It is this ocean circulation that, by redistributing heat, limits the temperature difference between the tropics and the poles to about 30 °C. If the atmosphere alone was responsible for moving heat, that difference would be something like 110°C. And, when it comes to anthropogenic global warming, the problem would be far greater without the buffering effect of the ocean.
The ocean not only absorbs heat that would otherwise remain in the air, it also swallows up a third of the carbon dioxide emitted by human activity. While this makes seawater more acidic (or, strictly speaking, less alkaline), which can harm some marine species, much of the CO2 added ends up in the abyss, where it does not cause any greenhouse effect. could, and where it is likely to remain for many centuries.
The poverty of human understanding of ocean circulation, as compared to that of the atmosphere, is therefore lamentable. And the AAAS meeting was hailed as an excellent lament on the matter by Susan Lozier of the Georgia Institute of Technology, who was also last year’s president of the American Geophysical Union.
Oceanographers discovered in the late 20th century that the system’s engine room was in the North Atlantic. Here, in a process called the Atlantic Meridional Overturning Circulation (AMOC), water moves up from the tropics, cools, thus increasing in density, until it becomes so heavy that it begins to sink. To replace, draw more water from the south. After descending for 3 km, it turns itself towards south.
Although some ocean overturning of this kind occurs elsewhere, 90% of it occurs in the North Atlantic. And it is the North Atlantic inversion that drives what is often described as a planet-spanning conveyor belt of connected currents.
That, at least, is the conventional thinking. But Dr. Lozier considers this a bad analogy. A conveyor belt conveys an image of smooth and linear progress. This belt, however, jerks everywhere, making it very hard to figure out what’s going on.
A smoothly running belt should only be checked occasionally to check if it has a varying rate of progress. Therefore, when in 2005, a paper in Nature reported a 30% decline in AMOC magnitude between 1992 and 2004, based on five relevant shipborne surveys conducted since 1957, there was serious concern. If such a decline continues, it will alter weather patterns by altering the planet’s heat distribution, particularly over Europe. It would also reduce the rate at which CO2 was transported to the deep ocean.
As it happened, however, 2004 was a turning point in the observations of what was going on, as it saw the beginning of the deployment of a set of recording equipment now known as Rapid AMOC. These overlook the Atlantic a few degrees north of the Tropic of Cancer, the part of the world where the surveys reported in the Nature paper were conducted. RAPID AMOC was joined in 2014 by an Arctic counterpart, OSNAP, the Overturning in the Subpolar North Atlantic Program.
The result is the finding that the reversal rate can vary, apparently randomly, by up to six times over the course of a year. The decline described in the Nature paper was an attribute of a poor data set.
Another finding of OSNAP has been that the details of where the reversal occurs in the North Atlantic are not as predicted by the models. Most of the turnover occurs on the east side of the ocean, not the west, as previously believed. While this doesn’t make much sense in the grand scheme of climate change, it’s another example of how poorly people understand what’s happening in the ocean.
The next step for OSNAP is to expand its range by looking at carbon dioxide absorption. More systematic studies are taking place in other parts of the ocean as well, as land-husbanding humans are finally paying proper attention to the hitherto neglected 71% of the planet’s surface, which they delight to call “Earth”. But what can really be called a “sea”.
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© 2023, The Economist Newspaper Limited. All rights reserved. From The Economist, published under license. Original content can be found at www.economist.com
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