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Fig. 73. The reconstructed temperature histories for GRIP and Dye 3: Top: Last 8ka B.P.; Bottom: last 2ka B.P. (Data by Dahl-Jensen et al., 1998.)

the Red has led the first settlers from Iceland to southern Greenland and between 986-1000 A.D. more and more inhabitants have arrived from Scandinavia and have gradually settled in the southwest coast of Greenland. During the Little Ice Age the settlers has deserted. "1408 - A wedding is held at Hvalsey Church". This was the last written record of Greenland's Norse population. Between 1480 and 1500 the Norse population of Greenland has disappeared (e.g. The Norse History of Greenland; www.greenland-guide.ge/leif2000/history.htm). Four most recent climatic events were warming that culminated around 1730-1740, cold episode that achieved its minimum at 1870-1875, the provisional return of warmth in 1930-1950, and cooling in the last decades. On the long scale results of the GST reconstruction by Dahl-Jensen et al. (1998) show that temperatures in Greenland generally decreased (even not monotonously) since the Climatic Optimum. Measurements of borehole temperatures also have allowed a re-calibration of the oxygen isotope-temperature relation for the GRIP ice core. Comparative study has indicated that the temperature change at the end of the last glacial period was more than 20 degrees. Thus, at the time of the last glacial maximum (25ka) the temperature in central Greenland was by two tens of degree colder that the present-day state. This result was found independently in the GISP2 borehole (Clow et al., 1996). The reconstructed climate history coincides well with the general paleoclimatic trend gained from different proxy sources available in the Arctic region, and can be used to verify previous climatic reconstructions.

The most recent course of the GST history inferred in Greenland is in good agreement with meteorological measurements in the area. Analysis of the data from eight stations located at coastal southern Greenland for the 1958-2001 period has detected even slight cooling trend in comparison to the global warming by ~0.5 K for the same period (Hanna and Cappelen, 2003). Przybylak (2000) has compared data from 37 available Arctic and seven sub-Arctic stations on both the annual and seasonal scales since 1950. Results of this study have shown that in the area under investigation the highest temperatures since the beginning of the instrumental record occurred in the 1930s and even in the 1950s the temperature was higher than in the last 10 years. Temperatures were lowest in the 1960s, while since the mid-1970s the annual mean temperature does not exhibit clear trend.

Borehole reconstructions in Greenland can also be compared with the GST histories inferred from other circumpolar boreholes. Taylor et al. (2006) presented GST inversions from three boreholes located in the northeastern Canadian High Arctic (for details see previous section). The 500-year long GST reconstructions resolved cold period that the authors have attributed to the Little Ice Age, during the mid eighteenth to the mid nineteenth century with the GSTs by ~1K below the long-term average as well as the two-three warming/cooling events in the recent 150 years. Notwithstanding that this study area is at a 1400-2100 distance from the Greenlandic borehole sites, results by Taylor et al. (2006) have demonstrated rather good coincidence with the GST reconstructions by Dahl-Jensen et al. (1998). The results show high correlation (0.9) with the GST histories inferred from both the Dye 3 and GRIP data. The reason for such high coincidence is probably the influence of the North Atlantic Oscillation22 (NAO) that significantly affects the mass balance in the area (Hanna and Cappelen, 2003).

The Greenland ice cap provides high-resolution climate history; however, the longest records of climate were obtained from the ice boreholes in Antarctica, e.g. 400 ka long Vostok ice core (Petit et al., 1999; see also Figure 1, Chapter 1). The borehole temperature-depth profiles has been measured and successfully applied for the paleoclimate reconstruction in Antarctica (e.g. Salamatin et al., 1994; Dahl-Jensen et al., 1999) using forward approaches and/or inversion based on the Monte Carlo methods. The temperature profile in the 1200m deep borehole near the summit of Law Dome (66.73°S, 112.83°E) was measured in 1996, 3 years after the cessation of the deep drilling. The 4 ka long surface temperature history has been inferred from the measured temperature log (Dahl-Jensen et al., 1999). The temperature record exhibits two well-developed minima around 1250 and 1850 A.D. Since then temperatures have gradually increased by 0.7K. Obtained past temperatures show good coincidence with the proxy climate history gained from the stable oxygen isotope ¿18O record from the same hole. Because high sensitivity of the Antarctic environment to small rises in the annual temperature and its role in the possible global change (e.g. Antarctic ice caps may begin to melt and cause global sea-level rise measured in meters), in the foreseeable future the global warming and/or climate change in this area will be likely a fairly popular scientific topic.

22The North Atlantic Oscillation (NAO) is the dominant mode of winter climate variability in the North Atlantic region stretching from central North America to Europe and much into northern Asia. It represents a large-scale flow of air between the subtropical high and the polar low. The NAO can be characterized by various types of indices, e.g. by the fluctuations of air pressure between Iceland and Azores.

