Major Atlantic Ocean current system may be approaching ‘critical tipping point’

A major Atlantic Ocean current may have been losing stability over the past century, and could be “nearing a critical threshold”, with profound global impacts on weather and climate patterns.

Scientists studying the Atlantic Meridional Overturning Circulation (Amoc), which contains the Gulf Stream, said “the finding is worrying as well as a surprise”, as the current is responsible for the relatively mild temperatures in Europe and influences weather systems worldwide.

A potential collapse of the system could have “severe consequences”, according to the team, which is made up of researchers from 18 institutions in 10 different countries.

The Amoc carries heat from the tropics up to the northern hemisphere by transporting warm water masses northward at the ocean surface, and returning as a cool current southward at the bottom of the ocean.

If this powerful current collapsed, the scientists hypothesise it would still exist, to some degree, but would function in a much-reduced “weak mode”, as opposed to its existing “strong mode”.

The change would mean plunging temperatures across Europe, and would disrupt monsoon systems.

Previous research suggests the current is now the weakest it has been for at least a thousand years, and the scientists now believe a sudden transition to its weak mode could be imminent.

“The Atlantic Meridional Overturning really is one of our planet’s key circulation systems,” said the author of the study, Niklas Boers from the Potsdam Institute for Climate Impact Research, Freie Universität Berlin and Exeter University.

“We already know from some computer simulations and from data from Earth’s past, so-called paleoclimate proxy records, that the Amoc can exhibit – in addition to the currently attained strong mode – an alternative, substantially weaker mode of operation.”

He added: “This bi-stability implies that abrupt transitions between the two circulation modes are in principle possible.”

Since it was established that the current has been weakening, scientists have debated whether the observed weakening corresponds to a change in the average circulation state, or whether it is associated with an actual loss of the essential stability of the current.

“The difference is crucial,” said Dr Boers, “because the loss of dynamical stability would imply that the Amoc has approached its critical threshold, beyond which a substantial and in practice likely irreversible transition to the weak mode could occur.”

Although long-term observational data of the strength of the Amoc does not exist, the ocean cycle does produce physical evidence which can be studied.

This includes the sea-surface temperatures and salinity patterns of the Atlantic Ocean, which can be traced through observations, and are considered the “fingerprints” of the current.

“A detailed analysis of these fingerprints in eight independent indices now suggests that the Amoc weakening during the last century is indeed likely to be associated with a loss of stability,” said Dr Boers.

“The findings support the assessment that the Amoc decline is not just a fluctuation or a linear response to increasing temperatures but likely means the approaching of a critical threshold beyond which the circulation system could collapse.”

The researchers said there are a number of factors in play, which could impact ocean circulation, but said they are all ultimately linked to the climate crisis.

Warming global temperatures have increased freshwater inflow from the melting of the Greenland ice sheet, melting sea-ice, increasing precipitation and river run-off.

Freshwater is lighter than saltwater and reduces the tendency of the water to sink from the surface to greater depths, which is one of the drivers of the overturning.

“I wouldn’t have expected that the excessive amounts of freshwater added in the course of the last century would already produce such a response in the overturning circulation,” said Dr Boers.

“We urgently need to reconcile our models with the presented observational evidence to assess how far from or how close to its critical threshold the Amoc really is,” he added.

The research team said: “While the respective relevance of the different factors has to be further investigated, they’re all linked to human-caused climate change.”

The research is published in the journal Nature Climate Change.