Fig. 96. Temperature logs measured at Kamchatka.
Borehole climate reconstructions in Asia are sparser in comparison with an extensive area of this continent. Near 100 GST reconstructions from the temperature-depth profiles measured in China are compiled in the database by the NOAA Satellite and Information Service and the National Climatic Data Center (www.ncdc.noaa.gov/paleo/ borehole/asi.html). Recent reconstructions in Korea are of special interest because despite intensive borehole logging that continues at present, until recently this country has been a virgin area for such kind of climate reconstructions. Successful attempts of joint study of the climate and groundwater effect on the shallow temperatures were made in Japan (Taniguchi et al., 1999; Uchida and Sakura, 1999). Even when these studies were primarily hydrogeologically motivated, they have recognized the effects of the global warming and the urbanization in the Tokyo metropolitan area on the subsurface T-z profiles. Goto et al. (2002) have inferred approximately 5000-year long GST history from repeated temperature logs measured in 1993 and 2002 in approximately 800 m deep Karasuma borehole, SW Japan. Reconstructed GST histories were highly coherent and represented slow steady warming from 3000 B.C. that culminated around 800 A.D. (Medieval Warm Period). Climatic excursions of the last two millennia were generally similar all over the world and contained the Little Ice Age with minimum temperature between 1400 and 1500 A.D., warm conditions from the middle to late nineteenth century, and approximately 1 K cooling since then. Neither the GST history inferred from 1993 temperature log nor the data measured in 2002 show any signature of the recent warming. Authors have concluded that detected temperature excursions can be partly attributed to the past environmental changes such as temporal expansions of the neighboring Lake Biwa. Recently, Okubo et al. (2003) have performed screening of the 50 borehole temperature logs (boreholes often show signatures of the groundwater movement) and obtained the first GST reconstructions using carefully selected T-z profiles.
More than 100 GST reconstructions were performed for the borehole temperature logs in India (Sukanta, 2003). These data provide distinct evidence of the GST warming of —0.7 ± 0.1K in India over the past two centuries. A joint analysis of borehole temperature logs and SAT records revealed POM of 0.5 K lower than the 1961-1990 mean SAT, which supported results of the GST reconstructions.
The summary of the past one to two millennia climate excursions revealed by the "geothermal" reconstructions can be generalized as follows. Temperatures were relatively high in the Northern Hemisphere as a whole during the earlier centuries of the millennium. A major difference of this "Medieval Warm Period" from the near global
warming of the late twentieth century is that it was not globally synchronous. Significant warming seems to have occurred in Europe, especially in the regions surrounding the North Atlantic. The Medieval Warm Period was changed by the hemispheric and/or global secular cooling since about 1300-1400 A.D. This cold period known as the "Little Ice Age" has persisted through the Middle Ages up to the nineteenth century. Both spatial and temporal patterns of the Little Ice Age were somewhat erratic including provisional returns of warmth. The peaks of cooling in different regions occurred in substantially different periods of time. In contrast, the twentieth century warming shows a much more homogeneous global appearance, especially in the Northern Hemisphere. Because significantly less data are available for the Southern Hemisphere, both long-term GST excursions as well as the conditions prevailing in the last millennium are less known for this half of the globe.
3.2 Temperature Trends Over Past Five Centuries Reconstructed From Borehole Temperature Data (Spatial and Temporal Patterns)
The previous section described collective attempts of the small spatial-scale GST reconstructions, generally in the separate locations or maximum at the country scale. No doubt, the reconstruction of the GST history in a single site and/or in limited area represents an achievement. These studies are crucial for assessing the spatial patterns of climate variability and for evaluation of linkages/differences between regions. The systematizing and synthesis of the results from previous reconstructions to obtain large-scale climatic trends is even more difficult and has only recently been achieved after considerable efforts. This section describes this kind of GST reconstructions.
As demonstrated in the previous chapter, the resolving power of the geothermal method for paleoclimate reconstruction is relatively low. Further going back into the past, less details can be resolved and only a smoothed trend of real temperature conditions can be gained. Thus, the most promising application of the GST reconstruction method is the last approximately 500 years. The surface temperature signal of this time interval is stored in the uppermost 200-300 m of the temperature-depth profiles. This depth is reached by most of the existing boreholes. The vast amount of the borehole temperature reconstructions of this kind inspired the researchers to accumulate obtained data and generalize them for the large spatial-scale climatic trends. There are numerical works trying to identify major patterns of the climate variability for the last 500 years on hemispheric to global scales. Because individual GST reconstructions exhibit site-to-site variability arising from different local influences (see Chapter 2, and "spaghetti diagrams" in the previous section) that can obscure the real climatic signal, the signal strengthening can be credibly achieved after averaging of a large number of individual results and their statistical analysis. On the other hand, as shown in the work by Beltrami and Burlon (2004), under the restrictions that are essential to obtain robust spatial averages on hemispheric or global scale, the merging of individual GST reconstruction results cannot retrieve reliable information on the climate variations at times before 1500. Information about more remote climate changes can be obtained only from individual borehole inversions.
The earliest cumulative analysis, performed by Pollack et al. (1998), used the underground temperature measurements from 358 boreholes in eastern North America (116), central Europe (98), southern Africa (86), and Australia (58). Data were extracted from published works and/or from the database of borehole temperatures. Authors have applied the quality control of the data used. Investigated boreholes were distributed over approximately 40-60°N and 10-40°S longitudes and have represented well both the Northern and Southern Hemispheres. Temperatures at most of the boreholes selected for the analysis were measured to 200-600 m depth, and thus surely included the GST changes of the last five centuries. Authors have examined only century-long trends. Obtained results have shown that approximately 80% of the 358 individual GST reconstructions exhibited a net warming over the past five centuries (Figure 99). The fact that near 20% of the investigated sites indicate a net cooling over the past 500 years reflects regional climate variability. Similar inconsistency can also be found in the meteorological SAT records. On the other hand, 80% warming against 20% cooling hints the universal global nature of the recent warming.
Calculated composite temperature change (relative to the present) for the last 500 years is shown in Figure 100 (most recent updated version of this diagram can be found on the web site of the University of Michigan, USA; www.geo.lsa.umich.edu/climate/core.html). This diagram shows a global trend of GST change over the last five centuries, averaged from 358 individual reconstructions. Thick line is the mean surface temperature since 1500 to the present date. The shading represents ±1 standard error of the mean. The evaluated total temperature increase over this time interval equals ~ 1K. Results also indicate that during the twentieth century alone the surface temperature of the Earth has increased by about 0.5K; thus, the characteristic velocity of warming for the last century was well above the temperature rise in the previous centuries. Authors affirm that
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