National Geographic : 2003 Jan
Forum September 2002 Many readers of "State of the Planet" and "Water Pressure"blame over populationfor dwindling natural resources. "We're told to conserve," wrote one reader. "But conservefor what? So a burgeoningpopulation can outstrip our most valiant efforts?" Others wonder if the threat of overpopulationis exaggerated,citing recent reportsthatfertil ity rates have stabilized-oreven dropped-inparts of the world. Shrinking Great Lakes You provided water level data that only go up to 2000, which we all remember as a "low" year. But you provide only a dismis sive admission that water levels have gone up since then. Public data show that in early 2002 Lakes Huron and Michigan were about 0.4 meters [1.3 feet] below their long-term historic average but above their all-time low. In August they were up about 0.3 meters from 2001. Lake Superior has tracked just 0.1 meters below its long-term average throughout 2002. Same for Lake St. Clair, the "poster child" for low lake levels. The only conclusion from over 140 years of data on water levels is that the water levels are variable. They always have been, and they always will be. There is no evidence to suggest that there is a long-term trend down ward that can be extrapolated into the future. EDWARD M. KARLS Ann Arbor, Michigan MEMBERSHIP Please call 1-800-NGS-LINE (1-800-647-5463). Special device for the hearing-impaired (TDD) 1-800 -548-9797. Online: nationalgeographic.com/ngm AOL Keyword: NatGeoMag There are natural patterns to the rise and fall of the lakes, but as a Michigan resident I can say with confidence: Recent changes aren't natural. I've lived here since 1961, and there was a per ceptible change in the weather patterns in the second half of the 1980s. The summers are about the same, but winters have largely stopped coming or are mild. Without snow and ice these northern lakes are doomed to shrink and dry. JACK WRIGHT Waterford, Michigan FROM OUR ONLINE FORUM nationalgeographic.com/ngm/0209 The author attributes declining water levels to decreased precipi tation, increased evaporation, and high consumption. These primary causes are exacerbated by continued uplift of the local topography. When vast con tinental glaciers overlaid the region, the great weight of the ice depressed Earth's crust by up to 300 meters. When the glaciers retreated 11,000 years ago, the area began the slow process of isostatic rebound, which contin ues today. Rebound rates in the region are as high as a quarter centimeter per year-small numbers that are significant NATIONAL GEOGRAPHIC * JANUARY 2003 NATIONAL GEOGRAPHIC SOCIETY "For the increaseand diffusion of geographic knowledge." The National Geographic Society is chartered in Washington, D.C., as a nonprofit scientific and educational organization. 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