 |
 cientists at 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
(PNAS) that strong oceanic tides are the engines behind this
warming-cooling cycle that may help determine future climates.
This report is the first comprehensive study of the effects of
tidal mixing on climate change spanning millennia. The current
phase in the cycle suggests that a natural warming trend began
a hundred years ago, picked up in the 1970s, and should continue
over the next five centuries.
 "We
have discovered an 1,800-year tidal cycle that appears to match
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 deep-sea sedimentary core records.
Previously (1997), they have reported on the effects of shorter
cycles of tidal forcing on global temperature at periods near
18, 90, and 180 years.
A maximum
in tidal cooling near 1974 might have produced more cooling,
but perhaps was masked by a simultaneous greenhouse warming,
according to Keeling, a professor of oceanography at Scripps.
"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,"
said Keeling. In addition to climate change recognition, the
research also represents 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 AD
to 1700 AD, major dust layers in Minnesota lake sediments spaced
about 1,800 years apart, a major drought in the Amazon Basin
around 2200 BC, and a 2000 BC 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.
"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 study
was supported by the National Science Foundation and the US Department
of Energy. 
|
| Scripps
Institution of Oceanography, at the University of California,
San Diego, is one of the oldest, largest, and most important
centers for global science research and graduate training in
the world. The National Research Council has ranked Scripps first
in faculty quality among oceanography programs nationwide. The
scientific scope of the institution has grown since its founding
in 1903 to include biological, physical, chemical, geological,
geophysical, and atmospheric studies of the earth as a system.
More than 300 research programs are under way today in a wide
range of scientific areas. The institution has a staff of about
1,300, and annual expenditures of approximately $100 million,
from federal, state, and private sources. Scripps operates the
largest academic fleet with four oceanographic research ships
for worldwide exploration and one research platform. Scripps
Institution of Oceanography on the World Wide Web: www-sio.ucsd.edu. |
