Nature this week has an excellent summary of the state of the science with regards to possible changes in the ocean thermohaline (or meridional) circulation in the Atlantic and its impact on climate. Even though it quotes a couple of us, it’s still worth reading if you want to understand how results like the Bryden et al paper – that suggested that the Atlantic overturning had reduced by 30% in recent decades – are assimilated into the scientific picture.
The [Bryden et al] result came as a surprise to those in the field. Few scientists had thought that such dramatic slowing of the thermohaline circulation could happen so soon. Models suggest that the increase in fresh water needed for a conveyor shutdown would not be expected without a global warming of 4–5 C; warming in the twentieth century is currently put at 0.6 C (ref. 3). The most complex computer models of the climate and oceans, the sort used to make climate predictions for the Intergovernmental Panel on Climate Change (IPCC), suggest that the flow might be expected to slow by an average of 25% by the end of the twenty-first century, but not to shut down completely.
The reason for this surprise is that other data of relevance, such as the changes in salinity or deep convection or lack of observed cooling in the North Atlantic, aren’t obviously consistent with such a large change:
[Ruth Curry's] recent analysis of 1950 to 2005 salinity data suggests that 4,000 cubic kilometres — eight times the annual outflow of the Mississippi river — of fresh water have accumulated in the upper ocean layers since the 1960s. “The extra freshwater input is beginning to affect density,” she says. But the amount of fresh water needed to shut down the thermohaline circulation in Rahmstorf ’s comparisons is an order of magnitude greater than the flux reported by Curry, and she agrees that the circulation will not be unduly affected this century. Peter Wadhams, an oceanographer at the University of Cambridge, UK, last year reported a substantial weakening of convection ‘chimneys’ down which surface water flows in the Greenland sea, but it is unknown how much of the observed effect is due to natural variability.
It is possible that the Bryden result was an unlucky statistical artefact: “The results are based, after all, on just five snapshots of an extremely noisy and under-sampled system,” says Carl Wunsch. His own analysis doesn’t suggest any significant change over the last decade or so (though it’s probably worth pointing out that Wunsch’s opinions of the relative (un)importance of the Atlantic circulation for climate change are probably not (yet?) mainstream in the community).
Everyone quoted is however agreed on one thing: “the notion that [a future change in the themohaline circulation] may trigger a mini ice age is a myth”. The evidence of previous changes for instance at the Younger Dryas or during the 8.2 kyr event is quite strong, and significant coolings were observed particular around the North Atlantic, but even such localised coolings are not predicted to occur if the circulation slows as an effect of global warming.
It is however a complicated business, and the stability of this circulation depends on many aspects of climate that are poorly observed and uncertainly modelled. So it may yet be some time before new observations (such as the permanant monitoring array recently installed along a section of the ocean), better modelling, and a better appreciation of the paleo-climatic data add up to a coherent understanding of this interestingly counter-intuitive aspect of climate change.