|
(*Note: the
following cribs liberally from responses to the film by the BBC &
George Monbiot - UK Guardian. Apologies to both.)
About 8,200 years ago, as the
last main glacial period was coming to an end, and the great global thaw
had permitted humans to move back into northern Europe, an ice dam in
north-eastern Canada burst. Behind it was a vast body of meltwater, which
roared through the Hudson Strait and into the North Atlantic. The story of
what happened next is now a matter of dispute, but some researchers
suggest the result was a 200-year ice age, during which humans once more
were driven out of northern Europe. The Day After Tomorrow starts with the
premise that it could be about to happen again. In fact, it now seems that
there simply isn't enough freshwater to shut down the circulation system,
and no means, such as the bursting of the great Canadian ice dam, by which
it could enter the ocean quickly enough to cause a dramatic effect. But
movies, of course, are all about dramatic effects, and a film about the
slow-rolling, complex transformations induced by climate change would be
about as gripping as a watching paint dry. We just have to accept
that a major movie house would never dream of tackling this subject if it
had to stick to the facts.
The facts
The Gulf Stream is a branch of the global thermohaline circulation
(THC), driven by differences in temperature and salinity of sea water,
that brings warm water north from the Caribbean to the UK and north west
Europe.
Global Warming will cause the Greenland ice cap to melt which, when
combined with increased rainfall at high latitudes, will potentially
disrupt the THC by adding freshwater and decreasing sea water salinity
in the North Atlantic.
The cooling of sea water and
its increased salt content makes the seawater in the Northern Atlantic
sink. The sinking action creates a pumping action as warm water
from the gulf moves in to take the place the sinking cool/salty
water. Changing the balance of the salt content by melting
glaciers stops the water from sinking. No sinking, no
pumping. No pumping, no gulf stream. No Gulf Stream, no warm
climate for Europe and northeastern North America.
Click
here for a scientific paper on this.
The experts
Dr Geoff Jenkins, senior climate researcher at the Hadley Centre, said
that the film does have a basis in theory. "If the Gulf Stream did
shut down, the UK and northwest Europe would become colder, but over a
much longer timescale - the film makers have crammed what would happen
in a decade into a few hours".
However, the UK would still be habitable! Some climate models suggest
that average temperatures could be 3-4°C lower following a slowdown or
shutdown of the THC. Winters would be much colder than now "along
the lines of the winter of 1962-1963" suggests Jenkins, with
summers being cooler and shorter. This would have many social
implications including (not surprisingly!) transport and agriculture.
3-4°C may not sound much, but the average air temperature difference
between the 'Medieval Warm Period' when vineyards thrived in southern
England and the 'Little Ice Age' when the River Thames regularly froze
over was only 1-2°C.
Some good news though, Dr Jenkins doesn't believe that the extreme
follow-on of events such as snow in New Delhi, tornadoes destroying Los
Angeles and grapefruit size hail in Tokyo are likely to happen.
In fact extreme events, such as those described above, can
potentially happen in our current climate. The largest hailstone ever
recorded (measured by circumference rather than weight) fell from a
storm in Aurora, Nebraska on June 22 2003. It measured 17.8cm wide and
47.6cm in circumference. And in May 2003, 562 tornadoes were recorded in
the USA, with almost 400 of those in one week - a new record!
In March of 2004, the first
recorded cyclone occurred in the Southern Atlantic
Dr Meric Srokosz is Science Coordinator of RAPID. "Greenland ice
cores and North Atlantic marine sediments show that rapid climate
changes have occurred repeatedly in the past. Changes as large as 5-10°C
over periods as short as a decade; fast for Climate Change where we
usually think in terms of centuries or millennia. These data also show
that the ocean circulation is implicated in many of these rapid
changes."
He said of the film "It telescopes events, such as the shutdown
of the North Atlantic circulation, into a few days (it would take years
to happen in reality) and amplifies the impact of the events (we are not
headed for another Ice Age). The ocean circulation might slow down in
the future and this might lead to somewhat cooler temperatures (by a few
degrees) in the UK and NW Europe, but not to the disaster depicted in
the film."
The future
Dr Jenkins said that some climate models indicate that the Gulf Stream
could slow by 20-25% by 2100. However, when considered against the
backdrop of Global Warming caused by the Greenhouse Effect, the UK would
still see overall warming. In a simulation of a total shut down of the
Gulf Stream, a fall in temperatures was only seen in a small area around
the North Atlantic in the UK and north west Europe, while the rest of
the world experiences warming.
Dr Srokosz said "Present day observations suggest that changes
are occurring in the North Atlantic circulation... however existing
observations are not adequate to answer the question of whether the THC
is slowing down."
"Rapid climate change is a low probability but high impact
event, so we need to improve our knowledge of the processes involved and
narrow the uncertainties on the prediction of potential future rapid
climate change."
To
learn more about global warming click here.
Or to find out what the Nat. Academy of Sciences has to say about
abrupt climate change you can buy their report here: http://books.nap.edu/catalog/10136.html
|
