New Scientist
A crucial ocean current, the Atlantic Meridional Overturning Circulation (AMOC), is unlikely to shut down entirely this century, even with extreme climate change. While it may weaken, Southern Ocean winds will help maintain its flow, preventing rapid cooling in Europe but still affecting sea levels and global rainfall patterns.
Vital ocean current is unlikely to completely shut down this century
Author: Madeleine Cuff
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 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 before 2100 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 fresh water 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 (Nature, doi.org/g86gqz). “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.”
Southern Ocean winds will continue to blow and that keeps the current running
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, they show that quadrupling carbon dioxide concentrations would lead to a significant reduction in the current’s strength, by 20 to 81 per cent. If the AMOC weakens by 50 per cent, the effects would be significant, Baker says, with rising sea levels on North Atlantic coastlines and changes to global rainfall. But this kind of weakening wouldn’t 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, says Boers. “That is of course an almost shutoff 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, agrees that the AMOC could weaken this century under extreme warming. Some studies even define an AMOC collapse as this kind of substantial weakening, he says. The study doesn’t change the assessment of the risk and impact of future AMOC changes in response to global warming, he says.
Credits: TCA, LLC.