Past 1,000 years in Asia
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From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
Contents |
China
Using a variety of climate records derived from peat, lake sediment, ice core, tree-ring and other proxy sources, Yang et al. (2002) identified a period of exceptional warmth throughout China between AD 800 and 1100. Yafeng et al. (1999) also observed a warm period between AD 970 and 1510 in δ18O data obtained from the Guliya ice cap of the Qinghai-Tibet Plateau. Similarly, Hong et al. (2000) developed a 6,000-year δ18O record from plant cellulose deposited in a peat bog in the Jilin Province (42° 20’ N, 126° 22’ E), within which they found evidence of “an obvious warm period represented by the high δ18O from around AD 1100 to 1200 which may correspond to the Medieval Warm Epoch of Europe.”
Shortly thereafter, Xu et al. (2002) determined from a study of plant cellulose δ18O variations in cores retrieved from peat deposits at the northeastern edge of the Qinghai-Tibet Plateau that from AD 1100-1300 “the δ18O of Hongyuan peat cellulose increased, consistent with that of Jinchuan peat cellulose and corresponding to the ‘Medieval Warm Period’.” In addition, Qian and Zhu (2002) analyzed the thickness of laminae in a stalagmite found in Shihua Cave, Beijing, from whence they inferred the existence of a relatively wet period running from approximately AD 940 to 1200.
Hong et al. (2000) also report that at the time of the MWP “the northern boundary of the cultivation of citrus tree (Citrus reticulata Blanco) and Boehmeria nivea (a perennial herb), both subtropical and thermophilous plants, moved gradually into the northern part of China, and it has been estimated that the annual mean temperature was 0.9-1.0°C higher than at present.” Considering the climatic conditions required to successfully grow these plants, they further note that annual mean temperatures in that part of the country during the Medieval Warm Period must have been about 1.0°C higher than at present, with extreme January minimum temperatures fully 3.5°C warmer than they are today, citing De’er (1994).
Chu et al. (2002) studied the geochemistry of 1,400 years of dated sediments recovered from seven cores taken from three locations in Lake Huguangyan (21°9’N, 110°17’E) on the low-lying Leizhou Peninsula in the tropical region of South China, together with information about the presence of snow, sleet, frost, and frozen rivers over the past 1,000 years obtained from historical documents. They report that “recent publications based on the phenological phenomena, distribution patterns of subtropical plants and cold events (Wang and Gong, 2000; Man, 1998; Wu and Dang, 1998; Zhang, 1994) argue for a warm period from the beginning of the tenth century AD to the late thirteenth century AD,” as their own data also suggest. In addition, they note there was a major dry period from AD 880-1260, and that “local historical chronicles support these data, suggesting that the climate of tropical South China was dry during the ‘Mediaeval Warm Period’.”
Paulsen et al. (2003) used high-resolution δ13C and δ18O data derived from a stalagmite found in Buddha Cave [33°40’N, 109°05’E] to infer changes in climate in central China for the past 1,270 years. Among the climatic episodes evident in their data were “those corresponding to the Medieval Warm Period, Little Ice Age and 20th-century warming, lending support to the global extent of these events.” In terms of timing, the dry-then-wet-then-dry-again MWP began about AD 965 and continued to approximately AD 1475.
Also working with a stalagmite, this one from Jingdong Cave about 90 km northeast of Beijing, Ma et al. (2003) assessed the climatic history of the past 3,000 years at 100-year intervals on the basis of δ18O data, the Mg/Sr ratio, and the solid-liquid distribution coefficient of Mg. They found that between 200 and 500 years ago, “air temperature was about 1.2°C lower than that of the present,” but that between 1,000 and 1,300 years ago, there was an equally aberrant but warm period that “corresponded to the Medieval Warm Period in Europe.”
Based on 200 sets of phenological and meteorological records extracted from a number of historical sources, many of which are described by Gong and Chen (1980), Man (1990, 2004), Sheng (1990), and Wen and Wen (1996), Ge et al. (2003) produced a 2,000-year history of winter half-year temperature (October to April, when CO2-induced global warming is projected to be most evident) for the region of China bounded by latitudes 27° and 40°N and longitudes 107° and 120°E. Their work revealed a significant warm epoch that lasted from the AD 570s to the 1310s, the peak warmth of which was “about 0.3-0.6°C higher than present for 30-year periods, but over 0.9°C warmer on a 10-year basis.”
Bao et al. (2003) utilized proxy climate records (ice-core δ18O, peat-cellulose δ18O, tree-ring widths, tree-ring stable carbon isotopes, total organic carbon, lake water temperatures, glacier fluctuations, ice-core CH4, magnetic parameters, pollen assemblages, and sedimentary pigments) obtained from 20 prior studies to derive a 2,000-year temperature history of the northeastern, southern and western sections of the Tibetan Plateau. In each case, there was more than one prior 50-year period of time when the mean temperature of each region was warmer than it was over the most recent 50-year period. In the case of the northeastern sector of the plateau, all of the maximum-warmth intervals occurred during the Medieval Warm Period; in the case of the western sector, they occurred near the end of the Roman Warm Period, and in the case of the southern sector they occurred during both warm periods.
