Temperature histories of specific locations and broad regions are regularly reconstructed from temperature-depth profiles obtained from "boreholes" drilled in soil and ice. In what follows, we briefly summarize what has been learned from the application of this technique and how the knowledge obtained from it compares with climate-alarmist claims about CO2-induced global warming, starting in 1997 with an impressive landmark study and moving forward in time year by year.
Huang and Pollack (1997) searched the large ground heat flow database of the International Heat Flow Commission of the International Association of Seismology and Physics of the Earth's Interior for measurements suitable for reconstructing an average ground surface temperature history of the earth over the last 20,000 years. Based on a total of 6,144 qualifying sets of measurements obtained from every continent, they produced a global climate reconstruction that they say is "independent of other proxy interpretations [and] of any preconceptions or biases as to the nature of the actual climate history." This effort revealed the existence of a global Medieval Warm Period that was as much as 0.5°C warmer than it was in the late 20th century, as well as a global Little Ice Age that was as much as 0.7°C cooler than it was in the early 1990s. Consequently, and contrary to the climate-alarmist claim that the Medieval Warm Period and Little Ice Age were neither real nor global, this study tells a very different story, which because of the massive data base upon which it rests must be the correct one.
Pollack et al. (1998) reconstructed a surface temperature history for the past five centuries from 358 boreholes spread throughout eastern North America, central Europe, southern Africa and Australia. Nearly 80% of these locations experienced a net warming over this period; but 20% of them experienced a net cooling. Consequently, the mean warming of this large area over the past 500 years was about 1°C, most of which occurred well in advance of the lion's share of the past century's anthropogenic CO2 emissions, suggesting that most of the 1°C of reconstructed warming was non-CO2-induced.
Dahl-Jensen et al. (1998) used data from two deep boreholes in the Greenland Ice Sheet to reconstruct the ice sheet's temperature history over the past 50,000 years. This exercise revealed that temperatures there were 23°C colder than at present some 25,000 years ago during the Last Glacial Maximum, but that they warmed to 2.5°C above current levels during the Holocene Climatic Optimum of 4,000 to 7,000 years ago. The Medieval Warm Period was also warmer than it is now, by about 1°C, while the Little Ice Age was 0.5 to 0.7°C cooler. After that time, the temperature of the Greenland Ice Sheet rose to another peak around 1930, whereupon it decreased to the end of the record. In contrast to the climate-alarmist claim that the latter part of the 20th century was the warmest the planet has been in the past two millennia, it is thus clear that for Greenland this claim is doubly false, as it was warmer there during the Medieval Warm Period (when there was 100 ppm less CO2 in the air than there is today), just as it was also warmer there a mere 75 years ago (when there was about 80 ppm less CO2 in the air than there is currently).
In a Canadian study of borehole temperature data obtained at ten southern Saskatchewan sites, Majorowicz et al. (1999) found that from 1820 to the present, surface temperatures there rose between 2.5 and 3.0°C. The primary significance of this finding, according to them, is that the borehole record suggests that "almost half of the warming occurred prior to 1900, before the dramatic buildup of atmospheric greenhouse gases." In fact, it has been noted by Idso (1982) that this buildup did not begin in earnest until the mid-1940s, which fact makes Majorowicz et al.'s contra-climate-alarmist point even more impressive.
Correia and Safanda (1999) reconstructed a five-century temperature history of a region some 150-200 km southeast of Lisbon, Portugal, by analyzing seven sets of borehole temperature data obtained there. Each of the seven borehole logs implied little temperature change over the first three centuries of record. Thereafter, however, four of them exhibited warming trends that began about 1800 and peaked around 1940, one showed a warming that peaked in the mid-1800s, another was constant across the entire five centuries, and one actually revealed cooling over the last century. Interestingly, none of these responses resemble the Mann et al. (1998, 1999) "hockeystick" temperature history promoted by the world's climate alarmists, where major warming does not begin until 1910, and where the warmest temperatures of the record occur at its end.