Numerous projects, e.g. the 2006-2012 West Antarctic Ice Sheet Divide Ice Core Project; http://waisdivide.unh.edu, are planned to collect various interrelated climatic signals stored in the deep ice holes. Concerning borehole temperature measurements, recently the USGS has suggested a new method to reconstruct past temperature changes in polar regions. This "borehole paleothermometry" technique involves high accuracy (0.0002 K) temperature measurements in boreholes drilled into polar ice sheets (or permafrost) and their interpretation. Future collaborative projects intend ice borehole paleoclimatic studies both in central Greenland and at numerous locations in Antarctica (http://esp.cr.usgs.gov/info/glaciers). Reconstructed by this technique 4ka long past temperature trends at Taylor Dome (East Antarctica, 77.83°S, 159.0°E) have shown that this sector of Antarctica has experienced cooling by 1.2K between 4000 and 1000 years ago, in agreement with the cooling which occurred that time in the Northern hemisphere. Further climate history of the area exhibits rapid strong (2K) approximately 500-years long warming. In other words, unlike the Northern Hemisphere, this region did not experience the Little Ice Age. Next short temperature decrease has occurred around 1900. And finally temperatures in the vicinity of Taylor Dome have increased by ~0.5K during the last 100 years. In general temperatures are now warmer than they have been over the last 4000 years. Temperatures in the site are still rising today. The course of the GST history revealed for Taylor Dome by precise "borehole paleothermometry" technique generally coincides with those inferred by Dahl-Jensen et al. (1999) using traditional GST reconstruction.

As can be seen, there exist noticeable difference between climatic histories detected for the southern and northern polar regions. The reason for such discrepancy is still not clear. On the other hand, southern surface temperature reconstructions concern only a tiny area of the Antarctic; thus, they could not account for the temperature history of the whole continent. For example, detected in the ice boreholes recent warming likely occurred due to the interaction with the southern ocean and is not characteristic for the interior of the Antarctica. Comiso (2000) has analyzed Antarctic temperature data measured at 21 surface stations and from satellite infrared measurements operating since 1979 and has revealed -0.008 to -0.042 K/year temperature decrease. Still higher cooling trends (approximately -0.07K/year) were reported by Doran et al. (2002) for the period 1986-2000. Further temperature measurements in the ice boreholes together with extensive determinations of the heat flow in the area, which have been planned by the USGS, can likely help to improve our knowledge of the past regional climate patterns and present understanding of the stability of the Antarctic Ice Sheet.

There exist also temperature measurements in shallower ice holes that can contribute to better knowledge of more recent climate change. Interesting reconstructions of the historical temperature trend were performed on the basis of the temperature-depth measurements in relatively shallow (120m deep) borehole that was drilled at the ice divide on the Lomonosovfonna Plateau (one of the highest ice fields in Svalbard) in 1997 (Van de Wal et al., 2001). Radar measurements have detected a ice thickness of ~127m at this site. The ramp/step inversion of the measured T-z profile using simple linear temperature trend assumption and the 1-D heat transfer model (because borehole is situated at the ice divide, the horizontal movement was neglected), described by Eq. (42), has shown that the temperature in the area under investigation has increased with the maximum rate of 0.02-0.025 K/year over the last 100 years. The total possible warming for historical period thus appears to be 2-3 K. Estimated temperature in the nineteenth century appears to be 2.4 K lower than the 1912-1996 mean value. Detected amount of cooling is in agreement with the meteorological observations at Svalbard airport.

Numerous attempts to reconstruct surface temperatures were performed also for boreholes drilled in high-latitude alpine environments. Probably the most well known of such projects is the 1998-2001 EU-Project "Environmental and Climate records from High Elevation Alpine Glaciers" (ALPCLIM; www.geo.unizh.ch/~hoelzle/alpclim.html). The scientific objective of this effort was the exploitation of Alpine glaciers to gain climate related records through the collection and interpretation of isotope-based temperature, atmospheric trace constituents, and especially the en-glacial temperature profiles virtually unexplored in this area. In this project en-glacial temperature profiles were measured with an absolute accuracy of ±0.01-0.03 K in a 29m deep borehole at Seserjoch (4300 m asl, Monte Rosa area), in a 25 m deep borehole at the saddle point of Colle Gnifetti (4450 m asl, Monte Rosa area) and in a 40 m deep borehole on top of Dôme du Goûter (4300 m asl, Mont Blanc area). Firn- and ice-temperature observations from Colle Gnifetti were performed since the early 1980s. This 20-year long time series of en-glacial suggests a surface temperature increase of approximately 0.6 K since about 1990. Obviously, only short scale GST histories could be inferred from the shallow T-z profiles. The GST inversions of the data from the above three sites suggest a surface temperature increase in the order of 0.5-1 K for the last decade.

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