One of the world’s great geophysical phenomena is the flow of warm water, and associated weather, from the Gulf of Mexico across the Atlantic towards western Europe. This flow is known as the Gulf Stream and the North Atlantic Drift and although huge in scale, it is merely a surface feature of a much bigger ocean process.
As this current travels north, warm water evaporates, leaving the surface water saltier and denser. Then, as it reaches the Arctic, the water begins to cool and freeze, making the remaining liquid even saltier and denser.
This salty, dense water sinks to the bottom of the Atlantic Ocean and begins the long journey back to the tropics and beyond, at depths of several thousand meters, where it begins the long cycle again. Any given parcel of water takes a thousand years to complete this conveyor belt ride.
The whole process is known as the Atlantic Meridional Overturning Circulation or AMOC. In the last twenty years or so, climate scientists have uncovered evidence that this vast ocean current has switched off in the past, such as during the last ice age. And recently, they have begun to worry that climate change is slowing it down so that it could once again come to a halt, with devastating impacts for Atlantic marine life and European land life.
Imminent Collapse
The question they desperately need to answer is over what timescale this is likely to occur.
Now we have an answer thanks to the work of Emma Smolders and colleagues at Utrecht University in the Netherlands, and it is not good news. These researchers say that global warming is almost certain to cause the collapse of the AMOC before the end of the century and more likely than not, before 2050.
The mechanism is straightforward. Global heating is melting the Greenland ice sheets and Arctic ice cap. This is increasing the flow of freshwater into the Atlantic and reducing the rate at which surface water freezes to form ice.
These processes are reducing the salinity and density of northern waters, making them less likely to sink and removing a key driver of the AMOC current. Indeed, there is growing evidence that the flow of water in various parts of the cycle is reducing in volume.
Scientists have long hoped that gradual change can be reversed by cooling the planet. That assumes, however, that the climate does not pass through so-called tipping points—where small changes in temperature lead to dramatic changes in climate patterns. The collapse of the AMOC is one of these feared tipping points.
So an important goal for climate scientists is to determine when their climate models predict the collapse will occur. The results make for worrying reading. Smolders and co say that climate models suggest the AMOC will switch off well before the end of the century.
“The collapse time is estimated between 2037-2064,” they say. “The probability of an AMOC collapse before the year 2050 is estimated to be 59 per cent ± 17 percent.”
That’s well within the lifespans of the majority of people alive today.
What’s more, the warning signs are likely to be few and far between. The sea surface temperatures in the sub-polar regions of the North Atlantic are unlikely to offer early warnings, say the team. Indeed, it turns out that the best place to look for early warning signs is the southern boundary of the Atlantic Ocean, one the remotest parts of the planet. That will need to be watched carefully in the coming years.
But there is a sense of inevitability about what happens next. “A collapse from its current strong northward overturning state to a substantially weaker or reversed state would have major climate impacts,” say Smolders and co.
Climate Carnage
The AMOC is hugely important because it carries nutrients across the Atlantic that support whole ecosystems of sea life. And it keeps western Europe’s climate moist and warm compared to similar latitudes elsewhere. New York, for example, has a similar latitude as Madrid and London as Moscow, but climates in these places are entirely different. The AMOC is the reason.
Any significant change would lead to substantial cooling in northwestern Europe while altering sea levels in the North Atlantic. The AMOC also influences the position of southern Asian tropical rain belts, so a collapse could cause widespread droughts in these areas.
Perhaps most worrying is that these consequences are likely to be upon us well before humanity reaches its goals of achieving net zero, let alone reducing atmospheric carbon levels. Whether or not such a reduction could restart the AMOC and over what kind of timescale is an open question.
Smolders and co are not the first to predict an imminent collapse of the AMOC. But their reanalysis of the data and their simulations with state-of-the-art climate models are consistent with previous warnings and significantly increase their urgency.
Last year, the Intergovernmental Panel on Climate Change, the IPCC, considered the collapse of the AMOC to be unlikely before 2100. Smolders and co say this now needs to be urgently. Reconsidered.
The IPCC’s next report is due towards the end of this decade. Whether that will leave enough time for action seems all too clear.
Ref: Probability Estimates of a 21st Century AMOC Collapse :arxiv.org/abs/2406.11738
