The dangers of a collapse of the main Atlantic Ocean circulation, known as AMOC, have been “greatly underestimated” and would have devastating and irreversible impacts, according to an open letter released at the weekend by 44 experts from 15 countries. One of the signatories, Stefan Rahmstorf, an oceanographer and climatologist who heads the Earth system analysis department at the Potsdam Institute for Climate Impact Research in Germany, explains here why he has recently upgraded his risk assessment of an AMOC breakdown as a result of global heating – and what that means for Britain, Europe and the wider world.
AMOC, or the Atlantic meridional overturning circulation, is a system of ocean currents that brings heat into the northern Atlantic. Warm surface water from the tropics flows north and releases its heat in the subpolar Atlantic, south of Greenland and west of Britain and Ireland. Then it cools and sinks to a depth of between 2,000m to 3,000 metres before returning south as a cold current. AMOC is one of our planet’s largest heat transport systems, moving the equivalent of 50 times the human energy use, and it has a particularly strong impact on the climate in Europe, affects the ocean’s CO2 uptake and oxygen supply, as well as rainfall patterns in the tropics.
There are indications that AMOC has been slowing down for the last 60 or 70 years due to global heating. The most ominous sign is the cold blob over the northern Atlantic. The region is the only place in the world that has cooled in the past 20 years or so, while everywhere else on the planet has warmed – a sign of reduced heat transport into that region.
Another indicator is a reduction in the salt content of seawater. In the cold blob region, salinity is at its lowest level since measurements began 120 years ago. This is probably linked to AMOC slowing down and bringing less salty water and heat from the subtropics.
When the water is less salty, it is less dense, which makes it harder to sink down. That is important because the sinking process is what drives AMOC. The fresher the water, the slower it gets.
Salinity is directly affected by global heating, which enhances the water cycle so there is more evaporation in the subtropics and more precipitation in the subpolar oceans. This leads to a freshening of the subpolar ocean. Then there are additional contributions from the melting of sea ice and the loss of continental ice from the Greenland ice sheet, which is freshwater that flows into the ocean.
It is an amplifying feedback: as AMOC gets weaker, the subpolar oceans gets less salty, and as the oceans gets less salty then AMOC gets weaker. At a certain point this becomes a vicious circle which continues by itself until AMOC has died, even if we stop pushing the system with further emissions.
The big unknown here – the billion-dollar question – is how far away this tipping point is. It is very difficult to answer because the process is non-linear.
Until a few years ago, the general thinking in the Intergovernmental Panel on Climate Change (IPCC) was that the probability of crossing the tipping point this century was less than 10%. Since then, there have been a number of studies suggesting a collapse would probably be triggered this century, possibly in the next few decades. So my risk assessment has really changed. I am now very concerned that we may push AMOC over this tipping point in the next decades. If you ask me my gut feeling, I would say the risk that we cross the tipping point this century is about 50/50.
That just means AMOC is then doomed and it will slowly die, but that process could take 50 to 100 years. Because the AMOC is already weakening we can’t be entirely sure whether we already passed a tipping point, but I would say this is most likely not the case, so it is not too late to prevent this.
We need to keep monitoring the flow of water in the Atlantic, which is being done with the Rapid project.
The effects [of AMOC breakdown] include a cooling of the northern hemisphere, particularly northwestern Europe. There would also be a shift of the tropical rainfall belt to the south, which is bad because the rains will move away from the rainforests to regions that are not used to so much rainfall. So this will mean droughts in some regions and floods in others.
AMOC collapse would also have a major impact on the northern Atlantic sea level, which would rise by half a metre or so, in addition to the rise caused by global heating. It would also reduce the CO2 uptake of the ocean because AMOC sinking in the northern Atlantic takes a lot of CO2 down into the deep oceans where it is safely locked away from the atmosphere.
AMOC collapse would also change the nutrient supply and reduce the oxygen of the deep oceans. This would have a massive effect on marine biology and the entire ecosystem of the northern Atlantic.
The last time, it took about 1,000 years to recover, though the past is not a direct analogue because there is also massive CO2 forcing this time – CO2 is already higher than any time in 15 million years. There are physical reasons why some form of deep overturning circulation will eventually come back.
One thing is for sure: humanity will not die out. But for some countries that will be in the midst of this, like Norway, and Scotland, the risks will be existential and raise the question whether people can continue to live there or whether most of them would rather move.
We have already crossed the tipping point of many coral reefs, which are now in middle of global die-off. This is very depressing because it is already too late to do anything about it, though marine biologists have warned about the risks for a long time. The Amazon rainforest is also dangerously close to a tipping point. As we speak, it is going through the worst drought on record.
Then in the very long run, we have the ice sheet tipping points in Greenland and west Antarctica. From Greenland alone, this will lead to a seven-metre global sea level rise that will wipe all major coastal cities off the map. But that will occur over many centuries because ice sheet melt is a slow process.
I am worried about all of these things to be honest. And the conclusion for all of them is the same: this is all driven mainly by fossil fuel emissions and also deforestation, so both must be stopped. We must stick to the Paris agreement and limit global heating as close to 1.5˚C as possible.