Asia's Medieval Warm Period
Arid Central Asia (ACA, an inland zone in central Asia from the Caspian Sea in the west to the southern Mongolian Plateau in the east), according to Chen et al. (2010), is “a unique dry-land area whose atmospheric circulation is dominated today by the westerlies” and is “one of the specific regions that are likely to be strongly impacted by global warming,” which could greatly impact its hydrologic future. In an attempt to understand such potential impacts, Chen et al. evaluated the “spatial and temporal patterns of effective moisture variations,” using 17 different proxy records in the ACA and synthesizing a decadal-resolution moisture curve for this region over the past millennium, employing five of the 17 records based on their having “reliable chronologies and robust proxies.”
The nine researchers report that the effective moisture (precipitation) in the ACA has a generally inverse relationship with the temperature of the Northern Hemisphere, as portrayed by Moberg et al. (2005); China, as portrayed by Yang et al. (2002); and Central Asia, as portrayed by Esper et al. (2007). That is to say, as they describe it, the “wet (dry) climate in the ACA correlates with low (high) temperature.” Stating it in yet another way, they indicate the ACA “has been characterized by a relatively dry Medieval Warm Period (MWP; the period from ~1000 to 1350 AD), a wet little Ice Age (LIA; from ~1500–1850 AD),” and “a return to arid conditions after 1850 AD,” which has been slightly muted—but only “in some records”—over the past 20 years by an increase in humidity.
Given such findings, Chen et al. propose that “the humid LIA in the ACA, possibly extending to the Mediterranean Sea and Western Europe, may have resulted from increased precipitation due to more frequent mid-latitude cyclone activities as a result of the strengthening and equator-ward shift of the westerly jet stream ... coupled with a decrease in evapotranspiration caused by the cooling at that time,” a cooling brought about by the gradual demise of the Medieval Warm Period. This in turn speaks volumes about the great significance of that centuries-long period of much-lower-than-present atmospheric CO2 concentration but of equivalent or even greater warmth than that of the Current Warm Period. This ultimately suggests the twentieth-century increase in the air’s CO2 content may have had little, or maybe even nothing, to do with twentieth-century global warming.
Also exploring the Medieval Warm Period in China, Hong et al. (2009) indicate that “because it is a distinct warm period nearest to the modern warming period and happened before the Industrial Revolution, it naturally becomes a [source of] comparison with modern warming.” And in this regard, they add, “a universal concern in academic circles is whether it also existed outside the European region and whether it is a common phenomenon.” In a study designed to broach both questions, they extracted cores of peat from a location close to Hani Village, Liuhe County, Jilin Province, China (42°13’N, 126°31’E) and used those cores to develop, as they describe it, “a peat cellulose δ18O temperature proxy record proximately existing for 14,000 years.”
Their efforts revealed, first, that the MWP had indeed held sway on the Chinese mainland over the period AD 700–1400, peaking at about AD 900. And the eight researchers report that phenological data from east China (Ge et al., 2006) and tree-ring records from west China (Yang et al., 2000) also indicate “the temperature on the Chinese mainland was distinctly warmer during the MWP.” In fact, they say MWP temperatures were as much as “0.9–1.0°C higher than modern temperatures (Zhang, 1994).”
With respect to the entire 14,000-year period, Hong et al. write, “sudden cooling events, such as the Older Dryas, Inter-Allerod, Younger Dryas, and nine ice-rafted debris events of the North Atlantic”—which are described by Stuiver et al. (1995) and Bond et al. (1997, 2001)—“are almost entirely reiterated in the temperature signals of Hani peat cellulose δ18O.” They state, “these cooling events show that the repeatedly occurring temperature cooling pattern not only appeared in the North Atlantic Region in the high latitudes, but also in the Northwest Pacific Region in the middle latitudes,” indicating the recurring cooling and warming pattern did indeed occur “outside the European region” and that this climatic oscillation was “a common phenomenon.”
