Get off the fence

"Are you sure?" is a common enough question. But it's often the wrong question to ask, especially when the stakes are really high, as they are when it comes to climate change.

Responsible leaders, whether in government or business, demand the best available information from their advisers and experts before making consequential decisions. But knowing when to act in the absence of certainty is the true test of wisdom. Few issues illustrate this truism better than the threat of the collapse of the thermohaline circulation.

The THC is what keeps Europe, and much of the East Coast of the U.S., warmer than most other parts of the world at similar latitudes. The Gulf Stream, which carries warm, salty water up from the tropics to the vicinity of Iceland, is part of this "conveyor belt" of heat, although the full story is more more complicated.

Should the THC shut off, Europeans and, to a lesser degree, New Englanders would find life very uncomfortable. It would be a Very Bad Thing, probably wrecking entire economies, depending on how fast it happened, and possibily threatening global food supplies.

And climate change is threatening to do just that, by melting freshwater ice in Greeland and elsewhere in the north polar region. If enough cold, fresh water pours quickly enough into the North Atlantic, the warm, salty water from the south will no longer be warm and salty enough to sink when it gets to the Arctic. And it is the sinking of that water that drives the THC.

Scientists believe it has shut down in the past, most notably 15,000 years ago when, as the world warmed itself out the last Ice Age, a lot of ice in Canada melted and poured out through the St. Lawrence River valley, stopping the THC dead in its tracks, and preventing it from restarting for about 2,000 years.

So what are the chances of this happening anytime soon? Until now, few climatologists would dare say. A few months ago, the conventional wisdom was that the probability of a complete shutdown was extremely low. But not anymore.

A new study (pdf), based on six years of research of the THC, was carried out by a team of climatologists at the University of Illinois at Urbana-Champaign, Wesleyan University and Princeton University. Not a shabby lot, I would say.

Lead author Michael Schlesinger, a professor of atmospheric sciences at Illinois, presented the group's findings to the global climate change conference in Montreal this week. It was well timed, coming on the heels of another report in Nature that describes a 30-percent decrease in the overall circulation of the THC in the North Atlantic. (Real Climate has a good summary of the Nature report here, and for the latest study, there's a good no-jargon story at Brightsurf News.)

Schlesinger et al. conclude that there is a 65 percent chance the THC will have shut down by 2200. Frightening, but still too far away to really scare anyone. The probability falls to 45 percent for sometime between the day after tomorrow and 2100. It might be possible to trim it down to about 28 percent, if we introduce immediate and severe economic countermeasures, such as a carbon tax of $100/tonne and a cap on emissions of all of fossil fuels, but given the resistance to such proposals evident at the Montreal talks ... well, you know how the US feels about that sort of thing.

The study's language is disturbingly blunt for a scientific paper:

One cannot but be taken by the fact that absent any climate policy, there appears to be a greater than 50% likelihood of a collapse of the Atlantic THC. Furthermore, even with the policy intervention of a carbon tax there appears to be a greater than 25% likelihood of a collapse. Such high probabilities are worrisome.

So: How much more confidence in their predictions will the climatologists have to supply before governments see fit to do something about this threat? When will we stop demanding the impossible 100 percent certainty of impending disaster?

Your guess is as good as mine.