There has been much consideration and controversy recently about the AMOC – the Atlantic Meridional Overturning Circulation – which is the Atlantic current that distributes heat around the world, and great confusion about how this differs from the Gulf Stream. Indeed, an opinion article in The Times even managed to confuse the two by saying quite untruthfully that “scientists predicted the Gulf Stream system could collapse by 2025”.
The origin of the concern was a paper that Peter and Susanne Ditlevsen, University of Copenhagen, published in Nature Communications in July. Sadly, as some people have clearly latched onto this topic as yet another thing that can scare people into climate action (a not altogether reliable motivator at the best of times), it seems important to be as clear as possible about the two phenomena and how they are related.
As this is bound to be a hot topic in every eco-club in UK schools after the holidays, we thought we’d inject the calming effect of an Economist article into discussions. This begins:
“Ask a climate scientist about possible “tipping points” and you are likely to hear about AMOC. The Atlantic Meridional Overturning Circulation is a stream of water which, as it flows from the southern to the northern (hence “meridional”) part of the Atlantic, grows cooler and saltier. Eventually it sinks to the ocean floor, 3km down, and flows back (hence “overturning”) across the abyssal plain. Mounting evidence suggests that the 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. The Gulf Stream is pushed along by winds which will persist whatever the climate. …”
If you want more detail, you could read the original paper. Very few on-line commentators seem to have done so …