For Homo sapiens, a dry land species, discussions about the climate and how it is changing tend to revolve around what is going on in the atmosphere. This is a dangerous regional view, because the atmosphere is only one of two fluid systems that circulate above the solid surface of the Earth. The other, the ocean, is in many ways more important than the pair.
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It is the circulation of the ocean which, by redistributing heat, limits the temperature difference between the tropics and the poles to about 30 ° C. If the atmosphere alone were responsible for heat transfer, the that difference more like 110 ° C. And, when it comes to anthropogenic global warming, the problem would be much greater without the buffer effect of the ocean.
Not only does the ocean absorb heat that would otherwise remain in the air, it also absorbs a third of the carbon dioxide emitted by human activity . Although that makes seawater more acidic (or, strictly speaking, less alkaline), which can harm some marine species, much of the CO2 it will be stuck in the dark, where it will not have the effect of a greenhouse, and where it will likely last for many centuries.
So sad is the poverty of people’s understanding of ocean circulation, compared to the feeling of the atmosphere. And the AAAS this meeting produced 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.
Mariners discovered in the second half of the 20th century that the system’s engine room is in the North Atlantic. Here, in a process known as the Atlantic background overturning circulation (AMOC). After going down as much as 3km, it then goes south itself.
Although this type of sea change is going on elsewhere, 90% of it is happening in the North Atlantic. And it is this turning of the North Atlantic that drives what is often described as a conveyor belt of connected currents that runs across a planet.
That, at least, is conventional thinking. But Dr Lozier believes it is a bad symbol. A conveyor belt provides an image of smooth and linear progress. This belt, however, jumps all over the place, making it much harder to figure out what’s going on.
A smoothly moving belt only needs to be checked occasionally to determine if its rate of progress is changing. So when, in 2005, paper in Nature reported, according to the five relevant surveys of shipping carried out since 1957, a 30% reduction in the number of AMOC between 1992 and 2004, there was great concern. If such a collapse were to continue, it would change weather patterns, especially in Europe, by changing the planet’s heat distribution. It would also reduce the level at which CO2 carried into the deep sea.
As it happened, however, 2004 was a turning point in looking at what’s going on, because it saw the beginning of the use of a set of recording instruments that are now known as. RAPID AMOC. These track the Atlantic a degree or two north of the Tropic of Cancer, the part of the world where the studies reported in the Nature paper was led. RAPID AMOC together in 2014 with an Arctic artist, OSNAPthe Return in the North Atlantic Subpolar Program.
The result found is that the turnover rate can change, apparently randomly, as much as sixfold in a year. The fall is described in the Nature paper was the stuff of a poor data set.
Another find of OSNAP it has been that the details of where the reversal is happening in the Atlantic are not as models predicted. Most of the rotation, it turns out, occurs on the eastern side of the ocean, not on the western side, as was previously believed. While this may not matter much in the grand scheme of climate change, it is another example of how poorly people have understood what is going on at sea.
The next step for OSNAP is to extend its remit to look at the use of carbon dioxide. And more systematic studies are underway in other parts of the ocean, too, as landers are finally taking proper notice of the 71% of the planet’s surface that has been neglected until now, they are happy to call it “Land”, but which, in truth, could be called “Sea”. ■
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