Download The Role of the Thermohaline Circulation in Climate Change

Progress in Climate Change Sciences
The Role of the Thermohaline Circulation
in Climate Change
Zhou Tianjun
LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences
2. Response of thermohaline circulation to climate
change
Response of the thermohaline circulation to global warming
has drawn great attention of the international climate research
community, particularly in European-USA countries. The basic
global change during global warming is the substantial warming
of high latitudes, and with this condition the reduction of the
equator-pole temperature gradient and the increase of poleward
moisture flux. Such a re-arrangement of heat and freshwater fluxes
tends to weaken the thermohaline circulation. If the meridional
overturning conveyor collapsed due to the warming climate, the
pole-ward oceanic heat transport would stop, and this undoubtedly
would be a serious disaster for the European people. In the past
1. Impact of thermohaline circulation on climate
decades, dozens of numerical experiments have been performed
Satellite measurements of the Earth radiation budget reveal the
to test the sensitivity of the thermohaline circulation to the increase
surplus of incoming absorbed solar energy over outgoing longwave
of atmospheric greenhouse gasses (e.g. CO2). Some results indicate
radiation in low latitudes, while the reverse is true at high latitudes
that in response to CO2 increasing, rainfall over high latitudes of
on an annual mean bias. Thus, radiative processes are continually
the North Atlantic would increase, the freshwater cap covering
acting to cool the high latitudes and warm the low latitudes, and it
the sea surface would prevent the down-welling of the surface
is only the poleward heat transport that serves to offset the energy
water, finally the thermohaline circulation would decrease in
loss, in which, the oceanic poleward heat transport has great
intensity and eventually shuts-down. This result is greatly not
contributions. Under current climate system, the Atlantic Ocean
convinced, since the climate system models used above are still
plays a key role in transporting heat from tropics to high latitudes,
far from the target of “state-of-the-art”, the ocean’s response is to
which is mainly due to the existence of the meridional overturning
great extent dependent on models (IPCC, 2001).
circulation, namely thermohaline circulation (Broecker, 1991).
As one example, response of the IAP LASG GOALS model to
Circulation of the 90% of ocean water beneath the surface zone is
CO2 doubling is shown in Figure 2. The initial concentration of
driven by the force of gravity as dense water sinks and less dense
CO2 is set to pre-industrialization level (345ppm), then it increases
water rises. Since the ocean water density is largely a function of
gradually with a rate of 1% per year and reaches the doubled level
temperature and salinity, the movement of deep water drived from
(i.e. 692ppm) after 70 years for integration. Following the increase
the density gradient is called “Thermohaline Circulation”. In
of atmospheric CO2, the global mean surface air temperature
comparison with other regions of same latitudes, the wintertime
increases gradually. When the concentration of CO2 is doubled,
North Europe experienced a warmer climate (Fig. 1,see color page 4 ),
the air becomes 1.65 warmer. Simultaneously, the North Atlantic
partially due to the climate impact of the North Atlantic
thermohaline circulation also decreases gradually. However, it does
thermohaline circulation by transporting meridional heat (Bryden,
not collapse, the mean annual intensity remains to be 19.8Sv
1993). Any kinds of potential changes of the thermohaline
(1Sv=106m3s-1) when the CO2 is doubled. If the model integration
circulation would cause serious impacts on the northern Europe
continues, when the CO2 concentration reaches a threshold value,
regional climate (Zhou, et al, 2000; Wang, et al, 2004).
the thermohaline circulation might be shut-down. Previous studies
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found that quadrupling the atmospheric CO2, the thermohaline
of the thermohaline circulation would lead to a colder North
circulation in the North Atlantic would eventually collapse.
Atlantic climate, in reverse, the recovering of the thermohaline
Signals in the Greenland ice core and the North Atlantic ocean
circulation would result in a rapid warming. Some scientists
bottom sediments indicated that the North Atlantic experienced
suggest that variability of the thermohaline circulation might be
rapid climate change in the past hundred thousands years, an
closely linked to the ice-sheet covering North America and Europe
example of which is the Younger Dryas event (1.1 ~ 1.0 kaBP)
during the glacial and interglacial periods. Associated with the
(Dansgaard,1989). However, in the recent 10 thousand years and
freshwater input resulted from the breakdown of ice-sheet or ice-
in particular 8.0kaBP, climate over this domain was relatively
melting, sea surface salinity decreased and the deep-water
stable. Also there are evidences indicating that the rapid climate
formation reduced, which led to a weakened or even collapsed
change events of the North Atlantic have affected the global climate
thermohaline circulation, thereby colder climate over the domain
system to some extent, relevant signals have probably been found
and vice versa (Alley, et al., 1999).
in other regions of the world. Generally, since the temperature
Conclusively, the North Atlantic domain indeed has experienced
changes happen within very short periods, they can not be
rapid climate change in the past time, which might be closely linked
Global mean temperature of sigma 0.991
to the mode transformation of the thermohaline circulation.
Progress in paleo-climate studies provides abundant evidence for
the people’s worrying about the potential response of the
thermohaline circulation to global warming, thereby the suddenly
rapid cooling of the western and northern Europe.
E-mail: [email protected]
Integration time (unit: a)
North Atlantic THC Index in Sverdrup
Integration time (unit: a)
Fig. 2 Response of the global mean surface air temperature (a) and
the North Atlantic thermohaline circulation index (b) to the increasing
of atmospheric CO2 in IAP/LASG GOLAS model. (Figure (a) in unit
of , Figure (b) in unit of Sv, solid square corresponds to the
sensitivity experiment, thin line corresponds to the control integration.
1Sv=106 m3s -1)
explained simply in terms of the traditional earth orbital parameters
theory. The exact nature of the Younger Dryas event is still under
intensive study. Scientists speculated that the abrupt climate change
results from the abrupt shift of the thermohaline circulation from
one mode of operation to another (Manabe and Stouffer, 1988;
Muscheler et al, 2000). In general, the weakening and collapsing
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