Bodri and Cermak (1999) derived individual surface temperature histories from the temperature-depth logs of 98 separate boreholes drilled in the Czech Republic. In describing their findings, they say "the existence of a medieval warm epoch lasting from AD 1100-1300 is clear," which epoch they describe as "one of the warmest postglacial times." They also note that during the main phase of the Little Ice Age (AD 1600-1700), "all investigated territory was already subjected to massive cooling." Hence, they concluded that "the observed recent warming may thus be easily a natural return of climate from the previous colder conditions back to a 'normal'," which again is something that is anathema to climate-alarmist dogma.
Huang et al. (2000) employed data from 616 boreholes obtained from all continents except Antarctica to reconstruct a global temperature history of the past five centuries. This undertaking revealed that the mean surface air temperature of the globe likely rose by approximately 1°C over the past 500 years, with about half of the warming coming in the last century. Once again, this analysis lets CO2 off the hook with respect to its being the cause of most of the indicated warming, since the atmosphere's CO2 concentration did not begin to rise in earnest until the mid-1940s, as noted earlier.
Similarly, Harris and Chapman (2001) analyzed 439 borehole temperature logs in an effort to determine the temperature history of the mid-latitude sector (30-60°N) of the Northern Hemisphere. As could be expected, the data revealed a vast array of results for the many different sites, with some even depicting cooling over the past two centuries. In the mean, however, their analysis indicated 0.7 ± 0.1°C of warming between pre-industrial times and the interval 1961-1990, which is pretty much in line with the findings of Huang et al. (2000).
Whereas most boreholes do not exceed a depth of 1 km, which limits the temporal duration of surface temperature reconstructions by this method to only the past few centuries, Demezhko and Shchapov (2001) studied a borehole extending to more than 5 km depth, which allowed them to reconstruct an 80,000-year history of ground surface temperature. This borehole was drilled in the Middle Ural Mountains within the western rim of the Tagil subsidence; and it revealed a number of climatic excursions, among which were the "Holocene Optimum 4000-6000 years ago, Medieval Warm Period with a culmination about 1000 years ago and Little Ice Age 200-500 years ago." Furthermore, the mean temperature of the Medieval Warm Period was determined to have been more elevated above the mean temperature of the past century than the mean temperature of the Little Ice Age was reduced below it. Once again, therefore, we have evidence for the reality of the Medieval Warm Period, as well as another example of its dominance over the past century in terms of its greater sustained warmth, which flies in the face of the contrary claims of climate alarmists, who strive desperately to make the planet's current warmth appear "unprecedented" over the past millennium or more.
Moving on, Romanovsky et al. (2002) describe "an emerging system for comprehensive monitoring of permafrost temperatures," which they say is needed for "detection and tracking of climatic changes" and "verification of GCM outputs." In doing so, they also present a 1924-2001 history of mean annual temperatures for Barrow, Alaska (USA) at soil depths of 0.08 m (the "active layer"), 0.5 m, and 1.0 m (about 60 cm below the permafrost table). These data reveal that permafrost temperatures were "very similar during the 1940s and 1990s (except for unprecedented warm extremes of 1998 and 1999)." However, even including these "unprecedented warm extremes," we calculate from one of their figures that the mean temperature about 60 cm into the permafrost (-9.15°C) - over what climate alarmists call the warmest period of the past millennium, i.e., 1990 and onward - was no warmer than, or possibly even cooler than, the temperature of the 16-year period 1937-1952 (-9.06°C). Hence, in spite of all the hype about recent dramatic warming in the permafrost regions of Alaska, real-world data demonstrate - at least for Barrow - that it is likely no warmer there now than it was half a century ago, and that the area's permafrost is in no more danger of being wiped out today that it was in the days of our grandparents. Furthermore, Romanovsky et al. note that degradation of permafrost does not proceed as rapidly as many climate alarmists would have one believe. As they describe it, "degradation of permafrost is a slow process," and "if recent trends continue, it will take several centuries to millennia [our italics] for permafrost in the present discontinuous zone to disappear completely in the areas where it is actively warming and thawing."
