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Temperature Trends (Asia: China) -- Summary
Controversy abounds over the temperature history of the earth, particularly that of the past one to two millennia. The original debate was sparked by the papers of Mann et al. (1998, 1999), which challenged the long-accepted view that there was nothing unusual about earth's climatic history subsequent to the inception of the Industrial Revolution; and it prompted the Intergovernmental Panel on Climate Change (IPCC) to disavow their original representation of the temperature history of the Holocene (Houghton et al., 1990), where several periods of time prior to the inception of the Industrial Revolution were depicted as having experienced temperatures that were higher than those of the latter part of the 20th century.

The revisionist history of Mann et al. depicts a long nearly-linear temperature decline that ends with a dramatic post-1910 warming, which in its final two decades takes the mean surface air temperature of the Northern Hemisphere to a level that appears to have been unprecedented over the past millennium, while the similarly-developed temperature history of Mann and Jones (2003) extends this "unprecedentedness" back a full two millennia and claims to represent the entire earth.

This latter climatic history of the planet was challenged by Soon and Baliunas (2003) and Soon et al. (2003a), who in turn were challenged by Mann et al. (2003a), who in turn were challenged by Soon et al. (2003b), who in turn were challenged by Mann et al. (2003b), and so on, as the arguing went back-and-forth seemingly ad infinitum. And so the debate continues. In addition, the revisionist temperature history of Mann et al. was challenged on totally different grounds by McIntyre and McKittrick (2003), who in turn were challenged by Mann and others (this time on the Internet), who in turn were challenged in the same medium by McIntyre and McKitrick. And on and on it goes, with no end in sight; but see McIntyre and McKitrick (2005).

Irrespective of this still-ongoing scientific bickering, what has had a beginning must eventually have an end; and so will this debate someday come to a screeching halt, most likely as a result of ever-accumulating masses of real-world data that allow ever more temperature histories of ever more parts of the world to be produced with ever more fidelity. And as a part of that slow but steady process, this summary of several pertinent findings from China may well play a role in helping to resolve this contentious issue.

Yang et al. (2002) used nine proxy climate records derived from peat, lake sediment, ice core, tree-ring and other proxy sources to compile a single weighted temperature history for the whole of China that spanned the past two thousand years. This record revealed five distinct climate epochs: (1) a warm stage from AD 0 to 240 (the last part of the Roman Warm Period), (2) a cold interval between AD 240 and 800 (the Dark Ages Cold Period), (3) a return to warm conditions from AD 800-1400 (which included the Medieval Warm Period between AD 800 and 1100), (4) a cool interval between 1400 and 1820 (the Little Ice Age), and (5) the Current Warm Period that followed the increase in temperature that began in the early 1800s. And, most importantly, this record also revealed that the warmest temperatures of the past two millennia were observed during the second and third centuries AD, not in the latter part of the 20th century.

Three years later, working with fully two hundred sets of phenological and meteorological data, Ge et al. (2003b) 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. In describing their results, they noted 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 is the period of time when the planet slipped out of the Roman Warm Period and entered into the Dark Ages Cold Period, such that "around the AD 490s temperature reached about 1°C lower than that of the present (the 1951-80 mean)." Then, "abruptly," as they describe it, "temperature entered a warm epoch from the AD 570s to 1310s with a warming trend of 0.04°C per century," which led to a peak warmth that "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," which finding pretty much speaks for itself, since for a considerable amount of time during the Medieval Warm Period, this large chunk of China enjoyed greater warmer than has yet to be experienced in modern times during the winter season.

"After the AD 1310s," as Ge et al. continue, "temperature decreased rapidly at a rate of 0.10°C per century," such that "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 period, of course, was the Little Ice Age, from which the world may still be in processes of recovering. Last of all, they report 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." Be that as it may, Ge et al.'s results clearly demonstrate that, during the Medieval Warm Period, winter temperatures rose higher still, and for several 10- and 30-year periods.

Working with warm-season (May-August) data, Tan et al. (2004) established an annual layer thickness chronology (LTC) for a stalagmite from Beijing's Shihua Cave and developed a 2650-year (665 BC-AD 1985) warm-season temperature record (WTR) for Beijing by calibrating the LTC with the observed WTR of Tan et al. (2003). And in doing so, they found that their warm season temperature reconstruction "is consistent with oscillations in total solar irradiance inferred from cosmogenic 10Be and 14C," and that it also "is remarkably consistent with Northern Atlantic drift ice cycles that were identified to be controlled by the sun through the entire Holocene [Bond et al., 2001]."

