- Distinct changes in Earth's climate can be tracked in
cycles of ocean conditions over thousands of years, report scientists from
Scripps Institution of Oceanography at the University of California.
- These cycles reveal that Earth is currently experiencing
a natural rise in global temperatures and this, combined with warming from
the greenhouse effect, will push the planet through an era of rapid global
- Strong oceanic tides are the engines behind this warming-cooling
cycle that may help determine future climate change, according to Charles
Keeling and Timothy Whorf of Scripps. Their research was published in the
March 21 online edition of the Proceedings of the National Academy of Sciences.
- "The greenhouse effect is probably the best known
of those factors influencing climate, but up to now any role played by
tidal mixing of the surface ocean has not been included in future scenarios,"
said Whorf. "The greenhouse effect may be the larger of these two
effects, but it is important to know how much of future warming might be
natural or manmade."
- Their research is the first comprehensive study of the
effects of tidal mixing on climate change spanning a 1,000-year period.
The current phase in the cycle suggests that a natural warming trend began
100 years ago, picked up in the 1970s, and should continue over the next
- 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, suggest the researchers. 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.
- The position of the Earth in relationship to the moon
impacts tidal forces
- "The 1,800-year (cycle) arises when tidal forces
become maximized near optimal alignment and closest approach of the sun
and moon with the Earth. Slow changes in the orbital parameters which control
how strong the tidal effect might be, bring about the 1,800-year cycle,"
said Whorf. "Such astronomical changes also occur on shorter time
scales, such as 90 and 180 years, causing episodes of cooling and warming
depending upon whether the tidal forcing is stronger or weaker."
- "One such episode of cooling during the period 1940-1975,
when tidal forcingwas stronger, may have temporarily masked the appearance
of the greenhouse effect in global temperatures, and contributed to the
controversy of whether greenhouse warming was occurring at all," said
- "If that is true, then 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,"
- In addition to offering clues to climate change, the
research also shows a new mechanism for analyzing events in world history.
- The paper reports on the near coincidence of major tidal
fluctuations with worldwide phenomena, including the Little Ice Age of
1400-1700 A.D., major dust layers in Minnesota lake sediments spaced about
1,800 years apart and a major drought in the Amazon Basin around 2200 B.C.
It also could explain a 2000 B.C. drought that may have contributed to
the collapse of Akkadia, a Mesopotamian civilization regarded as the world's
- The Vikings inhabited Greenland in temperate conditions
in the 10th 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.
- "One of the principle benefits of the tidal hypothesis
is that researchers can compare the timing of specific historical events
with predicted times of warming or cooling to see whether they coincide
or not," said Whorf, a research associate in the Geosciences Research
Division of Scripps. "If we are correct, then the 1,800-year tidal
cycle will be important in understanding future climates as well as events
of the past."
- The National Science Foundation and the U.S. Department
of Energy also supported the study. Copyright 2000, Environmental News
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