Changes in Dryness/wetness in China during the last 529 years.
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Historical written records of weather conditions that affect agriculture and living conditions in China can serve as a proxy for instrumental observations of the relative wetness and dryness, or precipitation amounts, for periods of time dating back to at least AD 1470. The dryness:wetness index (DW) data at 100 selected sites and at a subset of 25 sites with entirely continuous data indicate that during the growing season the number of sites with dryness increased rapidly in the 20th century, in contrast to a gradual increase and leveling off of wetness since the 18th century. Dry conditions existed mostly in northeastern China in the 16th and 17th centuries. In the 20th century, dryness prevailed in most areas in China, and the chance of occurrence at each site increased. Wet conditions have expanded from east coastal areas to inland since the 19th century. As a result of increased dryness and wetness, the number of sites experiencing normal precipitation conditions has dropped dramatically. The number of sites with extreme dryness: wetness has also increased during the 20th century, and the spatial distribution of the sites with a large chance of extreme dryness:wetness has also changed. The frequent occurrences in dryness:wetness as well as in extremes in the 20th century seem indicative of an abnormality in climate on a large scale. Spectral analyses of the DW data has revealed that cycles in dryness:wetness and their extremes have existed with periods on relatively short time scales as well as on the long time scales. The possible existence of a century-scale cycle suggests that caution should be used in relating variations in rainfall conditions to global warming.