Going backwards in time, both of the above records clearly depict the start of the Modern Warm Period, the Little Ice Age, the Medieval Warm Period, the Dark Ages Cold Period, the Roman Warm Period, and the cold climate at the start of both records. And in light of these findings, Tan et al. concluded that "the synchronism between the two independent sun-linked climate records therefore suggests that the sun may directly couple hemispherical climate changes on centennial to millennial scales." In addition, the cyclical nature of the millennial-scale oscillation of climate that is evident in both climate records further suggests that there is no need to invoke rising atmospheric CO2 concentrations as a cause of the development of the Modern Warm Period.

Utilizing the same warm-season temperature data as Ge et al. (2003b), Ge et al. (2003a) identified a millennial-scale oscillation with a periodicity of approximately 1350 years. Hence, to see if there was anything unusual or unnatural about the warming of the recent past, and especially the past three decades, they compared similar portions of the two most recent cycles of this natural oscillation by plotting 30-year anomalies of reconstructed temperatures from the AD 1500s-1990s against similar anomalies from the AD 150s-650s and fitting a linear regression to the resulting data. In so doing, they found that "the temperature anomaly in the 1980s-1990s is much higher than the regression value," stating that "if the regression value is regarded as the basic value of temperature changes in nature, the unusual high temperature in the 1980s-1990s could likely be regarded as the forcing of the greenhouse effect induced by human activities on climatic changes," which they say "coincides with the conclusion that the greenhouse effect induced by human activities has been increasing remarkably since the 1950s [as] concluded by the IPCC based on results of modeling."

This conclusion, however, is erroneous; for there are other data points that are even further removed from the regression line than is the 1980s-1990s data point; and these more aberrant data points all occurred during the 1600s, well before any major changes in the air's CO2 concentration. The data point labeled 1635, for example, extends further above the regression line than does the most recent data point, while the very next data point labeled 1665 deviates the most of all, but below the regression line. What is more, the next data point labeled 1695 reverses course to once again extend further above the line than does the most recent data point. Hence, the relative warmth of the 1980s-1990s is not at all anomalous; and, therefore, it is not a manifestation of CO2-induced global warming. It is merely a consequence of natural climatic variability about a well-defined climatic oscillation.

Contemporaneously, Zhai and Pan (2003) derived trends in the frequencies of warm days and nights, cool days and nights, and hot days and frost days for the whole of China for the period 1951-1999, based on daily surface air temperature data obtained from approximately 200 weather observation stations scattered across the country. This work revealed that over this period, and especially throughout the 1980s and 90s, there were increases in the numbers of warm days and nights, while there were decreases in the numbers of cool days and nights, consistent with an overall increase in mean daily air temperature. At the extreme hot end of the temperature spectrum, however, they report that "the number of days with daily maximum temperature above 35°C showed a slightly decreasing trend for China as a whole," while at the extreme cold end of the spectrum, the number of frost days with daily minimum temperature below 0°C declined at the remarkable rate of 2.4 days per decade. In terms of human health, these changes in extreme temperature are very beneficial, for extreme cold is by far a greater cause of death in humans than is extreme heat; and since extreme cold weather has become less frequent in China without any increase in the frequency of extreme hot weather, it is clear that the global warming of the last half of the 20th century was a positive thing for the people of the China in terms of their overall health and longevity.

Creeping ahead one year, Ge et al. (2004) introduced their important study of two thousand years of reconstructed winter half-year temperatures of eastern China by stating that "it is important to study the temperature change during the past 2000 years for understanding the issues such as the greenhouse effect and global warming induced by human activities," stating additionally that "China has advantages in reconstructing historical climate change for its abundant documented historical records and other natural evidence obtained from tree rings, lake sediments, ice cores, and stalagmites." That said, what did they find?

Perhaps the five climate scientists' most fundamental finding was the existence of "an about 1350-year periodicity in the historical temperature change," which revealed a number of multi-century warm and cold periods. Preceding the Modern Warm Period, for example, was the Little Ice Age (LIA), which "in China," in their words, "began in the early 14th century (the 1320s) and ended in the beginning of the 20th century (the 1910s)." It included four cold stages and three short warming phases. The LIA, in turn, was preceded by the Medieval Warm Period, which Ge et al. say "began in the 930s and ended in the 1310s." It was composed of two warm stages, each of over 100 years duration, and a shorter intervening cold stage.

Continuing further back in time, the Chinese scientists found a cold period from the 780s to the 920s and a warm period from the 570s to the 770s, which was in turn preceded by a cold period from the 210s to the 560s, which they say "was the only one comparable with [the] LIA for the past 2000 years." This ultra-cold spell, of course, was the Dark Ages Cold Period that followed on the heels of the Roman Warm Period.

