Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 346

6.6.6 Oceanicandatmosphericcirculation

As the Earth warms it is possible that new oceanic and atmospheric circulation
patterns may become manifest. For example, the Broecker thermohaline ocean cir-
culation (see Chapter 4) is one of the key mechanisms by which heat is transported
away from the tropics to higher latitudes. It is driven by warm water from the tropics
moving northwards and becoming saltier due to evaporation. At higher latitudes this
denser water becomes even more dense through cooling and so sinks. A current in
the ocean depths then carries it between oceans, where it surfaces elsewhere. One of
the key places where a warm current from the tropics sees considerable evaporation
and then sinking at high latitudes is the Gulf Stream in the North Atlantic, with sink-
ing taking place around Iceland and Greenland. However, freshening of this water,
such as by iceberg discharges, Arctic ice melt or changes in regular precipitation
patterns, would make the current less dense and so reduce sinking. If the reduction
was sufficiently great it could shut the conveyor down. This might block the Gulf
Stream's northern route. Of course, the Coriolis force from the Earth's rotation would
still continue, as would trade winds, so a warm current would still leave the Gulf of
Mexico, but it would not travel so far north, but rather circle around at a lower latit-
ude. Without the Gulf Stream off north-western Europe, the British Isles, the North
Sea continental nations and the western edge of Scandinavia would see far harsher
winters, just as are today experienced in Labrador, which is on the same latitude as
Great Britain.
Although we do not have a particularly thorough understanding of the conveyor's
precise operation, the current understanding is that a complete shut-down of the North
Atlantic driver of the conveyor is unlikely (but not impossible). However, a weak-
ening of the conveyor by around 30% is (again, currently) considered a reasonable
possibility. If this happened then seasonality in north-west Europe is likely to increase
as winters would not warm in pace with summers due to long-term climate change.
Whereas such a change in ocean circulation would result in agricultural dislocation
and the need for infrastructure investment, the effect would be regional. Overall the
Earth would still be experiencing global warming, it is just that the climatic patterns
would change more markedly on a regional basis in many places. This would have
profound consequences for north-west Europe.
It is known from salinity studies that more fresh water has been added to the North
Atlantic in the 20th century. In particular, during the late 1960s a large pulse of fresh
water entered the Nordic Seas (between Greenland and Scandinavia) through the
Fram Strait. This freshwater pulse was of the order of around 10 000 km 3 at a rate
of about 2000 km 3 year − 1 , implying a net flux anomaly of approximately 0.07 Sver-
drups (Sv; where 1 Sv

10 6 m 3 s − 1 ) during a 5-year period. It is known as the Great
Salinity Anomaly (GSA). Naturally there is always fresh water being added (early
during interglacials) into the Arctic from rivers and ice melt, but the GSA is thought
to represent roughly around a 40% increase of normal flow. In 2005 Ruth Curry, from
the Woods Hole Oceanographic Institution, and Cecilie Mauritzen, from the Nor-
wegian Meteorological Institute, used these long-term salinity data sets to ascertain
the likelihood of an effect on the Broecker thermohaline circulation. In a carefully

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