What, no polar bears?

A couple of weeks ago, I discussed Bryden et al.'s widely reported Nature paper, which claimed a significant slowdown in the thermohaline circulation, and suggested that we're going to have to wait to see whether the results reflect a real trend, or are just a combination of error and natural variability in a poorly described system. All very well, but then I concluded:

In a couple of years, maybe we'll know whether I'll retire to vineyards or polar bears in my back garden.

Flippant, perhaps, and as it turns out also rather foolish. Via Real Climate, I've just come across a nice discussion (subscription required, I'm afraid) of how, even if the conveyor is slowing, its impact is not quite as simple as on=vineyards, off=polar bears.

One interesting aspect of this article is the caution with which most climate scientists are treating these results. Some example comments:

• "The story is appealing, but it is a very extreme interpretation of the data."
  
• "Bryden's results are extraordinary, but this is exactly why they require extraordinary evidence."
  

If nothing else, this shows exactly how slanderous so-called 'critics' are being when they accuse climate scientists of being more interested in playing politics than objective scientific analysis. If all they were interested in was scaring people into their liberal, social-engineered worldview and securing research funding, such data is an open goal for spinning any number of apocalyptic scenarios, a la James Lovelock. Instead, they are quite open about their doubts, which stem from two main sources. The first is observational: a reduced thermohaline circulation should reduce heat transport into the North Atlantic, but there has been no drop in high latitude sea surface temperatures, and Europe has been warming, not cooling, in the last decade. Secondly, the estimated increase in fresh water added to the surface North Atlantic is lower, by an order of magnitude, than that required by modelling to stop NADW formation. Much more warming is required before the thermohaline circulation should be seriously affected.

But going back to the quote from my previous post, the other message from this article is that although reduced NADW formation can be linked to cooling of Europe in the past, it is by no means clear that a shutdown will also lead to cooling in the future, and in fact most modelling seems to indicate it will not. At the end of the last Ice Age many think that the presence of sea ice provided an important positive feedback (through both the familiar ice-albedo effect, and by preventing heat transfer between ocean and atmosphere) which exacerbated cooling in response to slowing of the thermohaline circulation; our present warming climate reduces sea ice cover and hence the effectiveness of this mechanism. As Wally Broecker, one of the pioneers of this whole idea, puts it:

"The notion that a collapse of the thermohaline circulation may trigger a mini ice age is a myth."

But even if the local effects we might have expected may not be on the cards, this does not mean that we should not be concerned: the thermohaline circulation also plays an important role in supplying nutrient rich bottom water to many parts of the world, and additionally enhances the ability of the oceans to absorb CO2 from the atmosphere (by transferring dissolved CO2-rich water to the deep ocean). A shutdown could still have serious consequences. But, whatever happens, it appears that my garden will remain polar bear free.

Newer developments:
The Case of the THC "Shutdown"
THC not as weak as we thought - most of the time