Since one of the purposes of their study was "to test whether the warming in the 20th century has exceeded the maximum magnitude in the past 2000 years," Ge et al. considered this question in some detail. At the centennial scale, they report that "the temperature anomaly of the 20th century is not only lower than that of the later warm stage of the Medieval Warm Period (the 1200s~1310s), but also slightly lower than that of the warm period in the Sui and Tang dynasties (the 570s~770s) and the early warm stage of the Medieval Warm Period (the 930s~1100s)."

On a 30-year scale, they likewise report that "the warmest 30-year temperature anomaly in the 20th century is roughly equal to the warmest 30-year one in the Sui and Tang dynasties warm period, but a little lower than that of the Medieval Warm Period." And on the decadal scale, they say that "the warmest decadal temperature anomaly in the 20th century is approximately at the same level of the warmest decade of the early stage of the Medieval Warm Period."

Last of all, Ge et al. additionally note that "although the warming rate in the early 20th century has reached 1.1°C per century, such a rapid change is not unique during the alternation from the cold period[s] to the warm period[s]" of the prior 2000 years. For example, they report that the per-century warming rate from the 480s~500s to the 570s~590s was 1.3°C, from the 1140s~1160s to the 1230s~1250s was 1.4°C, and from the 1650s~1670s to the 1740s~1760s was 1.2°C.

In discussing the implications of these several observations of pre-20th-century faster-than-recent warming and higher-than-recent temperatures, Ge et al. say that their analysis "gives a different viewpoint from that 'the 20th century is the warmest century in the past 1000 years', presented by IPCC, and is of great significance for better understanding the phenomena of the greenhouse effect and global warming etc. induced by human activities." And what would that "different viewpoint" be? In the words of Ge et al., "the temperature of the 20th century in eastern China is still within the threshold of the variability of the last 2000 years," which observation clearly indicates that the Chinese data provide no evidence for the hypothesis that the eastern part of the country's 20th-century warming - or even a small part of it - was human-induced.

Pausing for a moment to focus on a study that was not conducted in China, it is constructive to consider the work of Kalnay and Cai (2003), who determined differences between observed surface air temperature trends in the conterminous United States and trends for the same regions derived from reanalyses of global weather over the past 50 years, such as the NCEP-NCAR 50-year Reanalysis (NNR) project, and who used the results to estimate the impact of land-use changes on surface warming, which has historically been determined solely from surface air temperature measurements. The reanalysis-derived temperatures, however, were based on atmospheric vertical soundings derived from satellites and balloons. Over rural areas, the surface-derived and reanalysis-derived surface air temperature data sets were shown by the two scientists to yield essentially identical trends; and, therefore, they concluded that they "could attribute the differences between monthly or annually averaged surface-temperature trends derived from observations and from the NNR primarily to urbanization and other changes in land use."

Utilizing this approach in their effort to determine the impacts of land-use changes on surface air temperature throughout eastern China (east of 110°E) - where rapid urbanization, deforestation, desertification and other changes in land use had occurred over the prior quarter-century - Zhang et al. (2005) focused on daily mean, maximum and minimum air temperatures from 259 stations over the period 1960 to 1999. This work revealed that changes in land-use had had little to no influence on daily maximum temperatures, but that they explained about 18% of the observed daily mean temperature increase and 29% of the observed daily minimum temperature increase in this region over the past 40 years, yielding decadal warming trends of about 0.12°C and 0.20°C for the latter two parameters, respectively, convincingly demonstrating that changes in land-use can have significant impacts on surface air temperatures in addition to what increasing atmospheric concentrations of various greenhouse gases may be doing.

In a contemporary study, Chen et al. (2005) analyzed the chemical composition of sediments deposited in Lake Erhai (25°35'-25°58'N, 100°05'-100°17'E), which is the largest fault lake in the western Yunnan Province of China. More specifically, they applied Principal Component Analysis to the concentrations of 21 major and minor elements found in the sediments, thereby deriving historical variations in temperature and precipitation over the period AD 1340-1990. This work revealed an initial period (1340-1550) of relatively high temperature and low rainfall; but thereafter, the climate gradually become cooler and wetter, coincident with the development of the Little Ice Age, which lasted until about 1890. Then, from 1890 to 1950, another warm and dry period held sway, followed by a cooling since the end of the 1940s, which is at odds with the hockeystick temperature history of Mann et al. (1998, 1999).

