The AMOC brings warm water north from the tropics near the surface and takes cool water in the opposite direction in the deep ocean
NOAA
A critical ocean current is unlikely to shut down before the end of this century even under the most extreme climate scenarios, according to new findings that undermine doomsday predictions of an imminent catastrophic collapse.
The Atlantic Meridional Overturning Circulation (AMOC) transports warm water from the tropics northward, helping to keep northern Europe temperate. Warming temperatures and an influx of cold water from melting Arctic ice are weakening the current and, scientists fear, could shut it down altogether. This would plunge oceanic ecosystems into chaos and rapidly cool Europe’s climate by several degrees.
Some researchers say an irreversible shutdown of the AMOC could happen this century. But this worst-case scenario is unlikely, says Jonathan Baker at the UK’s Met Office.
To explore whether a complete AMOC collapse this century is possible, Baker and his colleagues used 34 climate models to simulate changes to the AMOC under extreme climate change, with greenhouse gas levels trebling from today’s levels overnight. The team also modelled huge volumes of freshwater entering the North Atlantic at many times the rate of current ice melt.
They found that although the AMOC weakens significantly under these two scenarios, the ocean current would continue in its weakened state, supported by upwelling of North Atlantic deep water driven by winds in the Southern Ocean. “Southern Ocean winds continue to blow and this pulls deep waters up to the surface, which acts like a powerful pump,” says Baker. “That keeps the AMOC running this century in the models.”
The findings help to explain why climate models generally simulate a more stable AMOC in a warming world compared with research relying on statistical methods, which tend to suggest the AMOC is more fragile.
Niklas Boers at the Potsdam Institute for Climate Impact Research in Germany says the findings are “good news” for those worried about an imminent AMOC collapse. “All the state-of-the-art climate models agree on not showing a full AMOC collapse within the 21st century – because of the stabilising mechanisms induced by the Southern Ocean,” he says.
But although the models don’t foresee a full collapse of the AMOC, they show that quadrupling CO2 concentrations would lead to a significant reduction in the current’s strength, by 20 to 81 per cent.
If the AMOC weakens by around 50 per cent, the effects on climate would be significant, Baker says, with disruption to marine ecosystems, rising sea levels on North Atlantic coastlines and changes to global rainfall patterns impacting crop yields around the world. But this kind of weakening would not bring rapid cooling to Europe, he says.
By comparison, an AMOC that is 80 per cent weaker than it is today would have catastrophic effects, Boers stresses. “That is of course an almost shut-off AMOC,” he says. “That will give all the impacts in terms of cooling Europe and changing tropical monsoon patterns and everything we have been concerned about.”
Stefan Rahmstorf, who is also at the Potsdam Institute for Climate Impact Research in Germany, agrees that the world could be left with a weak and shallow AMOC current this century under extreme warming. Some studies even define an AMOC collapse as this kind of substantial weakening, he says, rather than a total shutdown of all circulation. “The new study investigates the remaining wind-driven overturning [current] in more detail, which is a valuable contribution to the scientific literature,” he says. “It does not, however, change the assessment of the risk and impact of future AMOC changes in response to human-caused global warming.”
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