Damage from extreme low temperatures during the warm season in Northeast China is one of the major disasters that affect agricultural activity. Fengjin and Lianchun (2011) computed temporal trends in the frequency of occurrence of extreme minimum temperatures during Northeast China's warm season (May to September) over the period 1956-2005, while concurrently calculating trends in the region's annual average near-surface air temperature. The two Beijing Climate Center researchers report that for the entire 1956-2005 period, the overall rate of increase in the annual average temperature was 0.32°C per decade, but that "from 1990, the increasing trend in the annual average temperature has become much more significant." Somewhat similarly, they found that the average number of extreme minimum temperature days increased from the 1950s to the 1980s, but has since decreased. With fewer extreme minimum temperature events occurring in response to the warmer temperatures of the past few decades, farmers in Northeast China have been able to harvest greater amounts of rice, sorghum, corn, soybeans, and other major crops.
From these several studies, it is evident that for a considerable amount of time during the Medieval Warm Period, many parts of China exhibited warmer conditions than those of modern times. Since those earlier high temperatures were caused by something other than high atmospheric CO2 concentrations, whatever was responsible for them could be responsible for the warmth of today.
Russia
Demezhko and Shchapov (2001) studied a borehole extending to more than 5 km depth, reconstructing an 80,000-year history of ground surface temperature in the Middle Urals within the western rim of the Tagil subsidence (58°24’ N, 59°44’E). The reconstructed temperature history revealed the existence of a number of climatic excursions, including, in their words, the “Medieval Warm Period with a culmination about 1000 years ago.”
Further north, Hiller et al. (2001) analyzed subfossil wood samples from the Khibiny mountains on the Kola Peninsula of Russia (67-68°N, 33-34°E) in an effort to reconstruct the region’s climate history over the past 1,500 years. They determined that between AD 1000 and 1300 the tree-line was located at least 100-140 m above its current elevation. This observation, in their words, suggests that mean summer temperatures during this “Medieval climatic optimum” were “at least 0.8°C higher than today,” and that “the Medieval optimum was the most pronounced warm climate phase on the Kola Peninsula during the last 1500 years.”
Additional evidence for the Medieval Warm Period in Russia comes from Naurzbaev and Vaganov (2000), who developed a 2,200-year proxy temperature record (212 BC to 1996 AD) using tree-ring data obtained from 118 trees near the upper timberline in Siberia. Based on their results, they concluded that the warming experienced in the twentieth century was “not extraordinary,” and that “the warming at the border of the first and second millennia was longer in time and similar in amplitude.”
Krenke and Chernavskaya (2002) present an impressive overview of what is known about the MWP within Russia, as well as throughout the world, based on historical evidence, glaciological evidence, hydrologic evidence, dendrological data, archaeological data, and palynological data. Concentrating on data wholly from within Russia, they report large differences in a number of variables between the Little Ice Age (LIA) and MWP. With respect to the annual mean temperature of northern Eurasia, they report an MWP to LIA drop on the order of 1.5°C. They also say that “the frequency of severe winters reported was increased from once in 33 years in the early period of time, which corresponds to the MWP, to once in 20 years in the LIA,” additionally noting that “the abnormally severe winters [of the LIA] were associated with the spread of Arctic air masses over the entire Russian Plain.” Finally, they note that the data they used to draw these conclusions were “not used in the reconstructions performed by Mann et al.,” which perhaps explains why the Mann et al. temperature history of the past millennium does not depict the coolness of the LIA or the warmth of the MWP nearly as well as the more appropriately derived temperature history of Esper et al. (2002).
In discussing their approach to the subject of global warming detection and attribution, the Russians state that “an analysis of climate variations over 1000 years should help … reveal natural multicentennial variations possible at present but not detectable in available 100-200-year series of instrumental records.” In this endeavor, they were highly successful, stating unequivocally that “the Medieval Warm Period and the Little Ice Age existed globally.”
Other Asia Locations
In addition to China and Russia, the Medieval Warm Period (MWP) has been identified in several other parts of Asia.
Schilman et al. (2001) analyzed foraminiferal oxygen and carbon isotopes, together with the physical and geochemical properties of sediments, contained in two cores extracted from the bed of the southeastern Mediterranean Sea off the coast of Israel, where they found evidence for the MWP centered on AD 1200. In discussing their findings, they note there is an abundance of other evidence for the existence of the MWP in the Eastern Mediterranean as well, including, in their words, “high Saharan lake levels (Schoell, 1978; Nicholson, 1980), high Dead Sea levels (Issar et al., 1989, 1991; Issar, 1990, 1998; Issar and Makover-Levin, 1996), and high levels of the Sea of Galilee (Frumkin et al., 1991; Issar and Makover-Levin, 1996),” in addition to “a precipitation maximum at the Nile headwaters (Bell and Menzel, 1972; Hassan, 1981; Ambrose and DeNiro, 1989) and in the northeastern Arabian Sea (von Rad et al., 1999).”
