Background
"It is widely expected," according to the authors of this new model study, "that global warming would intensify the hydrological cycle, with more evaporation in the tropics and more rainfall in polar regions," and that "such a changed freshwater balance will impose a fresh water flux forcing to the ocean's surface and alter the large scale ocean density structure, which consequently cause the THC [thermohaline circulation] in the global oceans to slow down."  As a result of this hypothetical sceanrio, many climate alarmists are claiming that CO2-induced global warming will slow the flow of the Gulf Stream, causing it to provide less heat to Europe and plunging the continent into a new Little Ice Age that ultimately leads to all sorts of political tensions around the globe.  In fact, the purveyors of this scare scenario have become so strident in promoting it that even the U.S. Pentagon's strategic planners are said to be grappling with its implications (Stipp, 2004).

What was done
Noting that a freshening of the North Atlantic Ocean has in fact been observed by Dickson et al. (2002) and Curry et al. (2003), the trio of scientists from the UK's Hadley Centre for Climate Prediction and Research "analyzed a set of four model simulations under historical natural (solar irradiance and volcanic aerosol changes) and anthropogenic (greenhouse gases, sulphate aerosol and ozone changes) forcings," which they say "have successfully simulated 20th century global mean temperature variations."

What was learned
An ensemble of four simulations carried out with the Hadley Centre's coupled climate model HadCM3 successfully reproduced the systematic freshening of the deep North Atlantic Ocean that has been observed over the past four decades.  "However," as Wu et al. report, they "do not find a decreasing trend of the North Atlantic THC."  Quite to the contrary, they say that "accompanying the freshening trend, the THC unexpectedly shows an upward trend, rather than a downward trend."

What it means
As a result of their analysis, the Hadley Centre scientists conclude that the observed freshening trend of the North Atlantic Ocean "does not seem to be consistent with an anthropogenically forced climate change scenario."  And to be sure we get the import of their message, they reiterate a few sentences later that their analysis "does not seem to support an interpretation of the observed freshening trend as an early signal of climate change due to human activities."  In addition, we would further note that with no slow down of the THC, and maybe even an increase in its flow rate, the Gulf Stream should continue to keep Europe significantly warmer than what its latitude would suggest as far into the future as we can reasonably claim to see.

References
Curry, R., Dickson, B. and Yashayacv, I.  2003.  A change in the freshwater balance of the Atlantic Ocean over the past four decades.  Nature 426: 826-829.

Dickson, B., Yashayacv, I., Meincke, J., Turrel, B., Dye, S. and Holfort, J.  2002.  Rapid freshening of the deep North Atlantic Ocean over the past four decades.  Nature 416: 832-837.

Stipp, D.  2004.  Climate Collapse: The Pentagon's Weather Niare.  Fortune, 26 Jan 2004 (http://www.fortune.com/fortune/technology/articles/0,15114,582584,00l?cn).