperature histories and by selection of those GST history that best explains the measured temperature-depth profile. At present the GST changes are inferred by the more general data inversion techniques. The accuracy of the inversion depends on numerous a priori information, e.g. on the knowledge of conductive properties of the subsurface stratum. This technique is essentially multi-decadal and cannot provide information about annual temperature changes or for the times near the present. The advantages of the "geothermal" method are discussed in details in Paragraph 2.2. Here, we would like to point out only the two main advantages: (1) subsurface temperatures are measured directly and on the contrary with the proxy measures their inversion provides a direct evidence of past temperature change at the

Earth's surface, (2) because of great number of boreholes the method is applicable over most continents including polar ice caps.

From the middle of twentieth century numerous measurements of the temperature profiles in boreholes were performed for the terrestrial heat flow study. The recognition that past climate changes influence the GSTs, penetrate in the subsurface and could be recovered from the temperature-depth profiles measured at boreholes dates back to Lane (1923). The first attempt to infer past climate changes from measured temperature-depth profile dates back to Hotchkiss and Ingersoll (1934) and Birch (1948). More systematic studies of the possibilities of the geothermal method were undertaken only in the early 1970s (Cermak, 1971; Anderssen and Saull, 1973; Beck, 1982). However, even that time climatic perturbations to an otherwise equilibrium geotherm were regarded as "noise", and it was customary to eliminate them from the temperature profiles measured for the Earth's heat flow investigations and/or use the lower "undisturbed" sections of the temperature logs for the terrestrial heat flow determination. The real recognition of the method has been gained in the 1980s when the evidence of pronounced last century warming was clearly proved in a number of wells in the Alaskan Arctic (Lachenbruch and Marshall, 1986; Lachenbruch et al., 1988). In the recent two-three decades the geother-mal community had undertaken widespread re-examining of existing heat flow data in order to reveal the past GST changes and to construct systematic GST perturbation patterns. The previous "noise" was turned into a valuable signal of the climate change. Further studies of the geothermal method developed in three main directions:

1. Inversion methods - Numerous inverse methods based on different assumptions and used definite a priori knowledge have been developed that time.

2. Climate reconstructions using national borehole database - The ample worldwide geothermal database of temperature logs initially measured and compiled for heat flow studies has proved to be very useful for the GST reconstructions. The intensification of the borehole climate studies was supported by simultaneous significant growth of the global geothermal borehole network. Since the 1990s numerous borehole loggings were performed directly for paleoclimatic reconstructions (for more details visit web sites and/or

3. Integration of the obtained GST histories in the traditional paleoclimatic network (Harris and Chapman, 2001; Mann et al., 2003).

The first compilation of the studies inferring past climatic changes from underground temperatures has appeared in 1992 (Lewis, 1992). The reconstruction of the GST histories has drawn increasing attention under several international projects in the 1990s. The Project No. 428 carried out in the years 1998-2002 under the UNESCO International Geological Correlation Program "Borehole and Climate" was probably the most important of them. The next after the year 1992 current collection of the borehole climate reconstructions from a number of regions all over the world was compiled by Beltrami and Harris (2001). The analyses of the worldwide borehole data for the large-scale spatial-temporal reconstructions of the Earth's climate are presented in the works by Pollack et al. (1998), Huang et al. (2000), and Mann et al. (2003). Initially the paleoclimatic information was gained from conventional widespread land boreholes. Recently the 50000 years long GST history was recovered from temperature profiles measured in the ice borehole remained after successfully recovered ice core in the Greenland ice sheet (Dahl-Jensen et al., 1998). The ice borehole logs can provide valuable estimates of past temperature changes in polar environments that are complimentary to proxy reconstructions obtained from ice core oxygen isotopes (for details see Section 2.9). Superdeep boreholes belonging to the International Continental Drilling Program (ICDP; attracted special attention of the "borehole climatology" community. Geothermal and paleoclimatic investigations are among the most important directions of the ICDP scientific research (Section 3.5).

The common merits of the geothermal method are that it is based on a simple physical theory, that the past ground temperature conditions can be derived directly from measured temperature logs and do not need any additional calibration, its ability to recover continuous GST trends over the last millennium or longer, and a rather good terrestrial distribution of boreholes. Among the possible weaknesses is somewhat poor resolution that decreases back in time and non-climatic disturbances that could affect measured temperature-depth profiles. Numerous methods for diminishing of possible biases of the geothermal method are worked out. Obtained GST differs from the SAT that is of general use in meteorology/climatology. This complicates comparison of the GST and SAT based climatic reconstructions. The GST-SAT coupling depends on external factors (e.g. land surface cover and its changes, especially seasonal snow cover variations). Additional studies of this problem include experiments on the monitoring of meteorological and subsurface variables. These experiments were planned to reveal details of the air/ground energy exchange under various surface conditions (Putnam and Chapman, 1996; Beltrami et al., 2000; Cermak et al., 2000; see Chapter 4).

Despite the existing sources of bias, the results of the last two decades research confirmed the ability of the method to provide reliable GST history that is consistent with other paleoclimatic information. At present the geothermal method plays a new significant role in the investigations of climate of our planet. Borehole temperature profiles became one of the important sources of climatic information and contributed significantly to our knowledge of the millennial surface temperature changes. Some of the leading scientists give extremely positive evaluation of the geothermal method, e.g. "in my estimation, at least for timescales greater than a century or two, only two proxies can yield temperatures that are accurate to 0.5°C: the reconstruction of temperatures from the elevation of mountain snowlines and borehole thermometry" (Broecker, 2001). Using more modest expressions, one could declare that at present the 'borehole' method undoubtedly represents an independent well-developed research tool in the paleoclimatic studies and an important supplement to the climate reconstruction by proxy indicators.

The purpose of this book is to present our current best knowledge of the geothermal method for the past climate reconstruction. The book explains the capacity of the subsurface temperature field to "remember" what has happened on the surface and how this memory can be utilized. We therefore describe in details different methods of the GST inversion, make note on the strength and emphasize the potential weaknesses and caveats of the past climate reconstruction from geothermal measurements, particularly with respect to its resolving power, non-climatic disturbances to the measured temperature-depth profiles and GST-SAT coupling problems, and discuss the possibilities of further development. Significant part of this work summarizes the major results to reconstruct the climate scenario spanning from Holocene to recent and discusses their role in the improvement of the traditional paleoclimatic patterns. The final goal is to assess the magnitude of the present-day warming and to distinguish between the natural climate variability and the potential human contribution due to environmental pollution. We hope that this book will contribute to advance of the "Borehole Climatology" research in the coming years.

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