Further to the east, Kar et al. (2002) explored the nature of climate change preserved in the sediment profile of an outwash plain two to three km from the snout of the Gangotri Glacier in the Uttarkashi district of Uttranchal, Western Himalaya. Between 2,000 and 1,700 years ago, their data reveal the existence of a relatively cool climate. Then, from 1,700 to 850 years ago, there was what they call an “amelioration of climate,” during the transition from the depth of the Dark Ages Cold Period to the midst of the Medieval Warm Period. Subsequent to that time, Kar et al.’s data indicate the climate “became much cooler,” indicative of its transition to Little Ice Age conditions, while during the last 200 years there has been a rather steady warming, as shown by Esper et al. (2002a) to have been characteristic of the entire Northern Hemisphere.
At a pair of other Asian locations, Esper et al. (2002b) used more than 200,000 ring-width measurements obtained from 384 trees at 20 individual sites ranging from the lower to upper timberline in the Northwest Karakorum of Pakistan (35-37°N, 74-76°E) and the Southern Tien Shan of Kirghizia (40°10’N, 72°35’E) to reconstruct regional patterns of climatic variations in Western Central Asia since AD 618. According to their analysis, the Medieval Warm Period was already firmly established and growing even warmer by the early seventh century; and between AD 900 and 1000, tree growth was exceptionally rapid, at rates they say “cannot be observed during any other period of the last millennium.”
Between AD 1000 and 1200, however, growing conditions deteriorated; and at about 1500, minimum tree ring-widths were reached that persisted well into the seventeenth century. Towards the end of the twentieth century, ring-widths increased once again; but Esper et al. (2002b) report that “the twentieth-century trend does not approach the AD 1000 maximum.” In fact, there is almost no comparison between the two periods, with the Medieval Warm Period being much more conducive to good tree growth than the Current Warm Period. As the authors describe the situation, “growing conditions in the twentieth century exceed the long-term average, but the amplitude of this trend is not comparable to the conditions around AD 1000.”
The latest contribution to Asian temperature reconstruction is the study of Esper et al. (2003), who processed several extremely long juniper ring-width chronologies for the Alai Range of the western Tien Shan in Kirghizia in such a way as to preserve multi-centennial growth trends that are typically “lost during the processes of tree ring data standardization and chronology building (Cook and Kairiukstis, 1990; Fritts, 1976).” In doing so, they used two techniques that maintain low frequency signals: long-term mean standardization (LTM) and regional curve standardization (RCS), as well as the more conventional spline standardization (SPL) technique that obscures (actually removes) long-term trends.
Carried back in time a full thousand years, the SPL chronologies depict significant inter-decadal variations but no longer-term trends. The LTM and RCS chronologies, on the other hand, show long-term decreasing trends from the start of the record until about AD 1600, broad minima from 1600 to 1800, and long-term increasing trends from about 1800 to the present. As a result, in the words of Esper et al. (2003), “the main feature of the LTM and RCS Alai Range chronologies is a multi-centennial wave with high values towards both ends.”
This grand result has essentially the same form as the Northern Hemisphere extratropic temperature history of Esper et al. (2002a), which is vastly different from the hockey stick temperature history of Mann et al. (1998, 1999) and Mann and Jones (2003), in that it depicts the existence of both the Little Ice Age and preceding Medieval Warm Period, which are nowhere to be found in the Mann reconstructions. In addition, the new result—especially the LTM chronology, which has a much smaller variance than the RCS chronology—depicts several periods in the first half of the last millennium that were warmer than any part of the last century. These periods include much of the latter half of the Medieval Warm Period and a good part of the first half of the fifteenth century, which has also been found to have been warmer than it is currently by McIntyre and McKitrick (2003) and by Loehle (2004).
In commenting on their important findings, Esper et al. (2003) remark that “if the tree ring reconstruction had been developed using ‘standard’ detrending procedures only, it would have been limited to inter-decadal scale variation and would have missed some of the common low frequency signal.” We would also remark, with respect to the upward trend of their data since 1800, that a good portion of that trend may have been due to the aerial fertilization effect of the concomitantly increasing atmospheric CO2 content, which is known to greatly stimulate the growth of trees. Properly accounting for this very real effect would make the warmer-than-present temperatures of the first half of the past millennium even warmer, relative to those of the past century, than they appear to be in Esper et al.’s LTM and RCS reconstructions.
Treydte et al. (2009) "present a millennium-long (AD 828-1998), annually resolved δ13C tree-ring chronology from high-elevation juniper trees in northern Pakistan together with three centennial-long (AD 1900-1998) δ13C chronologies from ecologically varying sites," in the process of which they "define an 'optimum' correction factor that is best suited to remove non-climatic trends from [their] high-elevation trees in the Karakorum," in order to "provide new regional temperature reconstructions derived from tree-ring δ13C, and compare those records with existing regional evidence." The end result of the scientists' analysis was that the 1990s were substantially below Medieval Warm Period temperatures; their reconstruction provides additional suggestions that High Asian temperatures during the Medieval Warm Period might have exceeded recent conditions. This suggests that the magnitude and rate of 20th-century warming likely did not exceed the natural climate variability of the last millennium.
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Related Links
Past 1,000 years in North America
Past 1,000 years in South America
