By disturbing a massive ocean current, melting
Arctic sea ice might trigger colder weather in Europe and North
America, and make global warming a chilling possibility.
by Patrick L Barry
Global warming could
plunge North America and Western Europe into a deep freeze, possibly
within only a few decades.
That's the paradoxical
scenario gaining credibility among many climate scientists. The
thawing of sea ice covering the Arctic could disturb or even halt
large currents in the Atlantic Ocean. Without the vast heat that
these ocean currents deliver - comparable to the power generation
of a million nuclear power plants - Europe's average temperature
would likely drop 5 to 10°C (9 to 18°F), and parts of eastern North
America would be chilled somewhat less. Such a dip in temperature
would be similar to global average temperatures toward the end of
the last ice age roughly 20,000 years ago.
Some scientists believe
this shift in ocean currents could come surprisingly soon - within
as little as 20 years, according to Robert Gagosian, president and
director of the Woods Hole Oceanographic Institution. Others doubt
it will happen at all. Even so, the Pentagon is taking notice. Andrew
Marshall, a veteran Defense Department planner, recently released
an unclassified report detailing how a shift in ocean currents in
the near future could compromise national security.
"It's difficult to
predict what will happen," cautions Donald Cavalieri, a senior scientist
at NASA's Goddard Space Flight Center, "because the Arctic and North
Atlantic are very complex systems with many interactions between
the land, the sea, and the atmosphere. But the facts do suggest
that the changes we're seeing in the Arctic could potentially affect
currents that warm Western Europe, and that's gotten a lot of people
There are several satellites
keeping an all-weather watch on ice cover in the Arctic. NASA's
Aqua satellite, for instance, carries a Japanese-built sensor called
the Advanced Microwave Scanning Radiometer-EOS ("AMSR-E" for short).
Using microwaves, rather than visible light, AMSR-E can penetrate
clouds and offer uninterrupted surveillance of the ice, even at
night, explains Roy Spencer, the instrument's principal investigator
at the Global Hydrology and Climate Center in Huntsville, Alabama.
Other ice-watching satellites, operated by NASA, NOAA and the Dept.
of Defense, use similar technology.
The view from orbit
clearly shows a long-term decline in the "perennial" Arctic sea
ice (the part that remains frozen during the warm summer months).
According to a 2002 paper by Josefino Comiso, a climate scientist
at NASA's Goddard Space Flight Center, this year-round ice has been
retreating since the beginning of the satellite record in 1978 at
an average rate of 9% per decade. Studies looking at more recent
data peg the rate at 14% per decade, suggesting that the decline
of Arctic sea ice is accelerating.
Some scientists worry
that melting Arctic sea ice will dump enough freshwater into the
North Atlantic to interfere with sea currents. Some freshwater would
come from the ice-melt itself, but the main contributor would be
increased rain and snow in the region. Retreating ice cover exposes
more of the ocean surface, allowing more moisture to evaporate into
the atmosphere and leading to more precipitation.
Arctic ice, 1979-2003, based on data collected by the
Defense Meteorological Satellite Program (DMSP) Special
Sensor Microwave Imager (SSMI)
Because saltwater is
denser and heavier than freshwater, this "freshening" of the North
Atlantic would make the surface layers more buoyant. That's a problem
because the surface water needs to sink to drive a primary ocean
circulation pattern known as the "Great Ocean Conveyor." Sunken
water flows south along the ocean floor toward the equator, while
warm surface waters from tropical latitudes flow north to replace
the water that sank, thus keeping the Conveyor slowly chugging along.
An increase in freshwater could prevent this sinking of North Atlantic
surface waters, slowing or stopping this circulation.
AMSR-E is collecting
new data that will help scientists evaluate this possibility. For
one thing, it provides greatly improved ground resolution over previous
all-weather sensors. AMSR-E images reveal smaller cracks and fissures
in the ice as it breaks up in the spring. This detail allows scientists
to better understand the dynamics of ice break-up, says Cavalieri,
a member of the AMSR-E team.
"Other important pieces
of the puzzle, like rainfall, sea-surface temperatures, and oceanic
winds, are also detected by AMSR-E. Looking at those variables together
should help scientists assess the likelihood of a change in the
Atlantic currents," adds Spencer.
Once considered incredible,
the notion that climate can change rapidly is becoming respectable.
In a 2003 report,
Robert Gagosian cites "rapidly advancing evidence [from, e.g.,
tree rings and ice cores] that Earth's climate has shifted abruptly
and dramatically in the past." For example, as the world warmed
at the end of the last ice age about 13,000 years ago, melting ice
sheets appear to have triggered a sudden halt in the Conveyor, throwing
the world back into a 1,300 year period of ice-age-like conditions
called the "Younger Dryas."
Will it happen again?
Researchers are scrambling to find out.
On Feb. 13, an expedition
set sail from Great Britain to place current-monitoring sensors
in the Atlantic Ocean that will check the Gulf Stream for signs
of slowing. The voyage is the latest step in a joint US / UK research
project called Rapid Climate Change, which began in 2001. Another
international project, called SEARCH (Study of Environmental Arctic
CHange), kicked off in 2001 with the goal of more carefully assessing
changes in Arctic sea ice thickness.
Much depends on how
fast the warming of the Arctic occurs, according to computer simulations
by Thomas F. Stocker and Andreas Schmittner of the University of
Bern. In their models, a faster warming could shut down the major
Atlantic current completely, while a slower warming might only slow
the current for a few centuries.
The RRS Discovery, on a voyage to measure currents in the
And, inevitably, the
discussion turns to people. Does human industry play a major role
in warming the Arctic? Could we reverse the trend, if we wanted
to? Not all scientists agree. Some argue that the changes occuring
in the Arctic are consistent with large, slow natural cycles in
ocean behavior that are known to science. Others see a greater human
"The sea ice thawing
is consistent with the warming we've seen in the last century,"
notes Spencer, but "we don't know how much of that warming is a
natural climate fluctuation and what portion is due to manmade greenhouse
If the Great Conveyor
Belt suddenly stops, the cause might not matter. Europeans will
have other things on their minds - like global warming that causes
concerns as to how to grow crops in snow. Now is the time to find
out, while it's merely a chilling possibility.