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Area under investigation was situated close to the eastern margin of the Weichselian glacier (see Section 3.5.1, Chapter 3). During the maximum cooling phase (18-20ka ago) the permafrost in eastern Karelia has extended to 400-600m depth and was characterized by the temperature of about — 10°C. Forward models simulated by Kukkonen et al. (1998) have shown that the long-term effects of the Weichselian glaciation are sufficient to decrease the heat flow values to the measured levels and that detected lowered temperature gradients in the uppermost 1 km can be attributed exclusively to very low ground temperatures during the glaciation and to the thawing of the ancient permafrost. Permafrost probably existed in the area during the whole glaciation period (60-11 ka ago). Its melting in the post-glacial time has consumed heat. This process had retarding effect on the subsurface temperature response to the surface warming. Similarly low temperature gradients in the upper 3 km were reported for more than 300 heat flow measurements in the Urals (Kukkonen et al., 1997). The authors have shown that although this area was outside the glacier, observed phenomenon can be attributed to the periglacial21 conditions during the glaciation. Low thermal gradients were detected in the West-Siberian Platform at latitudes north of 62°N, where even a relic hidden permafrost layer was discovered below the thawed underground in the depth range of 100-300m (Duchkov and Devyatkin, 1992).

Safanda et al. (2004) have investigated temperature-depth profiles measured in a suite of deep (up to 2.3 km) holes located near the Polish-Lithuanian boundary. Measurements revealed a negative temperature gradient in the uppermost 400 m. The study of terrain effects in the vicinity of the boreholes have shown that the climate change can be the only plausible explanation of the peculiarities detected in the temperature logs. An average GST as low as — 10°C as well as more than 500m thick permafrost layer has existed in the area during the last glacial (Hartmann, 1994). Results of forward simulations of the past climate as well as the inverse GST reconstructions have shown that detected course of the T-z profiles can be attributed to the past climate changes alone and, similarly to the above-mentioned work by Mottaghy and Rath (2006), have emphasized the crucial role of the latent heat effect on the coupling of synthetic and measured temperatures. The authors have inferred temperature increase from — 10.3°C to +7°C at the end of the glacial (13.7 ka B.P.) and subsequent warming to + 8°C in the last 150 years. When the latent heat was not taken into account, the subsurface warming was more rapid and the negative temperature gradients have vanished some thousands of years earlier. According to the calculations by Safanda et al. (2004), observed low subsurface thermal gradients can be partly attributed also to a low (~40mW/m2) terrestrial heat flow incoming from the Earth's interior in the area under investigation. The reason for the low heat flow values is the low heat production of anorthosites, characteristic rocks of the area.

Geothermal gradients measured in the Scandinavian boreholes with the permafrost thickness of 220-380m have ranged from negative values of —6K/km to 10-38 K/km. Low geothermal gradients were also detected in a PACE borehole at Stelvio (Italian Alps), on Plateau Mountain (SW Alberta, Canada), at Swedish Lapland, and in certain parts of the Baltic Shield in Norway (Isaksen et al., 2001; and the references therein). Similarly to

21Several definitions exist for the term periglacial. More conventional one suggests that these environments were located at periphery of past Pleistocene glaciers. Broader modern usage encompasses a wide range of cold but essentially non-glacial climates, regardless of their proximity to glaciers on time or space. More than one third of the Earth's land surface can be included in this definition.

Safanda et al. (2004) the latter authors have studied all possible explanations for this phenomenon including terrain effects in the area under investigation and concluded that even in the presence of other influences (e.g. low internal heat production in the plutonic and metamorphic rocks in the mountainous areas of Scandinavia) the impact of paleoclimatic effects on geothermal gradients cannot be ignored. The above described low geothermal gradients are characteristic only for the upper parts of boreholes. Measurements in the deep holes (1 km and deeper) have detected that the low temperature gradients gradually increase with depth and achieve the steady-state values undisturbed by the glacial/interglacial cycles (Safanda and Rajver, 2001; Kukkonen and Joeleht, 2003; Safanda et al., 2004).

Numerous temperature logs measured in the boreholes penetrated permafrost were applied for the GST history reconstruction. Using a suite of 20 boreholes that were drilled up to 1800 m depth in the immediate neighborhood of the Kola superdeep hole (see Section 3.5.2, Chapter 3), Rath and Mottaghy (2003) have inferred GST history of the region back to 50 ka B.P. The study has detected that in the area under investigation, the GST increased by 4-5 K since the last glaciation to the Holocene. Somewhat smaller amount of warming than that detected in lower latitudes by other GST reconstructions can be attributed to the insulating effect of the ice cover. The authors also performed the joint inversion of the T-z profiles for seven deepest holes from this database. Reconstructed almost 100000-year long detailed GST history of the region indicated last glacial maximum with temperature —4K lower than nowadays, warming that began 15-17ka B.P. and culminated in 2000 B.P., subsequent cooling with the temperatures 2 K lower than the present level with minimum that occurred 300 years ago, and the warming since then. Recently Mottaghy and Rath (2006) have reprocessed the Poland temperature logs used by Safanda et al. (2004) for the GST reconstructions. The authors have applied the above-described mathematical model of the heat transfer in the freezing/thawing medium with more complex partition function r given by Eq. (41) and have reconstructed the GST history for the last 100ka. In spite of the different partition function applied in both works as well as the differences in other parameters, in general their findings agree well. The lowest estimated temperatures were — 10°C, and 650m (in comparison with 520m in the work by Safanda et al., 2004); thick permafrost has disappeared in the region about 4ka B.P.

