Fig. 12. Low-frequency component of the Northern hemisphere temperature anomalies (Moberg et al., 2005) compared with the reconstruction by Mann et al. (1998) and borehole ground surface temperature reconstruction by Huang et al. (2000).

Fig. 12. Low-frequency component of the Northern hemisphere temperature anomalies (Moberg et al., 2005) compared with the reconstruction by Mann et al. (1998) and borehole ground surface temperature reconstruction by Huang et al. (2000).

Recently Hegerl et al. (2007) have suggested a new calibration method that avoids the loss of low-frequency component in the multiproxy reconstructions. On the basis of updated proxy time series, these authors have reconstructed 1500 years long past temperature variations for the Northern hemisphere on decadal scale of aggregation. Obtained record shows substantial variability over the whole reconstructed period that is very similar to the Moberg et al.'s results. Hegerl et al. (2007) have tested their record with independent temperature reconstructions and with the climate model estimates. Good coincidence was found in both cases. The comparison of the reconstructed by Hegerl et al. (2007) temperature course with borehole estimates by Pollack and Huang (2000) and Pollack and Smerdon (2004) (for details see Section 3.2, Chapter 3) that revealed good agreement of both reconstructions appears to be most important for the borehole climatology. Using a conductive forward model and their SAT estimate as a surface forcing function (for details of calculus see Section 2.5, Chapter 2), Hegerl et al. (2007) have also calculated corresponding subsurface temperature anomaly. Its comparison with the average observed anomaly determined by Harris and Chapman (2001) has shown that the two temperature-depth profiles are almost identical.

All above cited works mean probably the end of the "hockey stick" representation and have inspired numerous responses in climatologic community, like "Is the hockey stick broken?" and index.php/2005/03/03/hockey-stick-2005-rip. As it is described in Section 3.2 (Chapter 3) (see also Figures 11, 12), borehole temperature reconstructions generally reveal more colder past and the warming that is more gradually distributed over past five centuries.

Being quite different from the "flat" curve accepted by the "mainstream" of the climato-logic community, these results were met with caution. Almost perfect coincidence of the GST reconstructions with the SAT changes presented in the works by Moberg et al. (2005) and Hegerl et al. (2007) represents also powerful verification of the "borehole climatology" with independent method and once more corroborates that borehole temperature reconstruction is a good indicator for the land annual SAT change.

In summary we can only mention that considerable scientific efforts have been done to reconstruct past climate from the biological and physical proxy sources. This task is a challenging one and its results represent the subject to many complications and potential uncertainties and confidence can only be gained after comparison of more and more independent sources of proxy data. It is clear that the fundamental limitations (both temporally and spatially) of large-scale proxy-based reconstruction for past centuries arise from increasing sparseness of proxy database available to provide reliable climate information back in time. This database can be completed in space and time to such state when significant improvements will be possible in proxy-based reconstruction of the global climate only through joint efforts of large number of paleoclimate researchers. The compilation of many proxies can somewhat extenuate this problem, however, even a "multiproxy" reconstructions can only give a general understanding of what the climate was like and identify large-scale changes which may be related to climatic forcing of hemispheric or global significance. The "multiproxy" reconstructions are best to indicate climate tendencies or trends rather than exact temperature changes. On the other hand, numerous proxy results indicating similar climatic trends represent a powerful evidence that these tendencies are significant and really occurred, even if the magnitude of the change cannot be quantified.

1.3 Borehole Climatology

Geothermics is the sub-branch of geophysics that studies terrestrial heat flow (Kappelmeyer and Haenel, 1974; Haenel et al., 1988; Jessop, 1990). Heat flow is the quantity of heat (generally expressed in mW/m2) transferred from the Earth's interior to the surface. The major source of the interior heat is the decay of radioactive elements in the Earth's crust and upper mantle. Up to 70% of continental heat flow may be generated within the upper 10-20km of the crust; while 96% of the oceanic heat flow comes from below the oceanic crust where the concentration of the radioactive elements is significantly poorer (Kearey and Vine, 1990). The distribution of heat flow is related to tectonic processes in the lithosphere. The average heat flow density is inversely correlated with the geologic age of a given tectonic unit or oceanic crust (Sclater et al., 1980; Condie, 1989). On a regional scale heat flow pattern depends on numerous factors, such as regional differences in crustal radioactivity, fault distribution, hydrogeology, and hydrothermal activity (Cermak, 1983). Knowledge of the subsurface temperature field is central for understanding of practically all geophysical processes. The variation of temperature with depth and amount of heat leaving the Earth's interior through its surface can be easily measured. Heat flow determinations in boreholes are made by combining sets of temperature-depth profiles and thermal conductivity data by the expression Q = K (dT/dz), where Q is heat flow, K the thermal conductivity, and T the temperature at depth z. The present global heat flow data set was compiled under aegis of the International Heat Flow Commission (IHFC; home-page www.geo.lsa.umich/IHFC) and contains more than 24000 measurements. Its description and analysis can be found in the work by Pollack et al. (1993). Temperatures obtained in boreholes, both the single values from maximum-reading instruments and/or continuous temperature surveys (temperature logs), are essential to many areas of scientific research and engineering.

