Climate alarmists hotly contend that the degree of global warmth experienced over the latter part of the 20th century was greater than that experienced at any other time over the past two millennia (Mann and Jones, 2003). Why? Because this contention helps to cement their claim that the "unprecedented" temperatures of the past few decades were caused by the historical increase in the air's CO2 content. Hence, they cannot tolerate the thought that the Medieval Warm Period of a thousand years ago could have been as warm as, or even warmer than, it has been recently, since there was so much less CO2 in the air at that time than there is now. Likewise, they are equally loath to admit that the temperatures of the Roman Warm Period of two thousand years ago may also have rivaled, or exceeded, those of the recent past, since the atmospheric CO2 concentrations of that still earlier era were also much lower than those of today. As a result, climate alarmists rarely even speak of the Roman Warm Period. In addition, they refuse to entertain the possibility that both of these prior warm periods were global in extent, claiming instead, with respect to the Medieval Warm Period of which they do speak somewhat, albeit disparagingly, that it was a purely local phenomenon restricted to lands surrounding the North Atlantic Ocean. In this Summary, therefore, we examine these contentions as they pertain to the Roman Warm Period on the other side of the Northern Hemisphere in Asia.
We begin with the study of Ma et al. (2003), who worked with a stalagmite from Jingdong Cave about 90 km northeast of Beijing, assessing the climatic history of the past 3000 years at 100-year intervals on the basis of ð18O data, Mg/Sr ratios, and the solid-liquid distribution coefficient of Mg. Between 200 and 500 years ago, they report that "air temperature was about 1.2°C lower than that of the present, corresponding to the Little Ice Age in Europe." Earlier, between AD 700 and 1000, there was an equally aberrant but warm period that peaked at about AD 900, which they say "corresponded to the Medieval Warm Period in Europe." This period of peak warmth had in turn been preceded by the Dark Ages Cold period that had been preceded by the Roman Warm Period, which in the stalagmite record is best defined by the much colder temperatures that preceded it.
Similar results were obtained by Xu et al. (2002), who studied plant cellulose ð18O variations in cores retrieved from peat deposits at the northeastern edge of the Qinghai-Tibetan Plateau of China. Following the demise of the Roman Warm Period, they observed the existence of three cold events centered at approximately AD 500, 700 and 900, during the Dark Ages Cold Period. Then, from AD 1100-1300, they report that "the ð18O of Hongyuan peat cellulose increased, consistent with that of Jinchuan peat cellulose and corresponding to the 'Medieval Warm Period'." Finally, they note the existence of three cold periods (AD 1370-1400, AD 1550-1610 and AD 1780-1880) that correspond to the Little Ice Age, after which modern warming begins.
In a much broader-based study, Yang et al. (2002) used nine separate proxy climate records derived from peat, lake sediment, ice core, tree ring and other proxy sources to compile a single weighted temperature history for all of China that spanned the past two thousand years. This composite record revealed five distinct climate epochs: a warm stage from AD 0 to 240 (the tail-end of the Roman Warm Period), a cold interval between AD 240 and 800 (the Dark Ages Cold Period), a return to warm conditions from AD 800-1400 (which included the Medieval Warm Period between AD 800 and 1100), a cool interval between 1400 and 1820 (the Little Ice Age), and the current warm regime (the Modern Warm Period) that followed the increase in temperature that began in the early 1800s. Another important aspect of Yang et al.'s study was that it demonstrated that the warmest temperatures of the past two millennia were observed during the second and third centuries AD.
Another broad-based study of a big chunk of China was published by Ge et al. (2003), who worked with 200 different 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), Sheng (1990) and Wen and Wen (1996) -- to produce a 2000-year history of winter half-year (October to April) temperature for the region of China bounded by latitudes 27 and 40°N and longitudes 107 and 120°E. Based on this record, they report that "from the beginning of the Christian era, climate became cooler at a rate of 0.17°C per century," which correlates well with the fact that this was the period of time when the planet slipped out of the Roman Warm Period and entered into the Dark Ages Cold Period, noting further that "around the AD 490s temperature reached about 1°C lower than that of the present (the 1951-80 mean)."