The GST inversions of temperature logs measured in boreholes drilled in permafrost also have detected more recent climatic changes. An analysis of the T-z profiles from Svalbard boreholes and from holes located in alpine regions of mainland Norway have shown that the permafrost has warmed up over the last 100 years. The reconstruction of the GST history at Janssonhaugen (Svalbard, Norway) from the borehole data has inferred a temperature increase of 1-2K over the past 60-80 years (Isaksen et al. 2000). An analysis of the temperature-depth profile from the borehole in Juvvassh0e (southern Norway) has indicated a temperature increase of 0.5-1 K over the last 20-40 years (Isaksen et al. 2001). Evidence for secular warming of European mountain permafrost was presented by Harris et al. (2003). The authors have analyzed mountain permafrost temperatures from six boreholes arranged in the latitudinal transect extending from the Alps, through Scandinavia to Norwegian Svalbard. As a part of the above-mentioned PACE European Union research project a number of at least 100m deep holes were drilled into the frozen bedrock during 1998-2000. All boreholes have exhibited clear "U-shapes", indicating recent warming. Inversion results have revealed GST warming of ~1K occurred in the last 100 years.

However, the most noticeable and/or celebrated evidence of the recent climate warming extracted from boreholes drilled in permafrost is connected to Alaskan Arctic region. Permafrost lies beneath about 80% of Alaska's surface. On the North Slope, permafrost ranges in thickness from about 200 to as much as 700m, having temperature from -8 to - 10°C. The geothermal data were obtained from the oil exploration holes distributed all over the Alaskan Coastal Plain and Foothills. Configuration of measured temperature-depth profiles revealed clear curvature towards warmer temperatures (U-shape) in the uppermost 200m (Figure 71). An analysis of this T-z profiles has provided the first evidence that Alaskan Arctic has warmed by 2-4K during the twentieth century prior to the mid-1980s (Lachenbruch and Marshall, 1986; Lachenbruch et al., 1988). Although the details of the climate warming that time could not be resolved, the recent warming of the permafrost was surely detected. The near-linear character of the deeper permafrost T-z profiles does not necessarily mean that an equilibrium thickness has been attained. Long characteristic times of the permafrost response to climate changes hint that it could still respond (freezing/thawing at the base) to the long-term climate changes that occurred in the last several tens of thousands years. Most recent GST estimates of the same authors using the ramp/step approach corroborated early results and have given 2.7 ± 1.0K amount of the last century warming.

Wide borehole temperature measurements performed over the last two decades in Alaska have shown that permafrost has warmed at all sites along a north-south transect

TEMPERATURE, deg.C

TEMPERATURE, deg.C

The individual T(z)-curves are shifted to avoid overlapping

Fig. 71. Borehole temperature observations from 14 holes in Arctic Alaska (Lachenbruch and Marshall, 1986). The curvature is consistent with a warming at the top of permafrost; the duration of the warming event varies for different sites, but it has a twentieth century onset in general. The shaded region for each curve represents the warming anomaly; the number above each profile indicates the magnitude of the local warming (in °C).

The individual T(z)-curves are shifted to avoid overlapping

Fig. 71. Borehole temperature observations from 14 holes in Arctic Alaska (Lachenbruch and Marshall, 1986). The curvature is consistent with a warming at the top of permafrost; the duration of the warming event varies for different sites, but it has a twentieth century onset in general. The shaded region for each curve represents the warming anomaly; the number above each profile indicates the magnitude of the local warming (in °C).

spanning from Prudhoe Bay to Glennallen (Osterkamp and Romanovsky, 1999). The total magnitude of the warming at the permafrost surface since the late 1960s was proved to be about 2K. Most recent data indicate that the last increase of the warming rate began on the Arctic Coastal Plane in the mid-1980s and in areas of discontinuous permafrost between 1989 and 1991. Maximum magnitude of warming (3-4 K) was measured at West Dock and Deadhorse near the Arctic Ocean. Detected in Alaska warming can likely be attributed to an anthropogenic influence. Most of the global warming scenarios derived from the GCMs predict that human-induced warming will be amplified in the high-latitude regions with the clear consequences for the permafrost regions (e.g. Woo et al., 1992; Flato et al., 2000). The GCMs predict that the doubling of atmospheric carbon dioxide concentrations should result in the 2-3 times greater warming in the polar regions than the global average. In the discontinuous permafrost region, where ground temperatures are within 1-2 degrees of melting, permafrost will likely disappear. This process will be accompanied by sizable environmental impacts. Its thawing, resulting in warmer soils, will speed decomposition reactions and release additional carbon dioxide and methane into the atmosphere. While permafrost limits water movement, thawing of any permafrost will increase groundwater mobility, the susceptibility of environment to erosion and landslides. Under climate warming, much of this terrain would be vulnerable to subsidence, particularly in areas of relatively warm, discontinuous permafrost.