Present book is devoted to the so-called 'geothermal or 'borehole' method of the climate reconstruction which represents the reconstruction of the past temperature changes from the temperature-depth profiles measured in boreholes. This method principally differs from conventional proxies since it provides direct estimates of the GST histories. Ground surface temperature itself represents one of the important climatic variables, thus, reconstructed GST histories do not require calibration against independent climatologic data.

The physics of the phenomenon is the next. At the constant surface conditions the underground temperature is governed by the outflow of heat from the Earth's interior. For the homogeneous stratum it increases steadily with depth. Temperature changes at the surface slowly propagate downward and appear superimposed on this background geo-therm. Figure 13 illustrates the ideal case of the penetration of sudden 1K increase in the surface temperature into the subsurface with zero temperature gradient. As seen, it creates noticeable curvature of the undisturbed geotherm (sometimes called "U-shape").

Fig. 13. Subsurface temperature distribution corresponding to a sudden 1 K increase in the surface temperature: 100, 300, and 500-years after its occurrence.

Surface warming manifests itself as a positive disturbance in the subsurface; cooling shows up as a negative disturbance. The perturbation has the maximum amplitude at the surface, and the subsurface effect is limited to progressively shallower depth ranges as the surface change duration is shortened. The velocity of perturbation is a function of the thermal diffusivity of the medium. For typical Earth's rocks with diffusivity of ~10-6m2/s perturbation can propagate approximately 20 m in one year and 650 m in thousand years (see examples in Section 2.2, Chapter 2). Thus, 500-650m deep hole archives climatic information for the last millennium or so. Whereas the depth of a subsurface temperature perturbation is related to the timing of the GST changes, its shape reflects details of the GST history. In other words, the borehole temperature log represents the transformation of the surface climatic events from the time into space coordinates. This transformation is performed by the nature and not by mathematics. Thus, the borehole temperature logging can replace the long-term surface temperature measurements.

Well-pronounced "U-shapes" occur only in the ideal case of a single powerful climatic event. Arising sequentially several changes in the surface temperature create more complex and/or less expressed patterns than the strong "U-shapes" presented in Figure 13. Example of the temperature-depth profile simulated for the real GST changes is shown in Figure 19 (Chapter 2).

Temperature changes at the Earth's surface occur at several temporal scales. The oscillations are more regular on diurnal, seasonal, and annual scales. The strongest of these changes are the daily and seasonal variations with the amplitude of approxi-mately10°C and the annual GST oscillations with typical amplitude of 20-30°C. Inter-annual and long-term temperature change patterns are generally irregular. As the surface temperature signal propagates downward, its amplitude decreases exponentially with depth due to the diffusive process of heat conduction. Each variation vanishes over a vertical distance related to the period of change and to the thermal diffusivity of the ground. Shorter period fluctuations attenuate more rapidly. Thus, the Earth selectively filters out high-frequency component of the surface temperature oscillations, and deeper we go, the more distant past can be inverted (unfortunately also more diffused and less credible). Figure 14 illustrates the amplitude attenuation of the temperature signal when propagating downwards and the delay of its phase by showing the results of the 12-year temperature monitoring at several shallow depths in the experimental borehole Sporilov (Prague, the Czech Republic) (Cermak et al., 2000). The daily temperature wave is practically not observable below 1 m depth. On the other hand, the temperature at 1 m represents integrated average of the daily signal of the previous day. Similarly, annual GST oscillations vanish near approximately 10-15 m depth and are not measurable below this depth. However, temperature measured above this depth is a proper index of the averaged temperature wave of the previous year. The temperature field below the 20-30m depth is free of any response to the annual and/or shorter temperature variations and contains exclusively the fingerprints of longer scale climatic events with characteristic time of at least several years. Such signal may well characterize the pattern of the long-term climate change.

Figure 15 illustrates the amplitude decrement and phase shift of the annual temperature wave with depth in more details. It shows the 1998-year interval of the long-term temperature time series from Sporilov presented in the previous figure. Temperature was monitored at several shallow depths from the surface to 7.5 m. As the

Fig. 14. Results of 12-year precise temperature monitoring at several shallow depth levels in the Sporilov hole (the Czech Republic; 50.04°N, 14.48°E, 274m asl.) clearly demonstrate the propagation of the surface temperature signal to the depth.

surface temperature signal propagates downward, it is delayed in time and its amplitude decreases exponentially with depth due to the diffusive process of heat conduction. Each variation vanishes over a vertical distance related to the period of change and to the thermal diffusivity of the ground. Thus, the amplitude of the annual wave decreases to 50% of its surface value at ~2m depth with time delay of about 40 days. It decreases to ~ 15% of its surface value at 5 m depth where it arrives with approximately three months delay. Repeated measurements of borehole temperature logs, e.g. the temperature loggings performed in borehole GC-1 (northwestern Utah) over a span of 14 years by Chapman and Harris (1993) have shown a slowly varying temperature field with remarkable similarity of the measured signals and the replication of their main details, remaining evidently coherent in space and time (Figure 16). Bottom panel (c) of Figure 16 shows the temperature differences between the individual logs (data points) together with the synthetic temperature-difference profile (solid line) computed from the 100 years long meteorological record of SATs in the nearby weather station. The deviations between three temperature logs do not exceed 0.1K. Synthetic temperature profiles exhibit high correlation with the measured temperature logs. In the case of above mentioned borehole GC-1 synthetic profile represented well systematic negative anomaly and significant curvature in the uppermost 60 m of borehole. This fact indicates that "U-shapes"

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