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NYT: In Ancient Ice Ages, Clues to Climate
All right, not topical, but perhaps of interest.
Copyright The New York Times; acknowledged.
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February 16, 1999
In Ancient Ice Ages, Clues to Climate
By WILLIAM K. STEVENS
More often than not over the last million years, the earth has been
locked in the deep cold of ice ages.
In the frigid depths of the most recent of these glaciations, which
lasted about 100,000 years and ended about 10,000 years ago, great
sheets of ice buried much of Europe and North America, including New
York, Chicago and everything to the north. In its expansion phase, the
ice sometimes advanced so fast that it bulldozed forests in its path.
Most experts believe the ice will come again, as surely as the earth
turns on its axis and revolves around the sun.
It will crush cities, freeze great stretches of northern lands and
suck up so much of the world's water that global sea levels will drop
by hundreds of feet. In some spots, the Northeast Coast will be as
much as 100 miles east of where it is now, as it was during the last
People will survive just as they did then, but the warm, salubrious,
all-too-brief interval in which civilization flowered will be over.
The question is: When?
Until recently, scientists who study past glaciations for clues to the
future thought the present warm period was nearing its end. Though
global warming from the emission of heat-trapping greenhouse gases
might complicate the situation, they believed that an uneven slide
into a new glaciation could begin at almost any time.
"Now most people have dropped that view," said Dr. Lloyd Burckle, a
paleo oceanographer at Columbia University's Lamont-Doherty Earth
Observatory, who is an expert on ancient climates.
New evidence has led scientists to believe that the present warm
period could last another 10,000 to 20,000 years, or even longer.
They say the new evidence, in the form of chemical tracers of past
climates contained in deep-ocean sediments, suggests that previous
warm periods lasted longer than had been thought.
In particular, a warm interval some 400,000 years ago appears to have
endured for 30,000 years or more.
Scientists call it Stage 11, after its designation in the standard
record of glacial-interglacial cycles discovered through study of the
deep-sea sediment tracers.
In some critical aspects, Stage 11 appears to have more in common with
today's interglacial, denoted as Stage 1 and known as the Holocene
period, than do other warm periods of the past.
Dr. David Hodell, a paleooceanographer at the University of Florida,
who has produced some of the new evidence on Stage 11, said, "If Stage
11 is a good analog and lasted 30,000 years, we might expect that the
present interglacial will continue for another 20,000 years."
Further evidence also suggests that the most recent warm period before
the Holocene began about 130,000 years ago and lasted some 20,000
years, twice as long as had been thought -- and twice as long as the
Holocene has endured so far.
That interglacial, sometimes called the Eemian period, appears to have
had two distinct phases.
One, roughly the first 12,000 years, was warm and stable.
The second half of the period saw a gradual growth of ice sheets in
the North Atlantic region -- but the climate of the region apparently
stayed warm for another 8,000 years or so before descending fitfully
into a new glaciation.
The evident lag between the growth of ice sheets and their impact on
climate is an indication that even if the earth's overall climate
stays warm for another 10,000 to 20,000 years, there may still be
severe weather changes. For instance, the intensity of storms in the
temperate zones is determined by the contrast in temperature between
clashing warm and cold air masses.
In the latter part of the last interglacial, some scientists theorize,
the juxtaposition of cold, growing ice sheets with a still-warm ocean
could have produced monster storms.
How the climate behaved in the earlier warm intervals is one of many
mysteries that today plague and fascinate those who study the ice ages.
The conundrum is complicated all the more by the potential impact of
humans' burning of fossil fuels like coal, oil and natural gas, which
produces the heat-trapping atmospheric gas carbon dioxide.
According to a calculation by Dr. James F. Kasting of Pennsylvania
State University, an expert on geochemical processes and ancient
climate, fossil fuels would be used up in about 800 years at present
rates of burning. If that happened, he said, there would be 4 to 8
times as much carbon dioxide in the atmosphere as in pre-industrial
times, and it would raise the earth's average surface temperature by 8
to 27 degrees Fahrenheit.
"It looks like we would completely break the glacial-interglacial
cycle," he said.
That much warming would probably melt all or most of the ice on earth,
raising sea level to heights unknown in the last 65 million years or
so and wiping out most of today's coastal zones, where about half of
It would probably make the tropics uninhabitable and return the
world's climate to conditions that prevailed in the era of the
dinosaurs, when crocodiles lived in the Arctic.
"It's hard to believe we'd ever be that stupid," said Dr. Wallace S.
Broecker, a geochemist at Lamont-Doherty. On the other hand, there has
so far been little progress in reducing the use of fossil fuels.
