April 24, 2008

Floods and Droughts and Global Cooling?

In nearly every presentation on global warming, we hear that floods and droughts will be more severe as the temperature rises. Believe it or not, and who would not believe it given thousands of websites on the issue, there are many scientists who believe the opposite. We have covered these topics in many previous essays, and a recent article in Quaternary Science Reviews reinforces our skeptical viewpoint.

The research was conducted by Chun Chang Huang and five associates from China’s Shaanxi Normal University. Their goal was to reconstruct major flooding events of the Sushui River (Figure 1) during the Holocene period (the Holocene began approximately 12,000 years ago when the last great glacial period ended).

Huang et al. explain “In semiarid zones, the piedmont alluvial plains are episodically inundated by overbank floodwater during heavy rainstorms.” They also state “However, in the semiarid loess region in the middle reaches of the Yellow River drainage basin, the elevation of alluvial plains also increases through dust accumulation, which is commonly followed by soil formation that together separate fluvial deposits produced during episodes of overbank flooding. Fine-grained slackwater deposits of the floods are therefore preserved between the architecture of loess and soils which also protect these flood deposit from subsequent erosion by overland water flows, wind erosion, and human activities.” Basically, they found a geomorphic sequence that beautifully preserves datable information about major floods over the past 12,000 years. They write “Thus, these loess–soil sequences provide unique information for investigation of Holocene climatic change, flood hydrology, geomorphic and pedogenic changes, and human impact in semiarid zones. This stratigraphic data can provide valuable hydrologic information to those working in engineering hydrology, flood hazard prevention and mitigation, geomorphology, Quaternary sciences and global change.”


Figure 1. Map showing the location (small square) of the Huang et al. (2007) study area

The authors note that “Field investigations were carried out in the study area during 2002–2006. Many pedo-sedimentary profiles observed along the steep gully banks and in road cuts and archaeological prospecting trenches on the flat land in the lower part of the piedmont alluvial plain contain evidence of deposition associated with extreme floods.” They conducted a surprisingly complex set of analyses, and they found six periods over the past 12,000 years when large floods were frequent. Huang et al. state “During the Holocene, there are six episodes of overbank flooding recorded over the alluvial plain. The first occurred at 11,500–11,000 a BP, i.e. the onset of the Holocene. The second took place at 9500–8500 a BP, immediately before the mid-Holocene Climatic Optimum. After an extended geomorphic stability and soil formation, the third overbank flooding episode came at about 3620–3520 a BP, i.e. the late stage of the mid-Holocene Climatic Optimum, and the floodwater inundated and devastated a Bronze-age town of the Xia Culture built on the alluvial plain, and therefore the town was abandoned for a period of ca 100 years. During the late Holocene, the alluvial plain experienced three episodes of overbank flooding at 2420–2170, 1860–1700 and 680–100 a BP, respectively.”

OK – so what, right? As it turns out, all six episodes occurred during cool periods, not warm ones. The first two occurred early in the Holocene and prior to the warm mid-Holocene Climatic Optimum period. Huang et al. explain that “during the early Holocene summers became hotter while winters got colder in mid-latitude of the north hemisphere in response to orbital forcing. The increased seasonal contrast must have probably intensified dust storms and dust falls during winter–spring in the middle reaches of the Yellow River, and the rapidly accumulated dust would be expected to experience only slight modification by weathering. On the other hand, such an increased seasonal contrast probably intensified heavy rainstorms in summer–autumn and resulted in overbank flooding on the tributaries and the mainstream of the Yellow River in response to great overland surface runoff on the gentle loess lands.” Their data “show that there was an absence of overbank flooding over the piedmont alluvial plain during this mid-Holocene Climatic Optimum.” Very interesting – during the warm Climatic Optimum, the flooding appears to have stopped.

Their discussion of flooding and usually cold period continued for the other episodes; they state “The overbank flooding episodes temporally overlap with the cold-dry stages. It implies that during these three episodes both extreme floods and droughts occurred in the tributaries and the mainstream of the Yellow River in its middle and lower reaches.” They found flooding during the Little Ice Age stating “During the sixth episode of overbank flooding on the piedmont alluvial plain, 73 out of 110 catastrophic floods on the Sushui River in the Yuncheng Basin during the last 2200 years occurred between the 14th and 19th centuries. Extreme overbank flooding on the Sushui River occurred in AD 1570, 1662, 1745 and 1761, each resulting in hundreds of fatalities and inundation and devastation of thousands of villages and towns.” Furthermore, they found “Our data show that the last three episodes of overbank flooding, including the catastrophic floods recorded in literature, coincide with the cold-dry stages during the late Holocene. During these three episodes there were not only catastrophic floods, but also extreme droughts over the middle and lower reaches of the Yellow River drainage basin. For example, the last episode of overbank flooding event corresponds with the well documented ‘Little Ice Age,’ during which there were frequent natural disasters including catastrophic floods, droughts, dust storms, heat waves, migratory locusts and frequent famines and plagues in the middle-lower reaches of the Yellow River drainage basin. Climate departed from its long-term average conditions and was unstable, irregular, and disastrous during these anomalous episodes.” They conclude “The persistence of geomorphic stability on the piedmont alluvial plain and the absence of overbank flooding during the mid-Holocene Climatic Optimum provide evidence that extreme floods were uncommon in the warm-humid period dominated by the southeastern maritime monsoon.”

We get the message – cold periods in China bring catastrophic floods, droughts, dust storms, heat waves, migratory locusts and frequent famines and plagues while warm periods bring a reprieve from the misery. We seriously doubt Huang et al.’s research will be very popular with climate alarmists, but their work certainly suggests that cooling should be avoided at all costs and warming can be seen as a blessing.

Reference:

Huang, C.C., J. Pang, X. Zha, H. Su, Y. Jia, and Y. Zhu. 2007. Impact of monsoonal climatic change on Holocene overbank flooding along Sushui River, middle reach of the Yellow River, China. Quaternary Science Reviews, 26, 2247–2264.




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