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Will CO2-Induced Global Warming Slow the Atlantic Branch of the Ocean's Thermohaline Circulation?
Bleck, R. and Sun, S.  2004.  Diagnostics of the oceanic thermohaline circulation in a coupled climate model.  Global and Planetary Change 40: 233-248.

With respect to a "possible slowdown of the Atlantic branch of the oceanic meridional overturning circulation (MOC) during global warming," Bleck and Sun (2004) note that "such a slowdown is projected by most of today's climate models according to results compiled in Fig. 9.21 of IPCC (2001)."  A notable exception is the study of Sun and Bleck (2001a), wherein, according to Bleck and Sun, "the Atlantic MOC maintains its strength as atmospheric CO2 concentration gradually increases at the rate of 1% per year to twice its initial value."

What was done
Following a course suggested by this intriguing result, Bleck and Sun decided to "search for possible changes in the geographic layout of the MOC that may have happened in the model during CO2 doubling, using analysis techniques originally presented in Sun and Bleck (2001b)."

What was learned
"In view of evidence presented in IPCC (2001)," the two modelers say they "had expected the Atlantic MOC to weaken in response to a doubling of atmospheric CO2."  Once again, however, they found that "the Atlantic overturning stream function appears to be stable," suggesting that "it is insensitive to global warming resulting from gradual CO2 doubling."

What it means
Bleck and Sun state that "the overarching climate dynamics question driving this work is whether the Atlantic MOC should, or should not, be expected to weaken under a CO2 doubling scenario."  With their work suggesting it should not, the contrary finding of the IPCC is brought into serious question; but in this regard they say that "it is too early to state with any degree of certainty whether the decline seen in most model simulations compiled in Fig. 9.21 of IPCC (2001) is due to numerical errors or reflects a legitimate response to changes in the atmospheric environment."  Nevertheless, in the concluding sentence of their paper, they pointedly note that "recent publications, such as Sun and Bleck (2001a) and others referenced therein, paint a picture which makes a strong decline of the Atlantic MOC during CO2 doubling appear less likely."  In fact, the recent study of Wu et al. (2004) suggests that the Atlantic branch of the oceanic thermohaline circulation could even increase in intensity in a warming world.

IPCC.  2001.  Third Assessment Report: Climate Change 2001.  The Scientific Basis.  Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P.J. and Xiaosu, D. (Eds.).  Cambridge University Press, Cambridge, UK.

Sun, S. and Bleck, R.  2001a.  Atlantic thermohaline circulation and its response to increasing CO2 in a coupled atmosphere-ocean model.  Geophysical Research Letters 28: 4223-4226.

Sun, S. and Bleck, R.  2001b.  Thermohaline circulation studies with an isopycnic coordinate ocean model.  Journal of Physical Oceanography 31: 2761-2782.

Wu, P., Wood, R. and Stott, P.  2004.  Does the recent freshening trend in the North Atlantic indicate a weakening thermohaline circulation?  Geophysical Research Letters 31: 10.1029/2003GL018584.

Reviewed 25 February 2004