Drought And Deluge
While warmer temperatures will bring an increase of rainfall on the average, theory as well as an increasing number of daily weather reports strongly indicate that changes in precipitation patterns may vary widely. Such changes will be highly uneven and sometimes damaging in intensity. Both droughts and deluges are likely to become more severe. They may even alternate in some regions, with deluges followed by drought and vice versa.
By 2000, the hydrological cycle (indicated by precipitation patterns) seemed to be changing more rapidly than temperatures. With sustained warming, usually wet places often seemed to be receiving more rain than before; dry places were often experiencing less rain and subject to more persistent droughts. Some drought-stricken regions occasionally were doused with brief deluges that ran off earth cracked by drought. In many places, the daily weather was increasingly becoming a question of drought or deluge.
On July 21, 2007, for example, D'Hanis, near San Antonio in Texas, was severely flooded after 17 inches of rain fell in 12 hours. The same day, locations in southern and central England witnessed their worst flooding on record after a month's worth of rain, as much as 5 inches, fell in one hour, following the wettest June in England's history. One such location was Shakespeare's Stratford-upon-Avon, which led some residents to remark that it had become Stratford-under-Avon. The floods displaced thousands of people and fouled the water for half a million. Once-in-a-century rains also flooded large parts of China, destroying 3.6 million homes and killing at least 500 people. At the same time, the Western United States was scorched by a record number of wildfires provoked by heat and drought.
Andrew Revkin of The New York Times summarized the situation: "A warmer world is more likely to be a wetter one, experts warn, with more evaporation resulting in more rain, in heavy and destructive downpours. But in a troublesome twist, that world may also include more intense droughts, as the increased evaporation parches soils between occasional storms" (Revkin, 2002, A-10). "In a hotter climate, your chances of being caught with either too much or too little are higher," said John M. Wallace, a professor of atmospheric sciences at the University of Washington (Revkin, 2002, A-10).
Even as rains inundated some places, the percentage of Earth's land area affected by serious drought more than doubled between the 1970s and the early 2000s, according to an analysis by the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. Increasing drought occurred over much of Europe and Asia, Canada, western and southern Africa, and eastern Australia. Rising global temperatures appear to be a major factor, said NCAR scientist Aiguo Dai (Dai et al., 2004, 1117). By 2007, Australia was suffering its worst drought in recorded history. The conditions were so dry that irrigation to some farms was being cut off, threatening their owners' livelihood.
Dai and colleagues found that the proportion of land areas experiencing very dry conditions increased from a 10 to 15 percent range during the early 1970s to about 30 percent by 2002. Almost half of that change was due to rising temperatures rather than decreases in rainfall or snowfall (Dai et al., 2004, 1117). "These results point to increased risk of droughts as human activity contributes to global warming," said Dai (2004, 1117).
The same patterns also hold for snowfall. "Lake-effect" snow on the southern and western shores of Lake Ontario in New York State can be a dramatic example. Oswego County, New York, received almost no lake-effect snow during December 2006 or January 2007 but was buried under more than 110 inches in seven days during February, as a relentless cold wind crossed an unfrozen Lake Ontario. Within eight days, some areas near Oswego had 10 feet of snow. In nearby Redfield, the National Weather Service reported that 141 inches had fallen in 10 days, a state record for a single storm event—the lake-effect championship, spurred by very cold air traversing relatively warm lake water, then colliding with colder earth that forced it to suddenly condense. In earlier years, the lake's surface usually froze by January, cutting off most of the snowfall. In Buffalo, New York, which is well known for its bursts of heavy snow, storms have been becoming more intense in recent years for the same reason. By the end of the century, most of Buffalo's snow may change to lake-effect rain. New York City's highest recorded snowfall in Central Park, is 26.9 inches, recorded, February 12, 2006.
P. C. D. Milly, writing in Nature about an increasing risk of floods in a changing climate, said, "We find that the frequency of great floods increased substantially during the twentieth century. The recent emergence of a statistically significant positive trend in risk of great floods is consistent with results from the climate model, and the model suggests that the trend will continue" (Milly et al., 2002, 514-515). The World Water Council report compiled statistics indicating that between 1971
and 1995, floods affected more than 1.5 billion people worldwide, about 100 million people a year. An estimated 318,000 people were killed and more than 18 million left homeless during the quarter-century. The economic costs of these disasters rose to an estimated US$300 billion in the 1990s from about US$35 billion in the 1960s (Greenaway, 2003, A-5).
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