Scientists relied on climate models, not direct measurements, to claim in a new study man-made global warming caused a slowdown in the Gulf Stream ocean current.
It’s the very same scenario posed in disaster movie “The Day After Tomorrow,” where a slowdown in the Gulf Stream turned North America into a frozen wasteland. A catastrophic scenario could be decades away, some scientists are saying.
“We know somewhere out there is a tipping point where this current system is likely to break down,” Potsdam Institute climate scientist Stefan Rahmstorf, a co-author of one of the studies, said in a statement.
“We still don’t know how far away or close to this tipping point we might be,” Rahmstorf warned. “This is uncharted territory.”
Rahmstorf’s study was one of two that garnered alarming media headlines, but experts are skeptical because of the scant observational evidence. Indeed, scientists have only been taking direct measurements of the Gulf Stream for a little over a decade.
“Climate model reconstructions are not the same as observed data or evidence,” libertarian Cato Institute’s Dr. and Atmospheric Scientist Ryan Maue told The Daily Caller News Foundation.
“We should be very wary of grandiose claims of ‘A Day After Tomorrow’ based upon very limited direct measurements,” Maue said.
The Gulf Stream, or Atlantic meridional overturning circulation (AMOC), brings warm water from the Gulf of Mexico to the North Atlantic, and in turn, cold northern water is brought southward.
Polar ice melt and enhanced rainfall put an increasing amount of cold, fresh water into the North Atlantic, reducing salinity, some scientists say. Less saline has a harder time sinking, throwing off the AMOC.
Climate models generally show a weaker AMOC as a result of warming, but observational evidence has been scant. Anomalous cooling south of Greenland is evidence of a weakened AMOC, some scientists say.
The weak AMOC is explicitly tied to “increasing atmospheric carbon dioxide concentrations” and “temperature trends observed since the late nineteenth century,” according to the study, Rahmstorf co-authored.
However, the “Labrador Sea deep convection and the AMOC have been anomalously weak over the past 150 years or so … compared with the preceding 1,500 years,” a second study published in the same journal found.
In other words, the AMOC began weakening before human activities could play a role.
“The specific trend pattern we found in measurements looks exactly like what is predicted by computer simulations as a result of a slowdown in the Gulf Stream System, and I see no other plausible explanation for it,” Rahmstorf, whose study relied on proxy-data from ocean sediment and calcareous shells, said.
But again, there’s limited observational evidence. Several scientists besides Maue were skeptical of Rahmstorf’s study.
Rahmstorf’s “assertions of weakening are conceivable but unsupported by any data,” Massachusetts Institute of Technology’s Carl Wunsch told The Associated Press.
The National Center for Atmospheric Research’s Kevin Trenberth “said his recent work faults regular cycles in the atmosphere more than the ocean” and the “study doesn’t explain year to year variability, while atmospheric cycles do,” the AP reported.
“Essentially, what view you take of the results depends on how good you believe the models used are and likewise how well the chosen proxies represent the AMOC over the time scales of interest,” National Oceanography Center oceanographer Meric Srokosz told The Washington Post.