I1iiiil

Fig. 99. Histogram of cumulative temperature change since 1500 A.D. (Drawn from the data by Pollack et al. (1998).)

CUMULATIVE TEMPERATURE CHANGE SINCE 1500 A.D., K

Fig. 99. Histogram of cumulative temperature change since 1500 A.D. (Drawn from the data by Pollack et al. (1998).)

Fig. 100. Composite temperature change since 1500 A.D. relative to the present conditions, determined from borehole GST reconstructions. Shaded boundaries represent ± 1 standard deviation about the mean climatic trend. The 5-year running average of SAT temperature anomalies from the same regions as borehole data is superimposed of the GST trend. (Redrawn from Pollack et al. (1998).)

Fig. 100. Composite temperature change since 1500 A.D. relative to the present conditions, determined from borehole GST reconstructions. Shaded boundaries represent ± 1 standard deviation about the mean climatic trend. The 5-year running average of SAT temperature anomalies from the same regions as borehole data is superimposed of the GST trend. (Redrawn from Pollack et al. (1998).)

the twentieth century has been the warmest of the past five centuries (Pollack et al., 1998). For comparison, SAT anomaly series, averaged using meteorological records from the same regions, is also presented in the figure. The SAT data are shown as anomalies to the 1961-1990 base period, while the GST composite surface temperature anomalies were calculated relative to the present-day level. For this reason the authors have moved the SAT series downward by 0.2K to ensure better by-eye comparison. As shown in the figure, both records exhibit similar trends. This fact confirms that the borehole temperature reconstructions based on different data and methodology can provide similarly significant results as the meteorological measurements and thus offer an independent verification of the unusual character of the twentieth century climate that has also emerged from the multiproxy studies (Pollack and Huang, 1998).

The project "Global Database of Borehole and Climate Reconstruction" was initiated by the Geothermal Laboratory of the University of Michigan (USA) and suggested wide collaboration of the geothermal community (www.geo.lsa.umich.edu/~climate). The goal of this project was the design, collection, and analysis of the geothermal measurements on continents relevant for understanding the nature and causes of climate change for the past five centuries. Proposed main directions of research work were as follows:

1. Performing/collecting the basic geothermal observations including both field and laboratory measurements. Data of at least 200 m deep holes and the measurements embracing depth interval of 20-600 m were included into the database. Because temperatures from the deeper sections of the temperature logs do not contain information on the last five centuries, they were excluded from the consideration. Quality control was performed to find probable inconsistency in the measured data within 20-600m range.

2. Reconstructing the five-century long GST histories for each site using standardized inversion procedure. Inferred GST histories were then presented as the century-long trends for each of the investigated past five centuries. Such representation emphasizes long-term variations of the GST history. The collection of the century-long GST trends contains valuable information on the past climate changes. It also represents an independent data source that is complementary to high-resolution proxies such as tree rings, ice cores, etc.

The first data arrived in 1998. Present-day database contains temperature logs and GST reconstructions of approximately 1000 boreholes (database is continuously growing) contributed by the scientists from countries all over the world. Data represent all continents except Antarctica. The updated version of the map showing locations of the boreholes that have been analyzed to date can be found on the web site of the project. Available data cover well the Eurasia, North America, South Africa, and Australia, while in South America and North Africa (above 0° latitude) there is only 20 and 1 GST reconstruction, respectively. The database develops continuously; thus, at times the attempts are undertaken to revise the earlier results by Pollack et al. (1998) using wider massif of the borehole data.

Huang et al. (2000) have assembled vast database of the 616 borehole GST histories (453 are in the Northern Hemisphere and 163 in the Southern Hemisphere) and reconstructed 500-year long average climatic trends on the hemispheric as well as on the global scales. Obtained results have corroborated findings of the above-cited work of the same authors (Pollack et al., 1998). The 78% of the investigated borehole temperature logs revealed warming over the past five centuries. This analysis also supported previous conclusions about its value and rate in the 500-year long interval as well as in the twentieth century alone, which were derived on the basis of the smaller amount of 358 boreholes. The authors have proved that the general warming of about 1.0 K is a common climatic tendency of the last five centuries and the twentieth century change is the largest of all the previous four centuries. Almost 0.5 K of the observed temperature increase occurred in recent 100 years, which was the warmest time of the past five centuries. The standard deviation of the derived mean values is lower in the latter analysis using increased number of borehole sites and does not exceed ±0.1K instead of ±0.25 to 0.3 K for the previous calculations by Pollack et al. (1998). Thus, this cumulative average of GST history bears more significance. Similarly remarkable is the coincidence of the obtained average GST histories with the trends in the SAT records. The most recent (continuously renewed) global averages of the GST history over the past five centuries are presented on the web site www.geo.lsa.umich.edu/~climate.

The hemispheric reconstructions performed by Huang et al. (2000) on the same database revealed that the 500-year long GST warming is somewhat higher in the Northern Hemisphere (1.1 K/500years) than in the Southern Hemisphere (0.8 K/500years). Similarly significant is the twentieth century change, namely 0.6 K in the Northern Hemisphere compared with 0.4K in the Southern Hemisphere. Detected by the authors, "geothermal" trends are highly consistent with 0.56 and 0.47 K/100 year the Northern and Southern hemispheric trends, respectively, calculated from the land SAT data only (Jones et al., 1998, 1999). Regional reconstructions by Huang et al. (2000) also revealed tentative spatial and temporal variability of the GST warming on the continental timescale (Table 8). Results presented in this table show that all continents exhibit the same features as the global average trend, namely the largest warming in the twentieth century than in any of the previous centuries. More rapid warming has occurred in America and Asia; the lowest warming rate is characteristic for Australia. This tendency is also preserved in the last 100 years. It should be mentioned, however, that because of the relative sparse geothermal observations and poor geographical coverage, above conclusions seem to be only tentative, especially for the Southern Hemisphere.

Recently, Mann et al. (2003) and Rutherford and Mann (2004) have re-processed the same 453 Northern Hemisphere borehole temperature reconstructions as were used in the

Table 8. Continental five- and one-century long cumulative GST changes

Continent

Last 500 years

Last 100 years

Europe

0.8a

0.4b

Asia

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