Background
Climate alarmists are currently claiming that CO2-induced global warming will likely intensify earth's hydrologic cycle, leading to increased rainfall and, hence, the transport of larger volumes of freshwater to polar seas via river discharge.  The resulting change in high-latitude ocean salinity, according to this scenario, will alter the large-scale ocean density structure and cause the planet's thermohaline circulation to slow and possibly stagnate.  As a result of thus losing the heat that is currently provided to Europe by the Gulf Stream, they foresee all sorts of problems for that continent and much of the rest of the world.

What was done
Ottera et al. explore this subject via the Bergen Climate Model (Furevik et al., 2003), which consists of the ARPEGE/IFS general circulation model (GCM) of the atmosphere (Deque et al., 1994) and the MICOM ocean GCM (Bleck et al., 1992), the latter of which is coupled with a dynamic and thermodynamic sea ice model.  They first ran a 300-year control integration of the BCM with greenhouse gas and aerosol particle concentrations held constant at present-day values.  At the 100-year point of this experiment, they also ran a similar integration but with a threefold increase in the freshwater flux to the Nordic Seas and Arctic Ocean.  This perturbation, they say, "is believed to be consistent with the meltwater entering the high northern oceans during the last deglaciation (Simonsen, 1996)," as well as with "the simulated increase in the freshwater input poleward of 50°N obtained at a quadrupling of the pre-industrial CO2 level (Manabe and Stouffer, 1997)."

What was learned
Over the course of the control integration, the maximum Atlantic Meridional Overturning Circulation (AMOC) exhibited decadal variations of 1.5 Sv about a mean value of 18 Sv, while in the freshening simulation the AMOC dropped by ~6 Sv over the first 50 years, resulting in a cooling "of more than 1.5°C poleward of 30°N."  Thereafter, however, the maximum AMOC rose by approximately 2 Sv over the next decade, after which it rose more slowly over the next 90 years to approach its original value, in spite of continued freshwater input to the Nordic Seas and Arctic Ocean.  [For more details, see the expanded study of Ottera et al. (2004).]

What it means
These results do not indicate the likelihood of the imminent development of anything similar to a major ice age, but they do provide some support for the possible development of a future climatic anomaly similar to the Little Ice Age that preceded the Modern Warm Period.  However, the driver of this potential development (a high-latitude freshwater flux equivalent to that entering the northern oceans during the last deglaciation) seems a bit unrealistic, as does a quadrupling of the pre-industrial CO2 level.  In addition, the study of Bleck and Sun (2004) found the AMOC to be "insensitive to global warming resulting from gradual CO2 doubling," while the 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.  Clearly, therefore, there is no compelling or consistent evidence, even from within the virtual world of climate modeling, to suggest that anticipated future increases in the air's CO2 content would ever be the cause of dramatic rapid cooling anywhere on the planet.

References
Bleck, R. and Sun, S.  2004.  Diagnostics of the oceanic thermohaline circulation in a coupled climate model.  Global and Planetary Change 40: 233-248.

Bleck, R., Rooth, C., Hu, D. and Smith, L.T.  1992.  Salinity-driven thermocline transients in a wind- and thermohaline-forced isopycnic coordinate model of the North Atlantic.  Journal of Physical Oceanography 22: 1486-1505.

Deque, M., Dreveton, C., Braun, A. and Cariolle, D.  1994.  The ARPEGE/IFS atmosphere model: A contribution to the French community climate modeling.  Climate Dynamics 10: 249-266.

Furevik, T., Bentsen, M., Drange, H., Kindem, I.K.T., Kvamsto, N.G. and Sorteberg, A.  2003.  Description and validation of the Bergen Climate Model: ARPEGE coupled with MICOM.  Climate Dynamics 21: 27-51.

Manabe, S. and Stouffer, R.J.  1997.  Coupled ocean-atmosphere model response to freshwater input: Comparison to Younger Dryas event.  Paleoceanography 12: 321-336.

Ottera, O.H., Drange, H., Bentsen, M., Kvamsto, N.G. and Jiang, D.  2004.  Transient response of the Atlantic Meridional Overturning Circulation to enhanced freshwater input to the Nordic Seas-Arctic Ocean in the Bergen Climate Model.  Tellus 56A: 342-361.

Simonsen, K.  1996.  Heat Budgets and Freshwater Forcing of the Nordic Seas and the Arctic Ocean.  Ph.D. thesis, Nansen Environment and Remote Sensing Center, Bergen, Norway.

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.