November 13, 2007

Centuries of Yellow River Climate

Filed under: Droughts, Floods, Precipitation

According to 1,000s of websites trumpeting the horrors of global warming, we find countless claims that the ongoing build-up of greenhouse gases is causing droughts and floods all over the world. Hardly a week goes by nowadays without a front page news story about some weather or climate calamity occurring somewhere on the planet, and global warming is repeatedly claimed to be the cause. Turn the radio to NPR for an hour or so, and you will certainly be told how the failure of the United States to sign the Kyoto Protocol has caused some disastrous flood or drought.

Several articles have been published recently allowing us to catch a glimpse of centuries of climate variations in China, and as we have seen in hundreds of other similar studies, nothing all that unusual has been happening lately. The first of these recent articles will soon appear in Climate Dynamics and was written by the team of Shen, Wang, Hao, and Gong of the Atmospheric Sciences Research Center of the State University of New York. They note that “In recent decades, eastern China has suffered increased droughts in its north and increased floods in its south. The studies of climate models suggested that this trend could probably be attributed to the climate effects of black carbon aerosols and human-induced land cover changes”. Holy smokes – black carbon and land use? Shen et al. apparently haven’t been listening to enough NPR for surely drought and floods in China are related to global warming!

The Chinese National Meteorological Administration regularly publishes the dryness/wetness index (DWI) for stations throughout China, and the DWI records are now available for the past 530 years. The DWI is based on the rainfall in the summertime rainy season with values ranging from -2 for “very dry” to +2 for “very wet.” Shen et al. used the records to calculate areas on northeastern China (NC) experiencing varying levels of wetness and dryness; their study area included the entire drainage area of the Yellow River.

The plots below (Figure 1) show the results, we have stared at them for hours (it was a long plane trip), and we still do not see anything out of the ordinary in the most recent 100 or so years. Shen et al. note that “Among these flood years, 1964 with a regional DWI of 1.54 was the wettest year, when the highest summer rainfall in the period from 1951 to 2000 (40% more than that 50-year average) was recorded and 85.8% of NC suffered very wet conditions. Among the droughts, 1640 was the driest year. The regional DWI in 1640 was as low as -1.9, and 93.6% of NC experienced very dry conditions. A noted hydrological event of that year was the desiccation of the Yellow River and some of its tributaries such as the Fenhe River, Xinhe River, and Yihe River.”


Figure 1. Plots of 530 years of the dryness/wetness index for northern China (from Shen et al., 2007)

The second recent article was published in Water Resources Research by a team of scientists from China’s Lanzhou University and the Tree-ring Laboratory of Lamont-Doherty Earth Observatory at Columbia University in New York. Guo et al. begin stating “Annual streamflow of the Yellow River has decreased in recent years (1980 to 2000) because of climate change and human activity. This decrease affects the environment and the lives of the people in the drainage area.” Furthermore, we learn that “As the main water resource for large parts of China, the Yellow River streamflow contributes 2% of the total water resource of China and supplies 15% of the water used for agricultural production and 12% of the water used by the population of the country.”

The scientists cored trees in the headwaters of the Yellow River and statistically linked the ring widths to historical flows of the river. The tree rings extend back 593 years, and just like magic, the trees allow for a very accurate reconstruction of the total flow of the all-important Yellow River. Examining the plot below (Figure 2), and we just don’t find anything curious going on recently that hasn’t occurred in the past. The Guo et al. team note that “There are several particularly dry periods indicated during the past 593 years, namely, 1480–1490s, 1590–1600s, 1700–1710s, 1820–1830s, and 1920–1930s” and that “[t]he reconstruction demonstrated that 1480–1490 is the driest decade.” Further, they note that “The streamflow at Tangnaihai increased during much of the twentieth century. However, since the 1980s, Yellow River streamflow has decreased significantly. Nevertheless, it is not yet out of the range of earlier streamflow fluctuations reconstructed for the past several centuries.”


Figure 2. Reconstructed streamflow of the Yellow River over the past 593 years. The thin grey line is the reconstructed annual streamflow, and the imposed bold black line is the 11-year moving average. The average during the reconstruction period (1409–2001) is also shown as straight dashed line (from Guo et al., 2007).

The Yellow River region will undoubtedly experience floods and droughts in the future, and when they occur, they will immediately be blamed on global warming (and possibly black carbon and land use). Of course, if we look at the facts, we discover that droughts and floods are a natural part of the climate of the Yellow River area, and we see no evidence of any increase in drought or flood severity or frequency in the past century.

Referecnes:

Gou, X., F. Chen, E. Cook, G. Jacoby, M. Yang, and J. Li. 2007. Streamflow variations of the Yellow River over the past 593 years in western China reconstructed from tree rings. Water Resources Research, 43, W06434, doi:10.1029/2006WR005705.

Shen, C., W.-C. Wang, Z. Hao, and W. Gong. 2007. Characteristics of anomalous precipitation events over eastern China during the past five centuries. Climate Dynamics, DOI 10.1007/s00382-007-0323-0.




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