Could an ocean circulation system − the Gulf Stream, say − sort of  shut down? And what would that do to the world’s climate?

LONDON, 9 August, 2021 − Once again, new research has warned that one of the great engines of global climate, known variously as the Atlantic conveyor belt, a current that spans the entire ocean from the surface to its lowest depths, or (not very accurately) the Gulf Stream, could be about to falter.

That is, thanks to global heating, it could be about to switch from a relatively stable state to a “critical transition” towards a much feebler regime.

If it does so, that’s bad news for Europe, because part of what oceanographers call the Atlantic Meridional Overturning Circulation, or AMOC, is the Gulf Stream, a surface flow that brings tropical warmth to what would otherwise be chilly north-western European nations.

And it could be very bad news for billions in tropical Africa, Asia and South America, because it could trigger changes in the tropical monsoon system.

Repeated warnings

Climate scientists have been measuring indicators of possible change in the ocean circulation system for at least two decades: any shift in ocean behaviour could signal a tipping point, a serious shift in climate for the terrestrial world.

The current brings warm, dense salty water north to the Arctic, where it meets less dense meltwater from Greenland and the Arctic glaciers and dives to the ocean floor, to flow south all the way to Antarctica before it surfaces again.

Researchers have warned on an almost yearly basis that as greenhouse gas emissions grow, and global temperatures creep up, the ocean currents could become less stable: Europe’s relatively mellow climate could cool; it could do so some time this century; and when it did, it would disrupt global weather patterns.

The latest study, in the journal Nature Climate Change, is partly based on long-term climate data and reconstructions of past climates, themselves based on ice cores, fossil evidence and ocean deposits.

“If AMOC shuts down, this could negatively impact the climate further afield, such as the West African monsoon system”

These suggest that AMOC can exist in a stable state, or a weak one: more to the point, as it weakens, it could suddenly shift or tip into a new circulation mode. And what could be one of the agents of sudden change might be the increasing flow of cold fresh water from the warming Arctic.

This is consistent with many of the observations of the last decade. What isn’t certain is whether a sudden change is imminent. Is the seeming weakening of the flow part of a long-term natural pattern, or does it herald a dramatic loss of stability? What is the Gulf Stream really up to?

“The difference is crucial, because the loss of dynamical stability would imply that AMOC has approached its critical threshold, beyond which a substantial and in practice likely irreversible transition to the weak mode could occur,” said Niklas Boers of the Potsdam Institute for Climate Impact Research in Germany, the author of the research.

“I wouldn’t have expected that the excessive amounts of fresh water added in the course of the last century would already produce such a response in the overturning circulation.”

Winners and losers

Dr Boers calls for more and more detailed research, and for better climate models that would allow climate scientists to make a more precise judgment of the consequences of what could be a shutdown of ocean circulation. The case is not closed, and Professor Tim Palmer of the University of Oxford, UK, points out that the study is based on indirect evidence.

Direct observations of the deep ocean current do not, he says, suggest that the Atlantic conveyor belt could be close to collapse or shutdown. But he too has argued for a concerted international effort to build better computer simulations of the planetary climate system. This could help to show what is happening to the Gulf Stream.

“The Gulf Stream is forced by atmospheric winds and these will continue to blow. If the AMOC does shut down, the Gulf Stream will flow a little further south than where it flows now. This will lead to cooler temperatures over the North Atlantic and hence over Northern Europe. This may help offset the effects of climate change in these regions (and potentially help stabilise Greenland ice loss − which would be a good thing),” Professor Palmer said.

“On the other hand, if AMOC shuts down, this could negatively impact the climate further afield, such as the West African monsoon system and the moisture flow into the Amazon.” − Climate News Network