Copyright 1999 The New York Times Company
December 7, 1999
By WILLIAM K. STEVENS
Evidence continues to accumulate that the frozen world of the Arctic and sub-Arctic is thawing, and the findings are spotlighting two increasingly important questions:
Can what is happening in the Great
White North touch off sudden shifts
in climate that will transform weather and disrupt life throughout the
Northern Hemisphere? Is the Arctic
a key to the way in which global
warming might be translated into
region-by-region climatic changes?
The theory behind this view holds
that the climate of the North Atlantic
region, including Europe and eastern
North America, is controlled by
great ocean currents that transport
heat northward from the tropics.
This oceanic conveyor belt is set in
motion when saltier, and therefore
heavier, surface water sinks to the
deep ocean in the vicinity of southern
Greenland.
It is replaced by warm
water from the tropics that warms
the North Atlantic region. Without it,
the relatively mild climate of England, for instance, might be as cold
as that of northern Canada.
The worry is that a great influx of
fresh water from the thawing Arctic
might dilute the salty current and so
either halt or weaken the heat-bearing conveyor belt.
This could result
in a sudden, long-term drop in the
North Atlantic region's temperature,
a climatic disruption that would
probably reverberate around the
hemisphere by altering large-scale
atmospheric circulation.
Last week, scientists reported that
the current did indeed weaken or
stop at least twice in the past, plunging the region, which each time had
been warming, into cold comparable
to that of an ice age.
Other researchers, focusing on the
present, last week produced the latest in a lengthening string of studies
documenting the thawing of the vast
sheet of ice that covers the far northern seas, a potential source of fresh
water that, once it melts, could help
weaken the heat conveyor.
A third research group, using computerized simulations of the ocean-atmosphere system, concluded that
the present-day thawing of Northern
Hemisphere sea ice could not be explained by natural causes.
The most
obvious possible cause, they said, is
global warming produced by heat-trapping waste industrial gases like
carbon dioxide, which result from
the burning of fossil fuels like coal,
oil and natural gas.
And they said the
rate of sea-ice melting was likely to
increase in the decades ahead if
emissions of the heat-trapping gases
continued at today's rates.
Sudden influxes of fresh water
from the Arctic appear to have shut
down or weakened the oceanic heat
conveyor twice in the relatively recent geological past, according to a
study reported in the current issue of
Nature by Dr. Carsten Rühlemann of
the University of Bremen, Germany,
and colleagues.
By examining chemical clues in ocean sediments, they
determined that in each case, the
western tropical North Atlantic was
relatively warm while waters farther north were relatively cold -- as
would be expected if the conveyor
stopped or weakened.
The apparent shutdowns occurred
as the world was coming out of the
last ice age.
The North Atlantic region's rebound was twice interrupted
by sudden reversions to glacial conditions, once around 15,000 years ago
and again around 12,000 years ago,
before settling into the relative
warmth that has prevailed for the
last 10,000 years.
The first reversion
to glacial cold coincided with an invasion of the North Atlantic by vast
quantities of fresh water in the form
of icebergs, which were released
from North America as temperatures warmed.
Another influx is believed to have
touched off the second reversion to
cold, called the Younger Dryas
event. That event set off far-flung
reverberations.
The climate of the
Middle East dried out so much that
food became scarce, forcing the invention of agriculture, many scientists believe.
The New York region
became so chilly that cold-weather
trees like spruce took over the landscape from species like oak.
Could an influx of fresh water
brought about by global warming
make that happen again? Recent
studies lend the question new urgency. In one, reported in the December
issue of the journal Geophysical Research Letters, scientists analyzed
data collected by sonar aboard nuclear submarines and found that the
floating ice cover of the Arctic Ocean
has become about 40 percent thinner
than it was two to four decades ago.
In another, reported in the current
issue of the journal Science, researchers led by Dr. Ola M. Johannessen of the Nansen Environmental and Remote Sensing Center
in Bergen, Norway, used satellite
data to measure the area of the Arctic ice sheet.
They found that the
perennial ice cover had shrunk by 14
percent over the last two decades.
The third study, by nine researchers headed by Dr. Konstantin Y. Vinnikov of the University of Maryland,
analyzed data from five different
sources and found that sea ice in the
Northern Hemisphere as a whole had
decreased by about 7 percent in the
last 46 years.
Computer models of the ocean-atmosphere system reconstructed this
trend accurately, said Dr. Alan Robock of Rutgers University, a member of the research team.
The model
simulations also determined that the
trend was much larger than what
would be expected to result from the
climate system's natural variability.
This assumes that the models can
reproduce natural variability with
some accuracy, an assumption that
many experts question.
Nevertheless, Dr. Robock said that
the study provides "strong support
for the theory that humans are causing the climate to change."
If the climate is indeed warming
because of human activity, many scientists say, the influx of fresh water
to the North Atlantic could increase.
Melting ice in the Arctic is not the
only potential contributor.
Water
from melting glaciers in Greenland
and other Arctic and sub-arctic islands could also contribute.
Last
year, scientists reported that the
southern half of the Greenland ice
cap was melting back by about two
cubic miles a year, enough to cover
Maryland with a sheet one foot thick.
(Runoff from melting glaciers contributes to a rising global sea level.
Melting sea ice does not; like an ice
cube in a glass of water, its change in
state does not raise the water level.)
A third contributor to an influx of
fresh water could be increased precipitation.
A warmer atmosphere
causes more water to evaporate
from the oceans, and some scientists
believe that some of this increased
atmospheric moisture would be
transported to the far north, where it
would produce a heavier runoff of
fresh water into the North Atlantic.
This alone, some computer models
indicate, would be enough to weaken
the North Atlantic heat conveyor
drastically in the 21st century.
How much fresh water might be
necessary to halt, slow or weaken the
conveyor is a crucial question that
scientists cannot yet answer.
Nor do
they know how close the world might
be to an abrupt change in the conveyor's behavior.
"I don't think there's any accurate
assessment out there that would tell
us," said Dr. Peter Schlosser, a geochemist at the Lamont-Doherty
Earth Observatory.
If a warming atmosphere pumped
enough fresh water into the ocean to
shut down the conveyor, what would
it mean? Dr. Schlosser and others
say the magnitude of the resulting
climate change might be different
from those observed in the last glacial cycle.
But, he said, "It might put
the system into a state that might be
a precondition for further change."
What might such a change entail?
Temperature and precipitation
patterns could be drastically altered,
much as El Niño alters them, and Dr.
Schlosser says the alteration's effects would probably be felt throughout the Northern Hemisphere.
But
exactly how the patterns would
change and therefore what their
down-to-earth effects would be, he
said, is an open question.