The Source of Europe's Mild Climate
The notion that the Gulf Stream is responsible for keeping Europe anomalously warm turns out to be a myth
A Sea Change in Climate?
Evidence from ocean sediments suggests that at times during the last
Ice Age the North Atlantic thermohaline circulation was considerably
weaker than it is today, or perhaps it even shut down entirely. One
such event took place about 12,900 years ago, during the last
deglaciation, and is called the Younger Dryas (after a European
cold-dwelling flower that marks it in some terrestrial records). The
Younger Dryas began with a dramatic reversal in what was a general
warming trend, bringing near-glacial cold to the North Atlantic
region. This episode ended with an even more dramatic warming about
1,000 years later. In Greenland and western Europe, the beginning
and end of the Younger Dryas involved changes in winter temperature
as large as 20 degrees taking place in little more than a decade.
But the Younger Dryas was not a purely North Atlantic phenomenon:
Manifestations of it also appeared in the tropical and southern
Atlantic, in South America and in Asia.
For many years, the leading theory for what caused the Younger Dryas
was a release of water from glacial Lake Agassiz, a huge, ice-dammed
lake that was once situated near Lake Superior. This sudden outwash
of glacial meltwater flooded into the North Atlantic, it was said,
lowering the salinity and density of surface waters enough to
prevent them from sinking, thus switching off the conveyor. The
North Atlantic Drift then ceased flowing north, and, consequently,
the northward transport of heat in the ocean diminished. The North
Atlantic region was then plunged back into near-glacial conditions.
Or so the prevailing reasoning went.
Recently, however, evidence has emerged that the Younger Dryas began
long before the breach that allowed freshwater to flood the North
Atlantic. What is more, the temperature changes induced by a
shutdown in the conveyor are too small to explain what went on
during the Younger Dryas. Some climatologists appeal to a
large expansion in sea ice to explain the severe winter
cooling. I agree that something of this sort probably
happened, but it's not at all clear to me how stopping the Atlantic
conveyor could cause a sufficient redistribution of heat to bring on
this vast a change.
In any event, the still-tentative connections investigators have
made between thermohaline circulation and abrupt climate change
during glacial times have combined with the popular perception that
it is the Gulf Stream that keeps European climate mild to create a
doomsday scenario: Global warming might shut down the Gulf Stream,
which could "plunge western Europe into a mini ice age,"
making winters "as harsh as those in Newfoundland," or so
claims, for example, a recent article in New Scientist.
This general idea been rehashed in hundreds of sensational news stories.
The germ of truth on which such hype is based is that most
atmosphere-ocean models show a slowdown of thermohaline circulation
in simulations of the 21st century with the expected rise in
greenhouse gases. The conveyer slows because the surface waters of
the subpolar North Atlantic warm and because the increased transport
of water vapor from the subtropics to the subpolar regions (where it
falls as rain and snow) freshens the subpolar North Atlantic and
reduces the density of surface waters, which makes it harder for
them to sink. These processes could be augmented by the melting of
freshwater reserves (glaciers, permafrost and sea ice) around the
North Atlantic and Arctic.
But from what specialists have long known, I would expect that any
slowdown in thermohaline circulation would have a noticeable but not
catastrophic effect on climate. The temperature difference between
Europe and Labrador should remain. Temperatures will not drop to
ice-age levels, not even to the levels of the Little Ice Age, the
relatively cold period that Europe suffered a few centuries ago. The
North Atlantic will not freeze over, and English Channel ferries
will not have to plow their way through sea ice. A slowdown in
thermohaline circulation should bring on a cooling tendency of at
most a few degrees across the North Atlantic—one that would
most likely be overwhelmed by the warming caused by rising
concentrations of greenhouse gases. This moderating influence is
indeed what the climate models show for the 21st century and what
has been stated in reports of the Intergovernmental Panel on Climate
Change. Instead of creating catastrophe in the North Atlantic
region, a slowdown in thermohaline circulation would serve to
mitigate the expected anthropogenic warming!