Moving ahead a few years, Cai et al. (2008) developed an April-September mean temperature history for the north-central Shaanxi Province of China for the period 1826-2004, based on ring-width analyses of 59 tree-ring cores obtained from 30 Pinus tabulaeformis trees growing on Huanglong Mountain in the southeastern part of the Shaanxi Loess Plateau. In doing so, they found that their temperature reconstruction showed an increase since the 1970s; but they also found that "the temperature of [the] last decade is not the highest in the reconstruction," noting that "the highest temperature period in [the] reconstruction is from 1928 to 1933," with 1929 being the warmest year of all. Once again, therefore, we have another real-world example of the late 20th century not being the warmest period of the last thousand or more years, as climate alarmists are wont to claim it was. And it is important to note that the truly warmest period in this part of China occurred when there was 20% less CO2 in the air than there was at the time of Cai et al.'s study.

Three years later, Ge et al. (2011) wrote that "studying climate change over the past 2000 years ... is essential to better understanding climate variability and provides background knowledge necessary for improving predictions of future changes," citing the IGBP (2009). More specifically, they rhetorically ask: "Has rapid warming such as that in the 20th century occurred previously, especially during the past 2000 years?" Noting that "this is a key issue in understanding the forces of climate warming in the 20th century," they go on to say that "few studies so far have addressed this question," which they, thus, proceed to do.

Using 24 preexistent proxy temperature series, the four researchers at the Institute of Geographic Sciences and Natural Resources Research of the Chinese Academy of Sciences analyzed rates of temperature change in China at 30- to 100-year time scales for the past 2000 years and at a 10-year time scale for the past 500 years. This work revealed, in their words, that "the warming rates at centennial and decadal scales in the 20th century were not exceptional for the past 2000 years." At the 30-year time scale, for example, they found that the peak rate was "less than rates for previous periods, such as the rapid warming from the Little Ice Age to the 20th century and from the 270s-290s to 300s-320s." Likewise, they say that "the peak rates of the 100-year scale warming in the AD 180s-350s in northeastern China, as well as those in the 260s-410s and 500s-660s in Tibet, were all greater than those from the mid-19th to 20th century." And they add that although "the rates of the most rapid cooling at scales of 30 to 100 years in the Little Ice Age were prominent," they state that they "were also not unprecedented in the last 2000 years."

"In conclusion," in the words of Ge et al., "it is demonstrated that although human-induced greenhouse effects may have contributed to rapid global warming in the 20th century, in the case of China such rapid decadal to centennial warming has occurred in preindustrial times," which significantly weakens the worn-out climate-alarmist claim of late 20th-century warming being unprecedented over the past one to two millennia.

Last of all, it is instructive to consider the study of Zhen-Shan and Xian (2007), who say they used a novel multi-timescale analysis method known as Empirical Mode Decomposition (EMD) to "diagnose the variation of the annual mean temperature data of the globe, Northern Hemisphere and China from 1881 to 2002." And what did they learn in the process?

First of all, Zhen-Shan and Xian report finding that the temperature histories they studied "can be completely decomposed into four timescale quasi-periodic oscillations including an ENSO-like mode, a 6-8-year signal, a 20-year signal and a 60-year signal, as well as a trend." This latter residual, which they determined could account for no more than 40% of the global temperature variation, was attributed by them to the historical increase in the air's CO2 content; but it is clear that some unknown portion of it could well be due to other factors. In addition, they report that "temperature variation in China precedes that [of] the globe and Northern Hemisphere," thereby providing what they denominated "a denotation for global climate changes." And by projecting the four oscillatory modes of temperature change that they identified into the future, together with the residual temperature trend, they came to the conclusion that "global climate will be cooling down in the next 20 years."

In light of their findings and what those findings imply, the two Chinese researchers say that "although the CO2 greenhouse effect on global climate change is unsuspicious, it could have been excessively exaggerated." Consequently, they conclude that "it is high time to reconsider the trend of global climate change." This warning is especially appropriate in light of Zhen-Shan and Xian's demonstration of CO2's less-than-dominant role in the global warming of the last hundred and twenty years (which may itself be inflated), plus their conclusion that if the atmosphere's CO2 content were to be suddenly stabilized, "the CO2 greenhouse effect will be deficient in counterchecking the natural cooling of global climate in the following 20 years," which puts a whole new slant on the question of what to do about CO2 ... which would appear to be all the more crucial in light of the fact that the globe has not warmed over the last decade and a half.

In conclusion, the results of these several studies demonstrate that the so-called unprecedented warmth of the late 20th century is simply a myth, as far as China is concerned. The apparent great warmth of this period in this particular part of the world appears to have been but a common manifestation of naturally-induced and regularly-recurring conditions similar to those that have been experienced in prior millennia. These results also serve as a testimony against those who would deny the existence of an extensive (hemispheric or global) Medieval Warm Period and Little Ice Age, as well as an extensive Roman Warm Period and Dark Ages Cold Period, and cyclical climate changes in general. Clearly, it's time for everyone to wake up! ... and to recognize the dynamic nature of real-world climate change!

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Last updated 28 August 2013