Gonzalez-Rouco et al. (2003) report that "borehole temperature profiles (Huang et al., 2000) are perhaps the only direct measurements of past temperatures, in contrast to the analysis of other climate proxies that have to be interpreted in terms of climate anomalies through transfer functions." However, they say "the validity of the interpretation of borehole temperature profiles has been questioned (Mann and Schmidt, 2003)." Consequently, they investigated the relationship between simulated surface air temperature (SAT) and terrestrial deep soil temperature (TDST) in a climate simulation of the last millennium with the state-of-the-art coupled climate model (ECHO-G) described by Legutke and Voss (1999), which was driven by historical forcing provided by "solar variability, atmospheric greenhouse gas concentrations, and radiative effects of stratospheric volcanic aerosols, in the period AD 1000-1990, derived from the estimations provided by Crowley (2000)."
This work revealed, in the words of the researchers, that "at long timescales, annual TDST is a good proxy for annual SAT, and their variations are almost indistinguishable from each other," in a significant rebuff to the challenge of Mann and Schmidt (2003). And in contradiction of the many Mann-inspired claims that late 20th-century warming produced temperatures that are unprecedented over the past millennium or more, Gonzalez-Rouco et al. report that "the simulated annual global SAT shows a period of temperatures roughly as warm as today around AD 1100 (the Medieval Optimum), a subsequent cooling trend until around AD 1850 (the Little Ice Age) punctuated by deeper temperature minima at around AD 1450, AD 1700, and AD 1820, coincidental with known minima of the solar output or periods of more frequent volcanic eruptions (the Spoerer, Maunder and Dalton minima, respectively)." They also note that "recent reconstructions on extratropical tree-ring chronologies indicate a better agreement with the borehole based reconstructions." As more and more data are collected and carefully analyzed, therefore, it is becoming ever more evident there was nothing unusual about the planet's thermal behavior of the past quarter-century, and that current temperatures do not materially exceed those of the Medieval Warm Period. If the warming of the past hundred or so years seemed unusually steep, it was simply because the Little Ice Age was anomalously cold, due largely to variations in solar activity.
Also noting that Mann and Schmidt (2003) and Mann et al. (2003) have questioned whether processes operating at the earth's surface bias the geothermal (borehole) method of reconstructing long-term surface temperature histories and thereby render them "unsuitable for climatic reconstruction," Beltrami et al. (2005) analyzed high-quality borehole temperature data retrieved from four boreholes in quasi-steady state located within a small region in northern Quebec, to ascertain whether records of surface air temperature collected nearby reproduce the subsurface temperature anomalies observed in the area. This exercise revealed, in their words, that "subsurface temperatures are consistent with nearly 70 years of surface air temperature data even though snow cover effects and soil moisture phase changes are present." As a result, they conclude that (1) "the ground tracks the variations of the surface air temperature," (2) "borehole data can be considered as a robust and independent indicator of past climatic conditions," and (3) "borehole records are robust long-term paleoclimatological indicators," further rebuffing Mann and his collaborators.
Last of all, in a study designed to deal with the presence of underground fluids that can distort the climatic signal contained in borehole temperature data, as described by (Lewis and Wang, 1992), Bodri and Cermak (2005) developed a corrective measure that accounts for vertical conductive and advective heat transport in a 1-D horizontally-layered stratum, as opposed to the purely conductive approach, which they then applied to data derived from four boreholes drilled near Tachlovice in the Czech Republic. This work revealed that the conductive/advective approach was far superior to the purely conductive approach, explaining 83-95% of the temperature signal in situations where the purely conductive model could explain no more than 27-58% of it. In addition, the purely conductive approach was found to significantly underestimate the pre-observational mean temperature by 0.3 to 0.5°C, which underestimate produced a significant overestimate of the degree of warming experienced from the pre-observational period to the present. What is more, the two scientists report that both pre-observational mean temperature values for 18th-century Bohemia (the one derived from the conductive/advective approach and the one derived from the purely conductive approach) "exceed the annual temperatures characteristic for the 19th/20th centuries" and "may indicate that the warming has still not achieved its earlier (late 18th century) level." Hence, they say that 20th-century warming may be merely "a recovery to previous warmer conditions after a noted cold period."
In light of these many eye-opening results of studies of borehole temperature data - which Broecker (2001) describes as one of "only two proxies" (the other being mountain snowlines) that for time scales greater than a century or two "can yield temperatures that are accurate to 0.5°C" - it is obvious that climate-alarmist claims of unprecedented CO2-induced global warming over the past few decades are wholly without significant real-world empirical support. In fact, they are refuted by it.
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