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Uncertainty over weakening circulation

Letter to Physics Today – March 2007

This is a must read letter to the editor. It really calls into question one of the major beliefs of those who state that global warming will cause cataclysmic damage to our way of life. Basically, many people believe that if the major ocean currents change dramatically, that we will have major changes to our local weather in parts of the world. While this belief has some legitimacy, there does not seem to be a preponderance of evidence that such a change is imminent.

The Atlantic Ocean circulation across 25 N latitude has been used as a benchmark for characterizing the mass and heat transport from the tropics to the northern latitudes. The upper portion of this transport includes the Gulf Stream, which is at least partially responsible for a moderate climate in Europe. A weakening of the Atlantic meridional overturning circulation and of the Gulf Stream might have the unpleasant consequence of cooling Europe’s climate.

…that the Atlantic meridional overturning circulation slowed by about 30% between 1957 and 2004. Their work inspired speculations that the anthropogenic increase in carbon dioxide may be responsible for the weakening of heat transport from the tropics, and that such an effect has now been detected.

The conclusion that the Atlantic meridional overturning circulation has decreased by 30% does not follow from the data presented by Bryden and coauthors, but is based on an incorrect treatment of measurement errors.

The observed change of 8.1 Sv is well within the uncertainty of the measurement. The correct conclusion from the data presented in Bryden’s paper should have been that no statistically significant change in Atlantic meridional overturning circulation at 25 N between 1957 and 2004 has been detected. Such a conclusion is in agreement with the earlier analysis of essentially the same data (between 1957 and 1999) by Alexandre Ganachaud and Carl Wunsch

there is no sign of any Meridional Overturning Circulation slowdown trend over the past decade, contrary to some recent suggestions

Bryden’s paper as submitted for publication to Nature included a question mark at the end of the title, suggesting only a possibility that the circulation might be slowing down. On the editor’s insistence, the question mark was removed, and the title was changed into a positive statement that caused a considerable stir

I first read about this error on this blog which in turn references the source which can be found here.

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