Several years earlier, Hong et al. (2000) had used a 6,000-year peat cellulose δ18O record derived from nearby Jinchuan Town, Huinan County, Jilin Province, China (42°20’N, 126°22’E) to identify δ18O periodicities of 86, 93, 101, 110, 127, 132, 140, 155, 207, 245, 311, 590, 820 and 1,046 years, which they described as being “similar to those detected in solar excursions,” and which they considered to be “further evidence for a close relationship between solar activity and climate variations on timescales of decades to centuries.” These findings were highly praised by Fairbridge (2001), who noted “almost identical equivalents are seen in solar emission periodicities and their harmonics, e.g., 86.884 years = 40 x 2.172 year Quasi Biennial Oscillation (QBO) as well as in the lunar tidal/apsides beat frequency (17.3769 years) which also matches closely with most of the longer spectral peaks, e.g., 140 (139) years, 207 (208.5), 311 (312.8), 590 (590.8) and 1046 (1042.6) years.” And for these spectacular spectral findings, Fairbridge wrote, “Hong et al. deserve the appreciation of the entire Holocene community.”
In another significant study, Liu et al. (2005) compared Ge et al.’s (2003) reconstructed winter half-year temperature anomalies in the central region of eastern China (25–40°N, east of 105°E) for the last 1,000 years with simulated anomalies of the same parameter, which they obtained from the ECHO-G global atmosphere-ocean coupled climate model that was driven by time-varying external forcings, including solar radiation, volcanic eruptions, and greenhouse gas concentrations (CO2 and CH4) for the same time period. And in conducting their analysis, they report, “the Medieval Warm Period (MWP) during 1000–1300 A.D., the Little Ice Age (LIA) during 1300–1850 A.D. and the modern warming period after 1900 A.D. are all recognizable from both the simulated and reconstructed temperatures.” In addition, they indicate the anomalies associated with the LIA and the modern warming simulated by the model are “in good consistency” with their reconstructed counterparts. However, they note that “in the earlier MWP, significant discrepancies exist between the simulation and the reconstruction.” More specifically, they say, “the simulated temperature anomaly in the 20th century is higher than that of the Medieval Warm Period, while the reconstructed temperature in the 20th century is lower.”
The seven scientists say the two different results “provide two different interpretations regarding the amplitude of recent global warming,” noting “one states that the 20th century warming has exceeded the normal range of the climate change, and it will result in catastrophic impact on human beings if warming continues,” whereas the other suggests “the current climate change has not yet exceeded the range of natural climate change in the past millennium.” As the real-world evidence for a warmer-than-present Medieval Warm Period continues to accumulate, it is becoming increasingly difficult to support the claim that current temperatures are unnaturally high due to rising anthropogenic CO2 emissions.
In one final study of China, Ge et al. (2010) developed three regional composite temperature reconstructions that extended back in time a full two millennia (Northeast, Tibet, Central East), one that extended back approximately 950 years (Northwest), and one that went back about 550 years (Southeast). With respect to the three reconstructions that extended through the Medieval Warm Period and the one that extended into but not through it, the six scientists report: (1) in the Northeast there was a warm period “between approximately 1100 and 1200 that exceeded the warm level of the last decades of the 20th century”; (2) in Tibet there was a “warming period of twenty decadal time steps between the 600s and 800s” that was “comparable to the late 20th century”; (3) in the Central East there were two warm peaks (1080s–1100s and 1230s–1250s) that had “comparable high temperatures to the last decades of the 20th century,” although the graph of their data indicates these two periods were in fact warmer than the last decades of the twentieth century; and (4) in the Northwest, “comparable warm conditions in the late 20th century are also found around the decade 1100s.” These findings make it clear there is nothing unusual, unnatural, or unprecedented about China's current level of warmth.
From China we proceed to Japan, where Aono and Saito (2010) “investigated documents and diaries from the ninth to the fourteenth centuries to supplement the phenological data series of the flowering of Japanese cherry (Prunus jamasakura) in Kyoto to improve and fill gaps in temperature estimates based on previously reported phenological data.” They then “reconstructed a nearly continuous series of March mean temperatures based on 224 years of cherry flowering data, including 51 years of previously unused data, to clarify springtime climate changes.” In addition, they estimated still other cherry full-flowering dates “from phenological records of other deciduous species, adding further data for six years in the tenth and eleventh centuries by using the flowering phenology of Japanese wisteria (Wisteria floribunda).”
