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
"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
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