The peak of whatever global warming might eventually result would come
well before the end of the Holocene if, as scientists now suspect, it
still has thousands of years yet to run.
The dominant view among scientists is that if greenhouse gas emissions
are not reduced, the average global temperature will rise by 2 to 6
degrees Fahrenheit, with a best estimate of 3.5 degrees, over the next
century -- and that further warming would take place after that if
By comparison, the average temperature has risen by 5 to 9 degrees
since the depths of the last glaciation some 20,000 years ago.
Some scientists believe the current glacial-interglacial cycle was set
in motion more than 2.5 million years ago by the gradual closing of
the isthmus of Panama, which re-routed oceanic currents that carry
water -- and heat -- around the globe.
Others say the cause was the rise about then of the Himalayas,
resulting in a realignment of atmospheric circulation. In any event,
the changes had vast repercussions. Until about a million years ago,
the alternating glacial and interglacial cycles were of roughly even
Since then, the glacial cycles have dominated.
The reigning theory about what sets the timing of the
glacial-interglacial oscillations says it involves periodic changes in
the earth's orbit and its position relative to the sun.
In one type of periodic change, the angle of tilt in the earth's axis
varies over periods of about 41,000 years.
In another, the magnitude of a wobble in the earth's rotation about
that axis (much like that of a spinning top as it slows down) changes
over periods of 19,000 and 23,000 years. In a third cycle, the shape
of the earth's orbit varies, from more circular to more elliptical,
over a period of 100,000 years.
In theory, the overlapping effect of the three orbital cycles alters
the angles and distances from which sunlight strikes the far northern
latitudes of the earth.
When less sunlight falls there, less snow melts in summer and over
time is compressed to form growing continental ice sheets. When more
sunlight falls, the ice melts back.
This elegant theory has been confirmed, in its adherents' view, by
studies of the relative abundance of different forms of oxygen
preserved in the fossilized shells of tiny marine animals called
foraminifera, or forams, in deep-sea sediments around the world.
>From the foram record, scientists worked out a timetable of the waning
and waxing of ice sheets.
They found that it neatly matched the timetable calculated by
astronomers on the basis of the earth's orbital cycles.
Later evidence, however, has suggested that the coming and going of
the ice is sometimes out of phase with the global or regional climate.
That is, it might remain warm for some time even as ice sheets grow,
as in the Eemian period.
Dr. George Kukla of Lamont-Doherty, Dr. Jerry McManus of the Woods
Hole Oceanographic Institution in Woods Hole, Mass., and others have
documented this lag by studying ocean sediments in the North Atlantic.
They looked for concentrations of gravelly debris dropped by melting
sea ice and fossils of a species of foram that is sensitive to
The researchers found that after the Eemian interglacial had lasted
some 12,000 years, a warm climate persisted in the North Atlantic
region for about another 8,000 years, even as the ice concentrations
continued to grow.
Dr. Kukla says that in the last half of the last interglacial, the ice
probably built up gradually in volume, but not necessarily in area,
while the ocean and the continental temperate zones remained warm.
Then the cold moved south, probably in a series of pulses.
Dr. McManus says the North Atlantic region may have stayed warm for so
long because great ocean currents that transport heat into the region
When the currents weakened, possibly because their salt content was
diluted at a critical juncture by fresh water from melting icebergs, a
global glaciation began.
It lasted some 100,000 years, ending about 10,000 years ago.
If the Holocene behaves like the last interglacial, Dr. Kukla and Dr.
McManus say, then the next onset of the cold associated with an ice
age lies thousands of years in the future.
But a number of experts are beginning to believe that the length of
the Stage 11 interglacial may be a better predictor of how long the
Holocene will last.
As Dr. Hodell explains, the shape of the earth's orbit in Stage 11 was
more circular then and is similarly so today.
The planet's spinning-top wobble was also less pronounced then and is
Together, the two cycles are allowing less solar radiation to reach
far northern latitudes in the summer, meaning that less ice is melting
each year as a result of the sun's direct action than melted 9,000
years ago: The die is cast for the next glaciation, but according to
the new thinking, the world will stay warm for about 10,000 to 20,000
Much more must be learned before scientists can be confident of what
happened in Stage 11, and what that might portend for humanity. Was it
warm enough then to melt the ice caps, as some scientists believe? How
did the climate vary over the 20 or 30 millenniums that Stage 11
apparently lasted? Firm answers will come only with difficulty, if
"If it was a nice neat story," says Dr. McManus, "we'd have known it a
long time ago and would have moved on to something else."
* * *
"Language is a virus from outer space."
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