Future prospects - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 359

circulation change) is taken increasingly seriously in the discussion on
anthropogenic climate change.''

Rahmstorf (2004), relying principally on models, then went on to discuss some
aspects of the effects of global warming on thermohaline circulation assuming that
rising CO 2 induces global warming. He concluded that warming would have to be
extreme (4-5 C) to cause such a shutdown. 18
The Day After Tomorrow was a 2004 apocalyptic science fiction film that
depicted the catastrophic effects of global warming leading into a new ice age via
the shutdown of thermohaline circulation. The poster for the film shows New
York City engulfed in a mountain of ice and snow (it is on sale at a number of
websites). This film grossed over $500 million. Weaver and Hillare-Marcel (2004)
stated this film was based to some extent on scientific studies that posed the poss-
ibility that global warming could lead to an ice age. A brief summary of the
concept is that enhanced freshwater discharge into the North Atlantic from
warming would shut down the AMO (the North Atlantic component of the over-
turning circulation of the global ocean). This would produce downstream cooling
over higher northern latitudes, leading to onset of the next ice age. Alley 19 also
referred to the film. He said:

''Are overwhelmingly abrupt climate changes likely to happen anytime soon,
or did Fox Studios exaggerate wildly? The answer to both questions appears to
be yes. Most climate experts agree that we need not fear a full-fledged ice age in
the coming decades. But sudden, dramatic climate changes have struck many
times in the past, and they could happen again. In fact, they are probably
inevitable.''

However, Weaver and Hillaire-Marcel (2004) asserted that only during periods
of extreme glaciation are there sources of freshwater at high latitudes that can
provide the required flows to shut down thermohaline circulation. They concluded:
''

it is safe to say that global warming will not lead to the onset of a new Ice
Age.''
Clark et al. (2002) pointed out that most GCM projections of the 21st century
climate show a reduction in the strength of the overturning circulation of the
Atlantic as a result of increasing concentrations of greenhouse gases, and ''if the
warming is strong enough and sustained long enough, a complete collapse cannot
be excluded.'' They concluded:

...

'' ... although the possibility of a reduced Atlantic thermohaline circulation

18 However, as pointed out earlier, C. Wunsch (pers. commun., December 2008) insists
(correctly) ''The sinking of high latitude water does not sustain the Gulf Stream. The Stream
is a wind-driven feature and a result of the torque exerted on the ocean by the wind. It would
exist even in a constant density fluid with zero convection.''
19 Abrupt Climate Change, see http://www.chicagocleanpower.org/alley.pdf