What is AMOC, the heat-distributing Atlantic current?
system that helps distribute heat around the world is weakening. Why do scientists find this so worrying? AMOC is something of a poster child for tipping points, which are notional thresholds beyond which systems that have been responding gradually and incrementally to global warming undergo sudden and dramatic changes. One reason for this is its sheer power and the scope of its influence.
The rate at which it transfers heat towards the pole—about one petawatt, or 1,000 terawatts, roughly 60 times the rate at which humans produce energy by burning fossil fuels in factories, furnaces, power stations, cars, aircraft and everything else—accounts for about a quarter of all the northward flow of heat from the tropics. At least half of the water that gets into the ocean depths does so in the North Atlantic. Another reason is that its tipping-point nature is not open to question.
Theory, modelling and reconstructions of prehistoric climate all support the idea that AMOC is “bistable". Rather than just gradually getting stronger or weaker, it can go suddenly from “on" to “off" if pushed too far, and does so in a way that makes it very hard to flip it back on again. It was one of the first such instabilities clearly demonstrated in the climate system.
And on top of all that there has long been good reason to think that global warming may be pushing that switch. On July 25th a paper published in Nature Communications suggested that the change of state could come about by the middle of this century. The overturning circulation is often confused with the Gulf Stream, which runs across the Atlantic in the same direction, but there is a crucial difference.
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