The Island of Doubt

"Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007-2008" (subs req'd) by a team of American, French, Danish and Canadian researchers, reports that things are back to what's presumably normal in northern waters after a decade or so of reduced or even absent convection.

Similar findings are scheduled to appear soon in Geophysical Research Letters (doi:10.1029/2008GL036162) by a separate team based in Canada.

This is potentially good news because the deep convection, or overturning, of oceanic waters is a major player in the global oceanic heat flux and removes any awful lot of carbon from the surface. The Gulf Stream is the most popular element of the global deep ocean conveyor that is controlled in part by the convection. But if the overturning stops, it won't be just the Gulf Stream that will be affected. Global warming will almost certainly kick into high gear. Indeed, the reduced convection reports had suggested to some climatologists that climate change was already wreaking havoc.

Unfortunately, this is a story that raises more questions than it answers. For example, the return of the convection may be tied to the massive transfer of ice from the Arctic Ocean to the North Atlantic last summer — an event widely thought to be tied to global warming itself. So which way things will go in the future is a big mystery. As the authors of the new paper write:

The myriad of factors involving the atmosphere-ocean-ice system that led to the return of deep convection in the winter of 2007-2008 highlights the complexity of the convective system in the North Atlantic, making it difficult to predict when deep mixing is likely to occur.

And here's an excerpt from a news item in Nature by Quirin Schiermeier

Whether the convection is back to stay, however, is not known. "There is a lot of natural fluctuation at play," says Detlef Quadfasel, an oceanographer and climate scientist at the University of Hamburg in Germany. "One nice day of ocean weather doesn't really tell you a lot about what you might have to expect in the future."

So how much data do we need? Schiermeier gave a possible answer in a story published last summer:

We won't know until we have collected more than 20 years' worth of continuous measurements, researchers said on Monday at the general assembly of the European Geoscience Union in Vienna, Austria.

In "After the Warming," the fate of the Atlantic's deep convection remains unknown until 2050. By the time our future climatologists finally figured out that it was irrevocably slowing to a halt, it was far too late to do anything about it. But science tends to move faster that most prognosticators anticipate -- kind of like climate change. So let's hope the 20-year timeframe turns out to be closer to the truth.

Meaning we shouldn't be scrimping on the funding for more research on this particular subject.

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Kjetil VÃ¥ge, Robert S. Pickart, Virginie Thierry, Gilles Reverdin, Craig M. Lee, Brian Petrie, Tom A. Agnew, Amy Wong, Mads H. Ribergaard (2008). Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007-2008 Nature Geoscience DOI: 10.1038/ngeo382