Floods in Asia
From ClimateWiki
From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
In the midst of 2002’s massive flooding in Europe, Gallus Cadonau, the managing director of the Swiss Greina Foundation, called for a punitive tariff on U.S. imports to force cooperation on reducing greenhouse gas emissions, claiming that the flooding “definitely has to do with global warming” and stating that “we must change something now” (Hooper, 2002). Cadonau was joined in this sentiment by Germany’s environment minister, Jurgen Trittin, who implied much the same thing when he said “if we don’t want this development to get worse, then we must continue with the consistent reduction of environmentally harmful greenhouse gasses” (Ibid.).
The IPCC seems to agree with Cadonau and Trittin. Its authors report “a catastrophic flood occurred along several central European rivers in August 2002. The floods resulting from extraordinarily high precipitation were enhanced by the fact that the soils were completely saturated and the river water levels were already high because of previous rain. Hence, it was part of a pattern of weather over an extended period” (IPCC, 2007-I, p. 311). While admitting “there is no significant trend in flood occurrences of the Elbe within the last 500 years,” the IPCC nevertheless says the “observed increase in precipitation variability at a majority of German precipitation stations during the last century is indicative of an enhancement of the probability of both floods and droughts” (Ibid.)
Research
In a study that covered the entire continent, Cluis and Laberge (2001) analyzed the flow records of 78 rivers distributed throughout the Asia-Pacific region to see if there had been any enhancement of earth’s hydrologic cycle coupled with an increase in variability that might have led to more floods between the mean beginning and end dates of the flow records: 1936 ± 5 years and 1988 ± 1 year, respectively. Over this period, the two scientists determined that mean river discharges were unchanged in 67 percent of the cases investigated; where there were trends, 69 percent of them were downward. In addition, maximum river discharges were unchanged in 77 percent of the cases investigated; where there were trends, 72 percent of them were downward. Consequently, the two researchers observed no changes in both of these flood characteristics in the majority of the rivers they studied; where there were changes, more of them were of the type that typically leads to less flooding and less severe floods.
Two years later, Kale et al. (2003) conducted geomorphic studies of slackwater deposits in the bedrock gorges of the Tapi and Narmada Rivers of central India, which allowed them to assemble long chronologies of large floods of these rivers. In doing so, they found that “since 1727 at least 33 large floods have occurred on the Tapi River and the largest on the river occurred in 1837.” With respect to large floods on the Narmada River, they reported at least nine or 10 floods between the beginning of the Christian era and AD 400; between AD 400 and 1000 they documented six to seven floods; between AD 1000 and 1400 eight or nine floods; and after 1950 three more such floods. In addition, on the basis of texture, elevation, and thickness of the flood units, they concluded that “the periods AD 400-1000 and post-1950 represent periods of extreme floods.”
What do these findings imply about the effects of global warming on central India flood events? The post-1950 period would likely be claimed by the IPCC to have been the warmest of the past millennium; it has indeed experienced some extreme floods. However, the flood characteristics of the AD 400-1000 period are described in equivalent terms, and this was a rather cold climatic interval known as the Dark Ages Cold Period. See, for example, McDermott et al. (2001) and Andersson et al. (2003). In addition, the most extreme flood in the much shorter record of the Tapi River occurred in 1837, near the beginning of one of the colder periods of the Little Ice Age. There appears to be little correlation between the flood characteristics of the Tapi and Narmada Rivers of central India and the thermal state of the global climate.
Focusing on the much smaller area of southwestern Turkey, Touchan et al. (2003) developed two reconstructions of spring (May-June) precipitation from tree-ring width measurements, one of them (1776-1998) based on nine chronologies of Cedrus libani, Juniperus excelsa, Pinus brutia and Pinus nigra, and the other one (1339-1998) based on three chronologies of Juniperus excelsa. These reconstructions, in their words, “show clear evidence of multi-year to decadal variations in spring precipitation,” with both wet and dry periods of 1-2 years duration being well distributed throughout the record. However, in the case of more extreme hydrologic events, they found that all of the wettest five-year periods preceded the Industrial Revolution, manifesting themselves at times when the air’s carbon dioxide content was largely unaffected by anthropogenic CO2 emissions.
