Climate alarmists contend that global warming is responsible for creating more frequent and greater extremes of various types of weather. This summary investigates this claim as it pertains to precipitation variability in Asia.
We begin with the work of Pederson et al. (2001), who developed tree-ring chronologies for northeastern Mongolia and used them to reconstruct annual precipitation and streamflow histories for the period 1651-1995. Working with both standard deviations and 5-year intervals of extreme wet and dry periods, they found that "variations over the recent period of instrumental data are not unusual relative to the prior record." They note, however, that their reconstructions "appear to show more frequent extended wet periods in more recent decades," but they say that this observation "does not demonstrate unequivocal evidence of an increase in precipitation as suggested by some climate models." Spectral analysis of the data also revealed significant periodicities around 12 and 20-24 years, which they suggested may constitute "possible evidence for solar influences in these reconstructions for northeastern Mongolia."
In another analysis focusing on Mongolia, Davi et al. (2006) used absolutely-dated tree-ring-width chronologies obtained from five sampling sites in west-central Mongolia to derive individual precipitation histories, the longest of which stretches from 1340 to 2002, additionally developing a reconstruction of streamflow that extends from 1637 to 1997. In the process of doing so, they discovered there was "much wider variation in the long-term tree-ring record than in the limited record of measured precipitation," which for the region they studied covers the period from 1937 to 2003. In addition, they say their streamflow history indicates that "the wettest 5-year period was 1764-68 and the driest period was 1854-58," while "the most extended wet period [was] 1794-1802 and ... extended dry period [was] 1778-83." For this part of Mongolia, therefore, which the researchers say is "representative of the central Asian region," there is no support for the claim that the "unprecedented warming" of the 20th century has led to increased variability in precipitation and streamflow. In fact, any tendencies that may be present in the data suggest just the opposite.
Writing as background for their work, Liu et al. (2012) state that "climate change is consistently associated with changes in a number of components of the hydrological cycle," including "precipitation patterns and intensity, and extreme weather events." Therefore, and in order to "provide advice for water resource management under climate change," they conducted a study of the subject in the Guangdong Province of Southern China, which occupies a land area of approximately 178,000 km2 and has a population of just over 96 million people (as of 2009). More specifically, the authors analyzed "trends of annual, seasonal and monthly precipitation in southern China (Guangdong Province) for the period 1956-2000 ... based on the data from 186 high-quality gauging stations," and they employed "statistical tests, including the Mann-Kendall rank test and wavelet analysis," in order to determine whether the precipitation series exhibited any regular trends or periodicities.
In discussing their findings, the four researchers report that "annual precipitation has a slightly decreasing trend in central Guangdong and slight increasing trends in the eastern and western areas of the province," but they say that "all the annual trends are not statistically significant at the 95% confidence level." In addition, they discovered that "average precipitation increases in the dry season in central Guangdong, but decreases in the wet season," such that "precipitation becomes more evenly distributed within the year." Last of all, they state that "the results of wavelet analysis show prominent precipitation with periods ranging from 10 to 12 years in every [italics added] sub-region in Guangdong Province." And comparing precipitation with the 11-year sunspot cycle, they find that "the annual precipitation in every [italics added] sub-region in Guangdong province correlates with Sunspot Number with a 3-year lag." Thus, rather than becoming more extreme in the face of 1956-2000 global warming, Liu et al.'s analysis of the pertinent data suggest that precipitation in China's Guangdong Province has become both less extreme and less variable. And the temporal precipitation patterns that do emerge upon proper analysis suggest that the primary player in their determination is the sun.
Taking a much longer look into the subject were Ji et al. (2005), who used reflectance spectroscopy on a sediment core taken from a lake in the northeastern part of the Qinghai-Tibetan Plateau to obtain a continuous high-resolution proxy record of the Asian monsoon over the past 18,000 years. This project indicated that monsoonal moisture since the late glacial period had been subject to "continual and cyclic variations," among which was a "very abrupt onset and termination" of a 2,000-year dry spell that started about 4200 years ago (yr BP) and ended around 2300 yr BP. Other variations included the well-known centennial-scale cold and dry spells of the Dark Ages Cold Period (DACP) and Little Ice Age (LIA), which lasted from 2100 yr BP to 1800 yr BP and 780 yr BP to 400 yr BP, respectively, while sandwiched between them was the warmer and wetter Medieval Warm Period, and preceding the DACP was the Roman Warm Period. Time series analyses of the sediment record also revealed several statistically significant periodicities (123, 163, 200 and 293 years, all above the 95% level), with the 200-year cycle matching the de Vries or Suess solar cycle, implying that changes in solar activity are important triggers for some of the recurring precipitation changes in that part of Asia. Hence, it is clear that large and abrupt fluctuations in the Asian monsoon have occurred numerous times and with great regularity throughout the Holocene, and that the sun played an important role in orchestrating them.
