The Medieval Warm Period (MWP) was a global climatic anomaly that encompassed a few centuries on either side of AD 1000, when temperatures in many parts of the world were warmer than they are currently. The degree of warmth and associated changes in precipitation, however, varied from region to region and from time to time; and, hence, the MWP was manifest differently in different parts of the planet. In this Summary we review what occurred in 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 6000-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 1400 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 1000 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 last 1270 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 3000 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 1000 and 1300 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 2000-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."
Last of all, 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 twenty prior studies to derive a 2000-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; while 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.
From these several studies, it is evident that for a considerable amount of time during the Medieval Warm Period, many, if not most, parts of China exhibited warmer conditions than those of modern times. And since those earlier high temperatures were obviously caused by something other than high atmospheric CO2 concentrations, whatever was responsible for them could well be responsible for the warmth of today.
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