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Columbia Scientist Warns That Global Warming
Could Trigger Collapse of Ocean Currents
Halt to Gulf Stream and Other Currents Could Freeze Europe;
Dublin Would Share Spitsbergen's Icy Climate
On the eve of the international meeting on global warming that opens Dec. 1
in Kyoto, Japan, one of the world's leading climate experts warned of an
underestimated threat posed by the buildup of greenhouse gases - an abrupt
collapse of the ocean's prevailing circulation system that could send temperatures
across Europe plummeting in a span of 10 years.
If that system shut down today, winter temperatures in the North Atlantic
region would fall by 20 or more degrees Fahrenheit within 10 years. Dublin would
acquire the climate of Spitsbergen, 600 miles north of the Arctic Circle.
"The consequences could be devastating," said Wallace S. Broecker,
Newberry Professor of Earth and Environmental Sciences at Columbia
University's Lamont-Doherty Earth Observatory, and author of the new research,
which appears in the Nov. 28 issue of the magazine Science.
A complex of globally interconnected ocean currents, collectively known as
the Conveyor, governs our climate by transporting heat and moisture around the
planet. But the Conveyor is delicately balanced and vulnerable, and it has shut
down or changed direction many times in Earth's history, Professor Broecker
reports. Each time the Conveyor has shifted gears, it has caused significant
global temperature changes within decades, as well as large-scale wind shifts,
dramatic fluctuations in atmospheric dust levels, glacial advances or retreats and
other drastic changes over many regions of the Earth, he said.
The Conveyor "is the Achilles heel of the climate system," Professor
Broecker wrote in Science. "The record . . . indicates that this current has
not run steadily, but jumped from one mode of operation to another. The changes
in climate associated with these jumps have now been shown to be large, abrupt
The ongoing accumulation of heat-trapping industrial gases blanketing the
Earth threatens to raise global temperatures, he said, but such a rise would occur
gradually. Far more worrisome is the buildup's potential to stress the climate
system past a crucial threshold that would disrupt the Conveyor and set off a
rapid reconfiguration of Earth's climate, predicted by existing computer models.
Professor Broecker also offered a new theory: Scientists generally agree
that periodic changes in Earth's orbit and the amount of solar radiation it receives
have paced fundamental climate changes on the planet over millions of years.
But the global climatic flip-flops may have been set in motion by sudden switches
in the operation of the Conveyor, he said.
Today, the driving force of the Conveyor is the cold, salty water of the North
Atlantic Ocean. Such water is more dense than warm, fresh water and hence
sinks to the ocean bottom, pushing water through the world's oceans like a great
plunger. The volume of this deep undersea current is 16 times greater than the
flow of all the world's rivers combined, Professor Broecker said, and it runs
southward all the way to the southern tip of Africa, where it joins a watery
raceway that circles Antarctica. Here the Conveyor is recharged by cold, salty
water created by the formation of sea ice, which leaves salt behind when it freezes.
This renewed sinking shoves water back northward, where it gradually warms
again and rises to the surface in the Pacific and Indian oceans.
In the equatorial Indian Ocean, surface waters are too warm to sink.
Northern Pacific waters are cold, but not salty enough to sink into the deep. This
is primarily because prevailing winds that whip around the planet hit the great
mountains of the western United States and Canada and drop their moisture.
The resulting snow and rain runs into the Pacific, adding a dose of fresh water
that dilutes the Pacific's saltiness, said Professor Broecker, a geochemist at
Lamont-Doherty, Columbia's earth science research institute in Palisades, N.Y.
Northern Atlantic surface waters have only about 7 percent more salt than
their counterparts in the northern Pacific, but that is just enough to reach the
threshold that allows them to sink. But if the North Atlantic waters warmed by
only a few degrees, or if they were diluted by just a bit more fresh water from
melting glaciers and sea ice or more rainfall, for example, the threshold would
not be achieved, and the waters would not sink. Computer models that simulate
the Earth's climate system show that the ocean's so-called thermohaline
circulation (from the Latin for "heat" and "salt") is sensitive to such small
changes, Professor Broecker said. The entire Conveyor might shut down or
rearrange in a different pattern, with serious effects on global climate, he said.
Today, the Conveyor comes full circle, eventually propelling warm surface
waters, including the Gulf Stream, back into the North Atlantic. In winter
months, this warm water transfers its heat to the frigid overlying air masses that
come off ice-covered Canada, Greenland and Iceland. Thus tempered, the
eastward-moving air masses make northern Europe noticeably warmer in winter
than comparable latitudes in North America. Without the Gulf Stream, nothing
would temper the Arctic air, and Europe would enter a deep freeze.
In recent years, evidence has mounted that the Earth frequently has
experienced rapid, large-scale climate changes. Greenland ice cores have shown
that during the last ice age Earth's climate switched back and forth every few
thousand years between periods of intense and moderate cold, with the transitions
occurring on a timescale of a few decades to as little as a few years. Each interval
of intense cold was matched by the launching of great armadas of icebergs in the
North Atlantic, seen in ocean sediment cores, and a great influx of dust into
Earth's atmosphere, indicating a pronounced change in wind and storm
patterns. Wetlands in tropical areas and mountain glaciers in Chile and New
Zealand expanded and shrank in synchrony with the North Atlantic changes.
There is also strong evidence, from tropical latitude glaciers, that the water
vapor content of Earth's atmosphere can shift, too. Water vapor is the most
abundant greenhouse gas in Earth's atmosphere and a marked reduction would
lower air and ocean temperatures significantly.
"Although the exact linkages that promote such climate changes have yet to
be discovered, a case can be made that their roots must lie in the ocean's large-
scale thermohaline circulation," Professor Broecker said. The most telling clue is
that the boundaries that mark climate changes in continuous sediment or ice core
records are sharp, not gradual. That is true even in climate change records
spanning millions of years, whose rhythms are governed by Earth's orbit.
Professor Broecker suggests that sudden switches in thermohaline circulation
may act as a trigger that sets off ice ages and other large-scale climate cycles.
Professor Broecker noted that abrupt climate changes have occurred not
only during ice ages but during warmer eras such as today's. The Eemian Period
- the last major warm period before the last ice age began about 115,000 years ago
- ended with a brief but intense cold period. A brief, intense cold spell also
occurred about 8,000 years ago - about 2,500 years after the last ice age ended -
when conditions were similar or warmer than they are now.
"Through the record kept in Greenland ice, a disturbing characteristic of
the Earth's climate system has been revealed, that is, its capability to undergo
abrupt switches to very different states of operation. I say 'disturbing' because
there is surely a possibility that the ongoing buildup of greenhouse gases might
trigger yet another of those ocean reorganizations and thereby the associated
large atmospheric changes," Professor Broecker said. "Were this to happen a
century from now, at a time when we struggle to produce enough food to nourish
the projected population of 12 to 18 billion, the consequences could be devastating."
Professor Broecker is one of the world's leading authorities on global
climate change. He has won nearly every major geological award, including the
Vetlesen Prize, considered by many to be the equivalent of the Nobel Prize in earth
sciences. Last year he was awarded the National Medal of Science and the Blue
Planet Prize, for achievements in global environmental research.
Lamont-Doherty Earth Observatory is part of the Columbia Earth Institute,
a new enterprise at Columbia University dedicated to creating innovations for
wise stewardship of our planet.
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