Also working on the Tibetan Plateau were Shao et al. (2005), who used seven Qilian juniper ring-width chronologies from the northeastern part of the Qaidam Basin to reconstruct a thousand-year history of annual precipitation there; and who in doing so discovered that annual precipitation had fluctuated at various intervals and to various degrees throughout the entire past millennium. Wetter periods occurred between 1520 and 1633, as well as between 1933 and 2001, although precipitation has declined somewhat since the 1990s. Drier periods, on the other hand, occurred between 1429 and 1519 and between 1634 and 1741. With respect to variability, the scientists report that the magnitude of variation in annual precipitation was about 15 mm before 1430, increased to 30 mm between 1430 and 1850, and declined thereafter to the present. These several findings suggest that either (1) there is nothing unusual about the planet's current degree of warmth, i.e., it is not unprecedented relative to that of the early part of the past millennium, or (2) unprecedented warming need not lead to unprecedented precipitation or unprecedented precipitation variability ... or both of the above. The findings of this study, therefore, do not provide support for the climate-alarmist contention that global warming leads to greater and more frequent precipitation extremes.
In yet another study focusing on Tibet, Zhang et al. (2007) developed flood and drought histories of the past thousand years in China's Yangtze Delta "from local chronicles, old and very comprehensive encyclopedia, historic agricultural registers, and official weather reports," upon which "continuous wavelet transform was applied to detect the periodicity and variability of the flood/drought series." In comparing their findings with two one-thousand-year temperature histories from the region, the authors report that "colder mean temperature in the Tibetan Plateau usually resulted in higher probability of flood events in the Yangtze Delta region." In addition, during AD 1400-1700 (the coldest portion of their record, corresponding to much of the Little Ice Age), the proxy indicators showed that "annual temperature experienced larger variability (larger standard deviation), and this time interval exactly [italics added] corresponds to the time when the higher and significant wavelet variance occurred" in the flood/drought series. In contrast, Zhang et al. report that during AD 1000-1400 (the warmest portion of their record, corresponding to much of the Medieval Warm Period), relatively stable "climatic changes reconstructed from proxy indicators in Tibet correspond to lower wavelet variance of flood/drought series in the Yangtze Delta region." And these findings once again illustrate that warmer climates tend to be less variable than colder ones.
In another part of the continent and based on analyses of tree-ring width data and their connection to large-scale atmospheric circulation, Touchan et al. (2005) developed summer (May-August) precipitation reconstructions for several parts of the eastern Mediterranean region (Turkey, Syria, Lebanon, Cyprus and Greece) that extend back in time anywhere from 115 to 600 years; and over the latter length of time, they found that May-August precipitation varied on multiannual and decadal timescales, but that on the whole there were no long-term trends. The longest dry period occurred in the late 16th century (1591-1595), while there were two extreme wet periods: 1601-1605 and 1751-1755. In addition, both extremely strong and weak precipitation events were found to be more variable over the intervals 1520-1590, 1650-1670 and 1850-1930. Consequently, the results of this study also demonstrate there was nothing unusual or unprecedented about late 20th-century precipitation events in the eastern Mediterranean part of Asia that would suggest a CO2 influence. If anything, as this region transited from the record cold of the Little Ice Age to the peak warmth of the Current Warm Period, May-August precipitation actually became less, as opposed to more, variable in the face of rising temperatures.
Examining a much larger region in their analysis, Kripalani and Kulkarni (2001) studied seasonal summer monsoon (June-September) rainfall data from 120 east Asia stations for the period 1881-1998. A series of statistical tests they applied to these data revealed the presence of short-term variability in rainfall amounts on decadal and longer time scales, the longer "epochs" of which were found to last for about three decades over China and India and for approximately five decades over Japan. With respect to long-term trends, however, none were detected. Consequently, the history of summer rainfall trends in east Asia does not support climate-alarmist claims of intensified monsoonal conditions in this region as a result of CO2-induced global warming. As for the decadal variability inherent in the record, the two researchers say it "appears to be just a part of natural climate variations."