The GST reconstructions used a set of temperature logs measured in 61 (109-620m deep) holes located in the latitudinal belt between 60° and 82°N in northern Canada have revealed the fingerprints of extensive recent warming (Majorowicz et al., 2004). Results of the simultaneous GST inversions of all borehole data have shown that the GST warming in this area has started in the late eighteenth century and have continued to the twentieth century. Cumulative amount of warming for this period equals to ~2K. The GST reconstructions by the same authors for more southern regions of northern Canada (Yukon, Northwest Territories and Northern Alberta) indicated a 1.1-2.5 warming in the twentieth century (Majorowicz and Safanda, 1998; Majorowicz et al., 2002). Recently Taylor et al. (2006) have described results of temperature monitoring at the Canadian High Arctic Permafrost observatories (77°-82.5°N). These sites represent the Canadian contribution to the above-mentioned GTN-P. They lie in the area of rigorous climate with mean annual air temperatures of — 18°C that is characterized by low precipitation and snow cover. This latter factor and the lack of vegetation result in a very low insulating effect. Various techniques of borehole measurements were used including single and multiple temperature logging as well as the temperature monitoring in the uppermost ~65m of boreholes. Investigations were concentrated on the detection of contemporary climate change through inversion of subsurface temperature time series (for more details see Section 4.2, Chapter 4) and on the extraction of the climate for the last two centuries to identify the long-term trend. Obtained results have corroborated early conclusions. Inversion resolved the Little Ice Age from the mid eighteenth to mid nineteenth century with the GST temperatures approximately 1K below the long-term mean and subsequent intensive warming that has produced approximately 3 K higher temperatures in the end of the twentieth century. These results generally coincide with similar reconstructions reported for Greenland ice cap GRIP holes and Dye-3 hole in the southeast (see the next section). Simultaneous inversion of the multiple temperature logs from shallower boreholes revealed fine structure of the recent warming. Calculations have detected two-three warming/cooling cycles in the GST change during the latter approximately 50 years.

In principle, available data, especially originated from the high latitudes, are sufficient to determine regional trends in the permafrost warming and corroborate the possibility of more intensive warming in the circumpolar regions suggested by the GCM simulations. However, from the pioneering work by Lachenbruch and Marshall (1986) to further extensive studies none of the researchers found any significant south-north trend in the magnitude of warming inferred from borehole temperature logs (e.g. Majorowicz et al., 2004). This fact is illustrated in Table 6 that accumulates some results on the permafrost warming magnitude detected in different locations of the Northern Hemisphere. As shown, all GST reconstructions show noticeable temperature increase in different parts of the permafrost environment during the last 30 years; however, any spatial trend is unrecognizable. This hints that the actual pattern of permafrost loss is more complex than a simple uniform northward retreat.

Summary: Long ago it was recognized that permafrost contains an abundant data for the proxy climate reconstructions (cryostratigraphic techniques, isotopic analysis, incorporated flora and fauna, etc.). Investigations of the last two decades have proved that temperature-depth profiles measured in boreholes drilled in permafrost contain also direct fingerprints of the past climate change and thus represent a useful tool for reconstruction of the past GST changes in the northern circumpolar areas as well as at high-altitude regions. Under proper choice of the mathematical description of the freezing/thawing processes an inversion of borehole temperatures from permafrost areas can provide accurate GST histories that are consistent with estimates of the temperature change in the permafrost areas gained from conventional proxies. Even if suggested forward and inverse methods for the reconstruction of the subsurface temperature distribution in the deeply frozen medium cannot handle the subtle features of the processes in the uppermost few meters of the ground (active layer); they generally give reliable results when the long-term climate variations associated with the last glaciation are considered. The GST signal for the past few tens thousand years can be reconstructed from the deep boreholes. An ensemble of results reported from the borehole temperature measurements in vast regions of the Alaskan, Russian Arctic, Scandinavian, and European mountains have revealed significant temperature increase since the last glacial as well as the warming during the twentieth century. It was also shown that an impact of the glaciation and fingerprints of the past permafrost can be detected from borehole temperature logs in many permafrost-free regions.

2.9 Climate from Ice Boreholes

Detection of the climate change in the circumpolar regions bears great importance. The Polish meteorologist Przybylak (2000) stated, "warming and cooling epochs should be seen most clearly here and should also occur earlier than in other parts of the world". Hence, as he continues, Earth's polar regions "should play a very important role in the detection of global changes". Vast climatic information can be obtained from a variety of stratified deposits, such as deep-ocean and lake sediments, polar ice sheets, speleothems, and peat deposits. This information is probably the most straightforward

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