Two years later, Jiang et al. (2005) analyzed pertinent historical documents to produce a 1,000-year time series of flood and drought occurrence in the Yangtze Delta of Eastern China (30 to 33°N, 119 to 122°E), which with a nearly level plain that averages only two to seven meters above sea level across 75 percent of its area is vulnerable to flooding and maritime tidal hazards. This work demonstrated that alternating wet and dry episodes occurred throughout the 1,000-year period, with the most rapid and strongest of these fluctuations occurring during the Little Ice Age (1500-1850).
The following year, Davi et al. (2006) developed a reconstruction of streamflow that extended from 1637 to 1997, based on absolutely dated tree-ring-width chronologies from five sampling sites in west-central Mongolia, all of which sites were in or near the Selenge River basin, the largest river in Mongolia. Of the 10 wettest five-year periods, only two occurred during the twentieth century (1990-1994 and 1917-1921, the second and eighth wettest of the 10 extreme periods, respectively), once again indicative of a propensity for less flooding during the warmest portion of the 360-year period.
The year 2007 produced a second study of the Yangtze Delta of Eastern China, when Zhang et al. (2007) developed flood and drought histories of the past thousand years “from local chronicles, old and very comprehensive encyclopaedia, historic agricultural registers, and official weather reports,” after which “continuous wavelet transform was applied to detect the periodicity and variability of the flood/drought series” and, finally, the results of the entire set of operations were compared with 1,000-year temperature histories of northeastern Tibet and southern Tibet. This work revealed, in the words of the researchers, that “colder mean temperature in the Tibetan Plateau usually resulted in higher probability of flood events in the Yangtze Delta region.”
Contemporaneously, Huang et al. (2007) constructed a complete catalog of Holocene overbank flooding events at a watershed scale in the headwater region of the Sushui River within the Yuncheng Basin in the southeast part of the middle reaches of China’s Yellow River, based on pedo-sedimentary records of the region’s semiarid piedmont alluvial plains, including the color, texture, and structure of the sediment profiles, along with determinations of particle-size distributions, magnetic susceptibilities, and elemental concentrations. This work revealed there were six major episodes of overbank flooding. The first occurred at the onset of the Holocene, the second immediately before the mid-Holocene Climatic Optimum, and the third in the late stage of the mid-Holocene Climatic Optimum, while the last three episodes coincided with “the cold-dry stages during the late Holocene,” according to the six scientists. Speaking of the last of the overbank flooding episodes, they note that it “corresponds with the well documented ‘Little Ice Age,’ when “climate departed from its long-term average conditions and was unstable, irregular, and disastrous,” which is pretty much like the Little Ice Age has been described in many other parts of the world as well.
The history of floods in Asia provides no evidence of increased frequency or severity during the Current Warm Period.
There is actually very ltitle doubt that the ice-core record is a good climate proxy. There are differences in detail between Antarctic and Greenland ice-cores, particularly that the latter show Heinrich events and Dansgaard-Oeschger events quite strongly. These are North Atlantic events that show up only weakly in Antarctica. There is also a phase shift between Antarctia and Greenland, often known as “the bipolar seesaw”.However the main climatic events, glaciations and interglacials and major stadials and interstadials show up in all the ice-cores at the same times and in the same proportions. The previous interglacial (MIS 5e) is always warmer than the present and followed by two cooler interstadials (MIS 5a and 5c), the next older interglacial (MIS 7) is always slightly colder than the present and multiple peaked and so on and so on. These are all features that can be checked by other data (ocean bottom drill cores, speleothems, loess profiles, cave sediments, pollen, fossils, ancient beaches, river terraces etc).And yes, all these proxies agree that most of this interglacial was significantly warmer than the present. The warm peak was reached quite early and the temperature has on the whole been declining since then (this seems to be a general feature of interglacials). But the decline has not been even. There has been warmer rallies (like the MWP) two quite sharp cold dips 9600 and 8200 years ago, and a downward step change about 4200 years ago.That the early and middle Holocene was markedly warmer than the present is nothing new. This has been known and universally accepted for a century ever since palynology was invented. It is as a matter of fact quite obvious if you are familiar with the fossil record and biogeography.
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