In concluding this discussion on precipitation variability in Asia, we turn to the work of Kishtawal et al. (2010), who added an interesting twist on the topic. Studying the Indian subcontinent (8.2°N to 35.35°N, 68.5°E to 97°E), Kishtawal et al. set out to assess the impacts of urbanization on the region's rainfall characteristics during the time of the Indian summer monsoon by analyzing in situ and satellite-based precipitation and population datasets. In doing so, the five researchers say their study showed "a significantly increasing trend in the frequency of heavy rainfall climatology over urban regions of India during the monsoon season," adding that "urban regions experience less occurrences of light rainfall and significantly higher occurrences of intense precipitation compared to non-urban regions." So what is the significance of these findings?
In their book entitled Dire Predictions: Understanding Global Warming, Mann and Kump (2008) write that most climate model simulations of global warming indicate that "increases are to be expected in the frequency of very intense rainfall events." Throughout most of the world, however, and as seen in the studies reviewed in this summary, such has not been found to have been the case to this point in time; and in places where there may have been a tendency for such to occur, the results of Kishtawal et al., plus the papers they cite in the introduction to their study, suggest that urbanization may have caused increases in intense rainfall events that are observed in some studies, while the study of Hossain et al. (2009) suggests that the proliferation of large dams may also be causing the same to occur. Thus, it is becoming ever more difficult to be able to distinguish what could be the primary cause of any net increase in the global frequency of intense rainfall events that might yet be detected over land. Nevertheless, and based on all of the work reviewed here, the only effect CO2-induced global warming may be having on precipitation variability in Asia is to make it less, not more, variable.
Davi, N.K., Jacoby, G.C., Curtis, A.E. and Baatarbileg, N. 2006. Extension of drought records for Central Asia using tree rings: West-Central Mongolia. Journal of Climate 19: 288-299.
Hossain, F., Jeyachandran, I. and Pielke Sr., R. 2009. Have large dams altered extreme precipitation patterns? EOS, Transactions, American Geophysical Union 90: 453-454.
Ji, J., Shen, J., Balsam, W., Chen, J., Liu, L. and Liu, X. 2005. Asian monsoon oscillations in the northeastern Qinghai-Tibet Plateau since the late glacial as interpreted from visible reflectance of Qinghai Lake sediments. Earth and Planetary Science Letters 233: 61-70.
Kishtawal, C.M., Niyogi, D., Tewari, M., Pielke Sr., R.A. and Shepherd, J.M. 2010. Urbanization signature in the observed heavy rainfall climatology over India. International Journal of Climatology 30: 1908-1916.
Kripalani, R.H. and Kulkarni, A. 2001. Monsoon rainfall variations and teleconnections over south and east Asia. International Journal of Climatology 21: 603-616.
Liu, D., Guo, S., Chen, X. and Shao, Q. 2012. Analysis of trends of annual and seasonal precipitation from 1956 to 2000 in Guangdong Province, China. Hydrological Sciences Journal 57: 358-369.
Mann, M.E. and Kump, L.R. 2008. Dire Predictions: Understanding Global Warming. DK Publishing, Inc., New York, New York, USA.
Pederson, N., Jacoby, G.C., D'Arrigo, R.D., Cook, E.R. and Buckley, B.M. 2001. Hydrometeorological reconstructions for northeastern Mongolia derived from tree rings: 1651-1995. Journal of Climate 14: 872-881.
Shao, X., Huang, L., Liu, H., Liang, E., Fang, X. And Wang, L. 2005. Reconstruction of precipitation variation from tree rings in recent 1000 years in Delingha, Qinghai. Science in China Series D: Earth Sciences 48: 939-949.
Touchan, R., Xoplaki, E., Funkhouser, G., Luterbacher, J., Hughes, M.K., Erkan, N., Akkemik, U. and Stephan, J. 2005. Reconstructions of spring/summer precipitation for the Eastern Mediterranean from tree-ring widths and its connection to large-scale atmospheric circulation. Climate Dynamics 25: 75-98.
Zhang, Q., Chen, J. and Becker, S. 2007. Flood/drought change of last millennium in the Yangtze Delta and its possible connections with Tibetan climatic changes. Global and Planetary Change 57: 213-221.
Zhao, C., Yu, Z., Zhao, Y. and Ito, E. 2009. Possible orographic and solar controls of Late Holocene centennial-scale moisture oscillations in the northeastern Tibetan Plateau. Geophysical Research Letters 36: 10.1029/2009GL040951.Last updated 23 January 2013