NSF PR 00-10 - March 22, 2000
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New Research on Long-Term Ocean Cycles Reveals Rapid Global Warming in Near Future
Scientists funded by the National Science Foundation (NSF) and affiliated with the Scripps Institution of Oceanography at the University of California, San Diego, report evidence of pronounced changes in the earth's climate that can be tracked in cycles of ocean conditions over thousands of years. These cycles reveal that Earth is currently in a period in which a natural rise in global temperatures, combined with warming from the greenhouse effect, will push the planet through an era of rapid global warming.
Charles Keeling and Timothy Whorf report in the March 21 online edition of the Proceedings of the National Academy of Sciences that strong oceanic tides are the engines behind this warming-cooling cycle. Their report is the first comprehensive study of the effects of tidal mixing on climate change over millennia. The current phase in the cycle suggests that a natural warming trend began a hundred years ago, increased in the 1970s, and should continue over the next five centuries.
"We have discovered an 1,800-year tidal cycle that appears to correspond with recent climate change," said Charles Keeling, the study's first author. "If this is a correct mechanism for understanding climate change over millennia, then temperatures will rise both because of weaker tidal mixing and because of the greenhouse effect, which is on the increase as well."
The researchers suggest that strong oceanic tides drive changes in climate due to their ability to increase vertical mixing in the ocean and thereby transport cold ocean water to the surface. The strong tides elicit cool conditions on the sea surface, which in turn lowers temperatures in air and over land, resulting in cooler climates around the planet, often accompanied by drought conditions. Weak tides lead to less cold water mixing and result in warmer periods on Earth.
Keeling and Whorf's 1,800-year cycle, which arises because of gradual changes in the astronomical alignments of the sun, moon, and earth, was proposed as an explanation for nearly periodic millennial changes in temperature seen in ice and deepsea sedimentary core records. "It becomes pretty clear that if today's natural warming trend is combined with the greenhouse effect, then we'll soon see the effect of combined warming all over the world," said Keeling.
The paper reports on the near coincidence of major tidal fluctuations with worldwide phenomena, including the Little Ice Age of 1400 A.D. to 1700 A.D., major dust layers in Minnesota lake sediments spaced about 1,800 years apart, a major drought in the Amazon Basin around 2200 B.C., and a 2000 B.C. drought that may have contributed to the collapse of Akkadia, a Mesopotamian civilization regarded as the world's first empire. The Vikings inhabited Greenland in temperate conditions in the tenth century near the end of a period of weak tidal activity, but perished or left Greenland when tides strengthened near the beginning of the Little Ice Age in the 13th century.
The study was also supported by the U.S. Department of Energy.
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