The Heat Before the Cold
This week’s unexpected heat wave across much of the Northeast
and Midwest, couple with recent reports about the surprisingly fast
collapse of an Antarctic ice shelf the size of Rhode Island, has heightened
fears of long-term rise in temperatures brought about by global warming.
But this fear may be misguided. In fact, paradoxically, global warming
could actually bring colder temperatures to some highly populated
areas like Eastern North America and Western Europe.
Here’s what might happen: In the North Atlantic, a 10-foot layer
of fresh water - some of which may be coming from melting ice in the
Arctic - has been accumulating and lowering the salinity of the ocean
to depths of more than a mile for the past 30 years. Fresh water in
the ocean may not sound cataclysmic, but it can upset the ocean currents
that are the key to our planet’s climate control system.
In February, oceanographers presented new evidence that this northern
freshwater buildup could alter currents in a way that would cause
an abrupt drop in average winter temperatures of about 5 degrees Fahrenheit
over much of the United States and 10 degrees in the Northeast. That
may not sound like much, but recall the coldest winters in the Northeast,
like those of 1936 and 1978, and then imagine recurring winters that
are even colder, and you’ll have an idea of what this would
be like. This change could happen within a decade and persist for
hundreds of years.
Under normal circumstances, the famous warm waters of the Gulf Stream,
carrying heat absorbed in the tropics, move up the East Coast of the
United States and southeastern Canada and then angle toward Europe,
warming the overlying atmosphere and surrounding land as they go.
As the Gulf Stream system carries warm, salty water north, the atmosphere
cools it, making it dense enough to sink to great depths. The plunge
of that great volume of water helps propel a global system of currents
sometimes called the great ocean conveyor. But add too much fresh
water, and North Atlantic waters become less salty and less dense.
They stop sinking. The Gulf Stream slows or is redirected southward.
Winters in the North Atlantic region get significantly colder.
Changes in the conveyor were responsible for some of the most noticeable
climate changes in scientific history. About 12,000 years ago, as
the earth emerged from the most recent Ice Age and the North Atlantic
region warmed, an influx of fresh water - perhaps from melting ice
sheets - shut down the great conveyor and plunged much of the Northern
Hemisphere back into ice-age conditions that lasted 1,000 years. About
500 years ago a reduction of the ocean conveyors may have turned the
climate in northern Europe and the northeastern United States much
colder, during what became known as the Little Ice Age, which lasted
for about 300 years. In America, the Little Ice Age coincided with
the notorious winter at Valley Forge.
There is not enough evidence for scientists to know for sure whether
the influx of fresh water in the North Atlantic has come from an already
altered ocean circulation, changes in rainfall patterns or rivers,
or glacial and Arctic Ocean ice. We don’t know the exact threshold
at which sinking, and the great ocean conveyor, could stop. A global
ocean-observing system would greatly enhance our ability to monitor
changes that can spawn major, long-lasting climate shifts like these
and lead to reliable predictions of what may follow. But the evidence
we do have suggests that global warming could actually lead to a big
chill.
Terrence Joyce is a senior scientist and chairman of the department
of physical oceanography at Woods Hole Oceanographic Institution.
Originally published: April 18, 2002