"Then," as they continue, "temperature entered a warm epoch from the AD 570s to 1310s with a warming trend of 0.04°C per century; the peak warming 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." This finding pretty much speaks for itself: during the Medieval Warm Period, this large chunk of China was warmer than it has ever been in modern times over a similar span of years.
"After the AD 1310s," Ge et al. determined that "temperature decreased rapidly at a rate of 0.10°C per century; the mean temperatures of the four cold troughs were 0.6-0.9°C lower than the present, with the coldest value 1.1°C lower." This, of course, was the Little Ice Age, from which the world appears to still be in processes of recovering. In this regard, they also note that "temperature has been rising rapidly during the twentieth century, especially for the period 1981-99, and the mean temperature is now 0.5°C higher than for 1951-80." Nevertheless, Ge et al. report that temperatures during the Medieval Warm Period rose higher still, and for several 10- and 30-year periods.
In another regional study of a second "big chunk of China," Bao et al. (2003) utilized proxy climate records (ice-core ð18O, peat-cellulose ð18O, 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 previously-published studies to derive a 2000-year temperature history of the Tibetan Plateau, after first developing similar temperature histories for its northeastern, southern and western sections. So what did they find?
The temperature histories of the three parts of the Tibetan Plateau were all significantly different from each other. In each case, however, they had one important thing in common: there was more than one prior 50-year period when the mean temperature of each of them was warmer than it was over the most recent 50-year period. In the case of the northeastern sector of the Tibetan Plateau, these 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. In the case of the southern sector, however, they occurred during both warm periods. Hence, for all three portions of the Tibetan Plateau, there has been nothing unusual or unnatural about their most recent warm temperatures.
With respect to the entire Tibetan Plateau, the story is pretty much the same: there has been nothing extraordinary about the recent past. For the whole region, however, there was only one prior 50-year period when temperatures were warmer than they were over the most recent 50-year period; and that interval occurred near the end of the Roman Warm Period, some 1850 years ago.
Another important contribution to the story of the Roman Warm Period in Asia comes from Bao et al. (2004), who collected and analyzed various proxy climate data derived from ice cores, tree rings, river and lake sediments, lake terraces and paleosols, as well as historical documents, which enabled them to determine the climatic state of northwest China during the Western and Eastern Han Dynasties (206 BC-AD 220) relative to that of the past two millennia. As they describe it, their analysis revealed "strong evidence for a relatively warm and humid period in northwest China between 2.2 and 1.8 kyr BP," during the same time interval as the Roman Warm Period. In fact, they determined that this period experienced higher temperatures than those of today. What is more, they report that "the warm-wet climate period during 2.2-1.8 kyr BP also occurred in central and east China, after [which] temperatures decreased rapidly (Zhu, 1973; Hameed and Gong, 1993; Yan et al., 1991, 1993; Shi and Zhang, 1996; Ge et al., 2002)," noting additionally that historical records report "an abrupt climate change from warmer and wetter to cooler and drier conditions occurred around AD 280 (Zhang et al., 1994)." Last of all, they state that "three alternate China-wide temperature composites covering the last 2000 years display an obvious warm stage in 0-240 AD (Yang et al., 2002)," and that "according to a 2650-year warm-season temperature reconstruction from a stalagmite from Shihua Cave of Beijing (Tan et al., 2003), the temperatures during 2.1-1.8 ka BP were basically above the average of the entire temperature series."
In light of these many evidences from a variety of locations across Asia, it is clear that the Roman Warm Period did indeed manifest itself in regions far removed from the North Atlantic Ocean, and that it did so in superlative fashion. In fact, Bao et al. (2004) state that "the warm and moist conditions during the Western and Eastern Han Dynasties [i.e., the Roman Warm Period] might have been responsible for the large-scale agricultural production and the local socioeconomic boom that is documented by the occurrence of the famous ruin groups of Loulan, Niya, and Keriya." Citing the existence of plant remains such as walnuts, rice, barley, millet and wheat grains found in the area, they also indicate that the water and temperature conditions of the Roman Warm Period in these parts of Asia "were suitable for rice cultivation and much better than today."
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