o in the case of snow deposited in ice sheets at high latitudes. Polar ice cores contain traces of the past atmosphere - temperature relations, data on precipitation, gas content, and chemical composition as well as a broad spectrum of the early environmental information (for details see Section 1.2.3). Past ice surface temperature reconstructions from the temperature logs measured in the ice boreholes provide valuable estimates of the surface temperature changes in polar environments that are complementary to paleocli-matic records obtained from ice core oxygen isotopes. Ice borehole information could spatially complete information provided by land boreholes. Boreholes drilled through the Greenland and Antarctica ice caps archive the magnitude and timing of climate change over the last 700 ka.
Greenland is the world's largest island. About 81% of its surface is covered by ice creating so-called the Greenlandic ice cap. Climate changes in this area and especially the fingerprints of the global warming are extremely important. Being entirely melted the Greenlandic ice could contribute 6-7 m to the global sea-level rise (Hvidberg, 2000). Numerical simulations of temperature-ice melting relation have indicated that an annual or only summer temperature increase of only by 1K will result in approximately 20-50% increase of ice melting (Janssens and Huybrechts, 2000). Systematic observations of the Greenland climate thus appear to be of the greatest importance for the studies of polar and global climate change. A unique opportunity for studying the Greenland past climate was offered when in 1966 the U.S. Army Gold Region Research and Engineering Laboratory drilled the first 1400 m long ice core at Camp Century on the Greenland ice sheet (www.ncdc.noaa.gov/paleo/icecore/greenland/gisp/campcentury/campc.html). The oxygen isotope measurement technique applied on the deep ice core provided detailed continuous climatic record over 130000 years (Johnsen et al., 1970). After that similar ice core projects have been performed in Greenland, Canadian Arctic, and Antarctica. From 1989 to 1994, the U.S. and European scientific communities initiated new intensive ice coring efforts in Greenland. These works, termed as the Greenland Ice Core Project (GRIP; www.ncdc.noaa.gov/paleo/icecore/greenland/summit/document/gripinfo.htm) and the Greenland Ice Sheet Project Two (GISP2; www.gisp2.sr.unh.edu), acquired deep ice cores from on and near the Greenland summit. The objective of both efforts was to reveal continuous, high-resolution, multi-parameter paleoclimatic/paleoenvironmental information stored in the ice.
The American-Danish-Swiss GISP1 program began in 1976 and produced a 2037 m deep core at the location Dye 3 in southeastern Greenland (65.2°N, 43.8°W; Johnsen et al., 1994). The temperature logging in this deep hole was performed in 1983. The GRIP and GISP2 drilling efforts essentially represent the renewed versions of the GISP1 program. The GRIP site is situated on the ice divide in central Greenland (72.58°N, 37.63°W). The depth to bedrock in this location is approximately 3 km, which corresponds to a stratigraphic record of at least 200000 years. This interval includes two glacial/interglacial cycles. The 3029m long GRIP ice core was drilled from 1989 to 1992. The GRIP ice core was successfully recovered in 1992, and 13 cm wide liquid filled borehole was left at rest for approximately one year. Temperatures were then measured to the bottom of the hole in 1993,1994, and 1995 (Dahl-Jensen et al., 1998). The GISP2 ice core (72.60°N, 38.50°W) 3053.4m in depth was recovered after 5 years drilling in 1993. At present it is the deepest ice core extracted from the polar caps. Details of the high-precision temperature logging performed in the GISP2 borehole are presented in the work by Clow et al. (1996).
All extracted ice cores contain a high-resolution archive of more than 100000 years of climatic history, embracing at least last interglacial-glacial cycle. It is the longest such record in the Northern Hemisphere. Under multi-institutional efforts scientists have performed a huge number of analyses, such as ice stratigraphy, trapped gases and their stable isotopes, stable isotopes in ice, particulates, major and trace element chemistry of ice, its conductivity, and other physical properties. Probably one of the most important findings of all projects was the recognition of the rapid climate changes at least of the regional extent (Dansgaard-Oeschger events) in the last glacial period (Dansgaard et al., 1993). They have been observed also in the previous ice cores; however, the recent drillings not only confirmed their potential existence, but also have detected such quantitative characteristics as their number and extremely fast onset that have occurred perhaps within a few decades. Investigations suggest that in contrast with the stability of the Holocene conditions, climate of the North Atlantic region is able to reorganize itself rapidly. Except for the recent times, instability was characteristic for the North Atlantic climate over the last 230ka. This fact hints the possibility of radical climatic changes under anthropogenic influence and present growing atmospheric pollution.