Their temperature reconstruction “showed two warm temperature peaks of 7.6°C and 7.1°C, in the middle of the tenth century and at the beginning of the fourteenth century, respectively,” and they say “the reconstructed tenth century temperatures [AD 900–1000] are somewhat higher than present temperatures after subtracting urban warming effects.” Finally, they note “the general pattern of change in the reconstructed temperature series in this study is similar to results reported by previous studies, suggesting a warm period in Asia corresponding to the Medieval Warm Period in Europe.”
In a separate study, Daimaru et al. (2002) wrote, “in snowpatch grasslands, plant distributions follow the contours of the snowmelt gradient around summer snowpatches,” producing “similarly steep gradients in plant productivity and topsoil (e.g. Billings and Bliss, 1959; Helm, 1982; Kudo, 1991; Stanton et al., 1994.)” In fact, they note “in the subalpine zone of northeastern Japan, sites where the snow cover disappears after July are usually occupied by ‘snowpatch bare grounds’ with extremely poor vegetation cover” that is “encircled by snowpatch grassland,” citing Yamanaka (1979). As a result, they write, “litter fall and the organic content in topsoil decrease toward the center of a snowpatch because the period for plant growth becomes shorter with delay in the time of snow disappearance,” so that in current “snowpatch grasslands, peaty topsoil is restricted to sites where snowmelt comes early.” And as a result of this, the unique situation provided by a snowpatch often can provide a good opportunity for paleoclimatic reconstructions based on vertical profiles of soil characteristics at various locations along transects moving outwards from summer snowpatches.
Consequently, working in a snowpatch grassland within a shallow depression of landslide origin on the southeastern slope of Japan’s Mt. Zarumori (~39.8°N, 140.8°E), Daimaru et al. dug 27 soil pits at various locations in and around the central location of the snowpatch, carefully examining what they found and determining its age based on 14C dating and tephrochronology. They state, “peaty topsoils were recognized at seven soil pits in the dense grassland, whereas sparse grassland lacked peaty topsoil” and “most of the buried peat layers contained a white pumice layer named ‘To-a’ that fell in AD 915.” This observation, plus their 14C dating, led them to conclude the buried peat layers in the poor vegetation area indicate “warming in the melt season” as well as “a possible weakened winter monsoon in the Medieval Warm Period,” which their data suggest prevailed at the site they studied throughout the tenth century, AD 900–1000. They write, “many studies have reported climatic signals that are correlated with the Medieval Warm Period from the 9th to 15th centuries in Japan,” suggesting the possibly weakened winter monsoon of AD 900–1000 also may have been a consequence of the warmer temperatures of that period.
In a Japanese study using sediment cores from Lakes Ni-no-Megata (39°57’N, 139°43’E) and San-no-Megata (39°56’N, 139°42’E) located on the Oga Peninsula of northeastern Japan, Yamada et al. (2010) measured several sediment properties, including sulfur content and coarse mineral grains. The former served as a proxy for paleo-Asian summer monsoon activity, and the latter was a proxy for paleo-Asian winter monsoon activity over the last two millennia. Upon examining these data, Yamada et al. found evidence for a cold/dry interval stretching from AD 1 to 750, a warm/humid interval from AD 750 to 1200, and another cold/dry interval from AD 1200 to the present. These intervals could represent, respectively, as they describe them, “the Dark Ages Cold Period (DACP), the Medieval Warm Period (MWP) and the Little Ice Age (LIA).”
In further discussing their findings, the six scientists say they complement those of Kitagawa and Matsumoto (1995), whose study of tree-ring records in southern Japan “suggested the existence of one warm interval at AD 750-1300 and two cold intervals at AD 200-750 and AD 1600-1800,” as well as the findings of Sakaguchi (1983), whose study of the pollen record of peaty sediments in central Japan revealed “an unusual warm interval (AD 700-1300) and a cool interval (ca. AD 250-700).” In addition, they write, the “strong summer monsoon and weak winter monsoon at Lakes Ni-no-Megata and San-no-Megata from AD 750–1200 correlates with the lower δ18O values from Wangxiang Cave (Zhang et al., 2008) and lower values of minerogenic clastic content (Chu et al., 2009).”
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