November 20, 2006

False Alarm

Filed under: Climate Changes, Gulf Stream

“False Alarm: Atlantic Conveyor Belt Hasn’t Slowed Down After All” is the title of a “News of the Week” piece in this week’s (November 17, 2006) Science magazine by science writer Richard Kerr (for those with a subscription, you can view the whole story, here).

Kerr’s piece starts off with the line “A closer look at the Atlantic Ocean’s currents has confirmed what many oceanographers suspected all along: There’s no sign that the ocean’s heat-laden ‘conveyor’ is slowing.”

The blockbuster paper that claimed otherwise was published in Nature (Science magazine’s primary competitor) about a year ago. Harry Bryden and colleagues at UK’s National Oceanography Centre concluded that “The comparison suggests that the Atlantic meridional overturning circulation has slowed by about 30 percent between 1957 and 2004.” And although the changes that Bryden et al. observed were very nearly of the same magnitude of natural variability, the researchers explained their confidence in their conclusions with “We accept that the uncertainty in transport structure for the 2004 section is ±6 Sv in the upper and lower layer transports and that the observed changes are uncomfortably close to these uncertainties. But the warmer waters near the western boundary in the 1998 and 2004 section leading to an increase in southward mid-ocean recirculation in the thermocline and the reduction in deep water flow only in lower NADW represent strong arguments that the observed changes are robust.”

These findings made worldwide headlines, because some argue (wrongly, but quite alarmingly) (as did the movie makers of “The Day After Tomorrow” and the author of “An Inconvenient Truth”) that a slowdown or cessation of the Atlantic Conveyor (aka, the Thermohaline Circulation (THC) or the Atlantic Meridional Overturning Circulation (MOC)) would plunge Europe into Ice Age like conditions.

Bryden et al. had used some (then) just-acquired data from a newly-installed system of moored ocean measuring devices the had been strung across the Atlantic as part of the UK’s Rapid Climate Change program (RAPID) that was designed to “improve our ability to quantify the probability and magnitude of future rapid change in climate, with a main (but not exclusive) focus on the role of the Atlantic Ocean’s Thermohaline Circulation.”

Richard Kerr, who has been skeptical of Bryden’s conclusions ever since they first were published (as we reported in our initial coverage of the Bryden results, see here) describes the genesis of Bryden et al.’s research as follows:

While waiting for the moored array to produce long-term observations, physical oceanographer Harry Bryden and his team at the National Oceanography Centre in Southampton, U.K., compared the 2004 snapshot with four earlier instantaneous surveys dating back to 1957. They found a 30% decline in the northward flow of the conveyor (Science, 2 December 2005, p. 1403), sparking headlines warning of Europe’s coming ice age.

Apparently, now, after having acquired a bit more data from the RAPID instruments, many oceanographers, think that Bryden jumped the gun (this should come as no surprise to avid WCR readers as we recently reported on the publication of new research that did not support Bryden’s original findings, see here). According to Kerr “The first year of RAPID array observations has now been analyzed, and the next European ice age looks to be a ways off.” Describing the general feeling of those in attendance of last month’s meeting of the RAPID scientists, Kerr quoted Dr. Martin Visbeck as stating that “more than 95% of the scientists at the workshop concluded that we have not seen any significant change of the Atlantic circulation to date.”

Renowned M.I.T. oceanographer Dr. Carl Wunsch told Kerr that “Scientific honesty would require records for decades” in order to pick out a greenhouse-induced slowing.

Interestingly, there has been no coverage thus far of any of this in Nature magazine (where the findings of Bryden et al. were first published).

Reference:

Bryden, H.L., et al., 2005. Slowing of the Atlantic meridional overturning circulation at 25ºN. Nature, 438, 655-657.

Kerr, R. A., 2006. False Alarm: Atlantic Conveyor Belt Hasn’t Slowed Down After All, Science, 314, 1064, doi: 10.1126/science.314.5802.1064a




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