Below we concentrate on the investigations that are most important from the topic of this book, namely on the surface temperature history reconstructions performed on the basis of the above-mentioned temperature-depth profiles measured in the holes after ice core recovering. These studies began only in the recent years (e.g. Johnsen et al., 1995; Dahl-Jensen et al., 1998). Figure 72 shows the temperature-depth profiles measured in the Dye3 and GRIP sites. Repeated logging was performed in both holes. The last profiles were used for both GST reconstructions that did not contain drilling disturbances. Measurement precision was ±30mK and ±5mK for the former and the latter temperature logs, respectively. Present mean annual surface temperatures at the sites range between approximately -(20-30)°C and grow to the bedrock to -(10-15)°C.
Forward modeling results by Dahl-Jensen et al. (1998) have shown that the basal temperatures have remained well below the ice melting point over at least the past 100ka. Generally, the temperature at the base of the glacier strongly depends on the accumulation and flow of ice and on the geothermal heat flow from below. In Antarctica, e.g. near zero temperature was measured at the base of the moving ice stream at holes B, C, and D (Parizek et al., 2002, and the references therein).
Both temperature profiles that are shown in Figure 72 exhibit significant climate-induced disturbances. Because of the nearly flat topography, the homogeneity of the medium and absence of the hydrological disturbances, ice boreholes appear particularly well for the GST history reconstruction. An important shortcoming of the ice boreholes in comparison with the holes drilled in rock is that the ice is moving. Initially, the ice moves downward as the result of snow accumulation at the surface and eventually it creeps laterally to the sea. Thus, temperature distribution along deep ice holes does not represent (like in permafrost regions) the result of pure conductive heat transfer and depends both on the surface temperature and on the geothermal heat flow as the boundary conditions. Except for the climate change measured in the ice boreholes T-z profiles also reflect the ice flow pattern and accumulation rate histories that could not be recovered from the temperature logs and need additional study. Available information can help to simplify physical model used for description of the heat transfer in both holes. Detailed investigations at Greenland summit have shown that (1) the basal temperatures have been below the melting point during the past 100 years and (2) the ice movement has been essentially vertical in the past. Thus, for the surface temperature reconstruction one can use 1-D time-dependent equation of heat transfer where t is time, z the depth, T the in situ ice temperature, and vz the vertical velocity of ice movement. Physical parameters of ice are its density (p), thermal conductivity (K), thermal diffusivity (k), and specific heat capacity (cp), respectively. Internal heat production is usually assumed to be zero. The model does not account for the long-term surface elevation changes. Past accumulation rates and ice flow pattern can be found by ice core studies (e.g. Johnsen et al., 1995; Cuffey and Clow, 1997). As previously, the ice surface temperature and the geothermal heat flow from below are regarded as unknowns of the problem (see Section 2.3, Chapter 2). Solution for combined ice flow/heat transport inversion problem was investigated in the work by Dahl-Jensen et al. (1998) on the basis of a Monte Carlo method.
Figure 73 demonstrates best-fit surface temperature histories back to 8 ka, which have been obtained in this work. As shown, results for both sites are highly coherent. They show the post-glacial Climatic Optimum that occurred 8000 to 5000 years ago with temperatures 3-4 K higher than now. The surface temperature histories also reveal the Medieval Warm Period around 1000 A.D., while the Little Ice Age has culminated near 1550 A.D. According to Lamb (1969) "climatic history must be central to our understanding of human history". It is especially a case in such regions as Greenland. Around 1000 A.D. the Vikings have established a colony in Greenland. Thus, in 986 A.D. Erik
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