age to warming - and back?
By Peter N. Spotts | Staff writer of The Christian
Science Monitor - from the March 18, 2004 edition
The Little Ice Age and "the 8,200-year event"
are not exactly household terms. Once only a handful
of climate scientists puzzled over these episodes
of abrupt climate change. Now, the topic is getting
close scrutiny from the Pentagon, the halls of
Congress, and even Hollywood - where a disaster
movie set for release in May depicts a sudden
reason for all the interest? While policymakers
have worried long and hard about global warming,
which might raise Earth's temperature 1.4 to 5.8
degrees C by century's end, a growing body of
evidence suggests natural forces could just as
easily plunge Earth's average temperatures downward.
In the past, the planet's climate has changed
10 degrees in as little as 10 years. That may
not sound like much. But the last time the planet
was 10 degrees colder, it was still in an ice
age. "There's the very real potential of
the climate system changing dramatically and rapidly"
in ways that lie outside modern human experience,
says Mark Eakin, who heads the paleoclimatology
program of the National Oceanic and Atmospheric
possibility of a sudden freeze doesn't mean mankind
can relax efforts to curb global warming, many
scientists warn. Indeed, given the complexity
of Earth's climate, human activities that spew
greenhouse gases into the atmosphere may increase
the potential for an abrupt cooling.
example: Regional and global climates have undergone
quick and dramatic changes even after what would
appear to be only gentle prodding by natural influences,
Dr. Eakin says. In many cases, that prodding has
been far less severe than the changes humans have
wrought via industrial emissions of carbon dioxide.
the absence of better knowledge, we have to assume
that humans are making abrupt climate change more
likely - not because humans are worse than nature,
it's just because we're changing the system,"
says Richard Alley, a Penn State University paleoclimatologist.
Dr. Alley led a 2002 National Research Council
panel that examined abrupt climate change and
laid out recommendations for research priorities
and possible adaptation strategies.
are beginning to pay attention. Last week, the
Senate Commerce, Science, and Transportation Committee
sent to the full Senate a bill that would give
NOAA $60 million for research into the causes
of abrupt change. The work could help provide
more accurate modeling of past and future climate
change, perhaps yielding clues that could serve
as an early warning to abrupt change.
a report prepared for the Defense Department bids
Pentagon planners to elevate the study of abrupt
climate change "beyond a scientific debate
to a US national security concern." The study
was prepared by the Global Business Network, a
corporate strategic planning and consulting firm
in Emeryville, Calif.
actions are fueled by a growing body of evidence
over the past five years that Earth's climate
has a history of rapid variations - and that if
the paleoclimate record is any indication, this
history repeats itself. Some periods, like the
Little Ice Age, would cause hardships today, but
industrial countries probably could adapt, researchers
say. The Little Ice Age lasted roughly from 1300
to around 1870 and dropped temperatures in parts
of the northern hemisphere by about 1 degree C.
the casual observer, that drop may seem small.
But from a climate and a social standpoint "it
was huge," says Lloyd Keigwin Jr., a senior
scientist at the Woods Hole Oceanographic Institution
(WHOI) in Woods Hole, Mass.
year without summer
Little Ice Age - actually three distinct cooling
periods - chilled northern Europe and parts of
the United States. It sent the Vikings back to
Europe from their outposts in Greenland. Farms
in Norway were covered with glaciers and crop
failures around Europe caused famines and spikes
in grain prices.
1816, New England experienced its "year without
summer," when many crops failed. One researcher
argues that the storm that wiped out a large part
of the Spanish Armada - and made Sir Francis Drake's
job easier - was part of the Little Ice Age pattern.
Little Ice Age is the only abrupt climate change
that people have experienced in industrial times,"
says Dr. Keigwin.
abrupt changes, like the rapid cooling event that
peaked 8,200 years ago, could be far more disruptive.
scientists have studied the climate record trapped
in glacial ice from Antarctica and Greenland,
and in mud samples extracted from beneath the
ocean floor, their respect for the speed of change
has grown. In the mid-1950s, a change of roughly
3 degrees C over more than 1,000 years was deemed
abrupt. In 1999, a team led by Jeffrey Severinghaus
at the Scripps Institution of Oceanography in
La Jolla, Calif., determined that the last ice
age ended with a temperature burst that raised
the thermostat at Greenland by some 9 degrees
C over a mere decade.
still don't understand the causes" behind
the increase, he says. A range of abrupt regional
and global changes poses the same challenge.
potential source of change may be the North Atlantic,
researchers say. There, warm water moves north
along the surface, cooling as it travels. By the
time the surface water reaches the far northern
portions of the North Atlantic basin, it has cooled
and grown denser than the underlying layers of
ocean, and it begins to sink. The water then travels
south along the bottom, driving an aquatic "conveyor
belt" that spans the globe.
researchers suspect that if enough fresh water,
perhaps from melting ice, is injected into key
spots in the North Atlantic, it can virtually
shut down the conveyor. Fresh water is more buoyant
than salt water and can form a layer that blocks
the circulation. The Northern Atlantic region
would then cool. Reduce the fresh water, researchers
say, and the reverse can happen, warming the North
much of the evidence for this comes from records
when the planet's climate was already cool, some
scientists argue. So the evidence may not hold
lessons for today, when the planet is in a warm
hiatus between glacial periods. A more timely
test of the North Atlantic's role in abrupt climate
change, they say, may come through unraveling
the mystery of a rapid cooling event that peaked
8,200 years ago.
the standards of the Little Ice Age, the 8,200-year
event was frosty and global. Although the event
lasted only about 100 years, Greenland's temperatures
dropped by about 3 degrees C. Indeed, the consultants
who argue for upgrading the national security
status of abrupt climate change used this event
as their model.
researchers say the cooling 8,200 years ago may
have been triggered by the collapse of ice dams
holding back the waters of Lake Agassiz - a vast
glacial reservoir covering the Great Lakes region
of the US and Canada at the end of the last ice
age. It would have flushed enough fresh water
into the right places in the North Atlantic to
shut down the conveyor.
here, too, the picture grows murky. At a meeting
last fall at the WHOI, researchers from the US
and Canada looked at the problem and raised more
questions than answers. For instance, no one has
yet seen conclusive topographical evidence of
such a huge outflow. If the waters of Lake Agassiz
did surge into the Hudson Straits and the Labrador
Sea, they surely would have carved a path in the
land. This lack of evidence has sent some searching
for an outflow path to the north, into the Arctic
samples from the ocean floor also fail to confirm
the Hudson Straits-Labrador Sea theory, says Mr.
Keigwin of the WHOI. Instead, he has found evidence
of freshening in the water much farther south
and hugging the coast. This suggests that the
water may have entered the Atlantic too far south
to affect the conveyor - at least directly.
is an important problem," he says. If scientists
can prove that freshening of the ocean did occur
during this current period between ice ages, then
the possibility of global warming triggering an
abrupt climate change would have to be taken more
seriously, he adds. "We need a coordinated
effort" to answer the riddle.