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Thursday, October 29, 2009

Atlantic meridional overturning circulation (AMOC) and glacier runoff from Greenland

Glaciers and the Atlantic

by Graham Cogley,, October 26, 2009

The Atlantic keeps cropping up when we try to understand why glaciers change. If you look at the right kind of map, you can see that the Atlantic, including the Arctic, is an enormously long inlet in the shore of the world ocean. (The Bering Strait, between Alaska and Siberia, is too shallow to make a difference.)

The ocean is hot near the equator and not so hot near the poles. The heat flows down the temperature gradient, which drives the ocean currents. The water has to go somewhere once it has got where it is going and has surrendered the heat to the atmosphere. So it sinks. The sinking has a natural explanation: now that the water is colder, it is also denser.

At this point, the first two of several intertwined complications alter the picture. First, as it surrenders heat, the north Atlantic also surrenders water vapour. The warmer the water, the faster it evaporates. Second, during evaporation the salt stays behind, making the ocean water denser than it was to begin with, and that makes it yet more likely to sink.

The water that sinks gets back where it started from by flowing southwards at depth. Eventually it finds its way out of the Atlantic inlet and wells up, or is dragged up by the wind, in broad regions of the low-latitude ocean, where it is reheated by the Sun and thus begins the cycle over again. This is the essence of the meridional overturning circulation, or MOC. The Atlantic part of this circulation, predictably abbreviated AMOC, is a sensitive part of the machine because its northern end is where most of the Northern Hemisphere overturning happens. There is nothing comparable in the north Pacific.

The water vapour released from the north Atlantic adds greatly to the poleward deliveries of vapour through the atmosphere, and helps to make the northern shores of the Atlantic snowy. The glaciers around the north Atlantic today, and the much bigger ice sheets we had during the ice ages, owe a lot to the AMOC.

But now we come up against the third of the intertwined complications. After the water vapour has condensed and fallen on the bordering landmasses, it flows back sooner or later to the ocean, where it dilutes the salt that helps the AMOC through the crucial sinking part of its cycle. In principle, a large enough return flow of fresh water from rivers and glaciers could reduce the density of the surface waters sufficiently to stop them from sinking, in which case the whole AMOC would stop.

And now, enter the fourth of the intertwined complications: in one word, us. If we heat the whole system by enough to shrink the Greenland Ice Sheet significantly, flooding the north Atlantic with fresh water, we raise the prospect of just such a switching-off of the AMOC.

All the climate models suggest that, if the AMOC collapsed, the northward heat transfer would also be greatly reduced and the shores of the north Atlantic would suffer cooling. But fears of a new ice age being triggered by a collapse of the AMOC, itself triggered by a collapse of the Greenland Ice Sheet, are not realistic.

In the first place, these collapses would happen in a context of global warming, and again the climate-model evidence shows that they would not suffice for the job. Secondly, we have lots of palaeoclimatic evidence for abrupt changes in the AMOC, which are leading candidates to explain Dansgaard-Oeschger transitions during the last ice age, and the cold snap 8,200 years ago. They didn’t last all that long, and they were all reversible. Thirdly, models of ancient climates suggest persuasively that the AMOC is not implicated as a mechanism for starting ice ages.

And finally, the models agree that, without actually collapsing, the AMOC is nevertheless very likely to weaken over the next century. Even decanting the Greenland Ice Sheet into the ocean would not switch it off. But several metres of sea level rise, and a weaker AMOC in a warmer world, are enormous problems in themselves. That they are not harbingers of a colder world is not a good reason for relaxing.

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