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We Know: Underlying Processes |
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The warm ocean
current known as the Gulf Stream,
shown in this satellite image of sea
surface temperatures, transports heat
from the Gulf of Mexico to northwestern
Europe. In this image, yellow indicates
a temperature of 23ºC (73ºF, green
is about 14ºC (57ºF), and blue is
about 5ºC (41ºF).
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The Gulf Stream, for example, is
a wind-driven surface ocean current originating
in the Gulf of Mexico and terminating in northwest
Europe. When water from this warm current evaporates,
it warms the air—which is why northwestern
Europe enjoys a milder climate than Canada at the
same latitude.
The driving force behind the Gulf Stream and other
ocean currents is simple physics: In the waters
west of Europe, the evaporation of water makes the
sea saltier and colder, which makes the water more
dense.
The denser water sinks and warmer surface water
streams in to replace it, providing the current’s
sustaining pull. |
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| Since this simple physical process is based
on the interaction of warmer and colder masses of water, global
climate change could seriously disrupt that interaction. |
| Evidences
and Uncertainties |
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Changes in water temperature could disrupt ocean
currents, slowing or even shutting them down. A shutdown of
the Gulf Stream would drastically change the climate of the
countries ringing the North Atlantic Ocean, leading to significant
cooling in these areas. Some fossil evidence suggests that past
shutdowns in the Gulf Stream were associated with previous ice
ages. For this reason, some researchers argue that a small increase
in global temperatures could actually initiate a sudden cooling
trend—a negative feedback loop in which changes in one
direction spark larger shifts in the opposite direction. Another
possible effect of continued warming on the oceans is a significant
rise in sea levels. Some of this would result from the melting
of polar ice, but the largest proportion of the increase would
be caused by thermal expansion. (Water, like all things, expands
as its temperature rises.)
Evidence suggests that sea levels have risen by 10 to 15 cm
(about the width of your fist) over the past one hundred years.
Continued rises in sea levels could have devastating effects
on islands whose entire living space lies only a few meters
or less above sea level. The Indian Ocean’s Maldive Islands,
for example, have a mean height of 1 m above sea level; even
small increases in sea levels could completely devastate tiny
nations like this. But forecasting the amount of sea level rises
must allow for a wide range of uncertainty: One recent estimate,
drawn from a report by the Intergovernmental Panel on Climate
Change (IPCC), suggests a possible range of between .09 and
.88 meters of increase over the next century. (See “Average
Annual Global Number of People Flooded under Three Emissions
Scenarios” on this site for information on the consequences
of rising sea level.) |
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glossary
terms
