If greenhouse emissions continue at current levels, key ocean currents in the Atlantic will completely collapse by the year 2060, leading to “serious consequences” across the globe.
This is the warning of a pair of researchers from Denmark, who extrapolated from ocean surface temperature data going back some 150 years.
The large system of ocean currents in question is the “Atlantic Meridional Overturning Circulation” (AMOC), which transport warm waters from the tropics into the North Atlantic.
The team’s statistical analysis indicated a 95 percent certainty that the current system will fail between 2025 and 2095 — with 2057 being the most likely date for the collapse.
The findings directly contradict those of the latest report from the UN’s Intergovernmental Panel on Climate Change (IPCC), which — based on climate modelling — concluded that an abrupt change in the AMOC to be very unlikely this century.
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The AMOC can be thought of as being like part of a giant conveyor belt of water trundling about the Earth’s ocean.
The current system is driven by density and temperature changes — a process scientists call “thermohaline circulation”.
As warm water flows northwards, it cools and some evaporates, making what’s left behind also saltier and denser.
Now colder and denser, the water precedes to sink to a depth of several miles and to slowly spread southwards.
Eventually, it is drawn upwards to form the “start” of the conveyor system again and warmed up in a process known as “upwelling”.
Climate models have long suggested that the AMOC will weaken over the course of the 21st century in response to the emission of greenhouse gases.
This, as the Met Office explains: “is because as the atmosphere warms, the surface ocean beneath it retains more of its heat. “Meanwhile, increases in rainfall and ice melt mean it gets fresher too.
“All these changes make the ocean water lighter and so reduce the sinking in the ‘conveyor belt’, leading to a weaker AMOC.”
This weakening is anticipated when making projections for how the UK’s climate will change going forward.
The Met Office added: “A weaker AMOC will bring less warm water northwards, and this will partly offset the warming effect of the greenhouse gases over western Europe.
“For the gradual weakening that is likely over the 21st Century, the overall effect is still a warming.”
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The study was undertaken by climate physicist Professor Peter Ditlevsen of the Niels Bohr Institute and mathematician Professor Susanne Ditlevsen of the University of Copenhagen.
Prof. Peter Ditlevsen said: “Shutting down the AMOC can have very serious consequences for Earth’s climate, for example, by changing how heat and precipitation are distributed globally.
“A cooling of Europe may seem less severe as the globe as a whole becomes warmer and heat waves occur more frequently.”
However, he added, “this shutdown will contribute to an increased warming of the tropics, where rising temperatures have already given rise to challenging living conditions.
“Our result underscores the importance of reducing global greenhouse gas emissions as soon as possible.”
The researchers’ prediction that the AMOC will collapse sometime in the mid-century is based on “early warning signals” — signs that the currents involved are growing unstable.
These signals have been reported previously, the duo note, but it is only now that sufficiently advanced statistical methods have allowed us to predict when the collapse will happen.
Prof. Susanne Ditlevsen said: “Using new and improved statistical tools, we’ve made calculations that provide a more robust estimate of when a collapse of the Thermohaline Circulation is most likely to occur, something we had not been able to do before.”
The team’s analysis centred on sea surface temperatures in a specific area of the North Atlantic that acts as a metaphorical barometer for the strength of the AMOC.
The AMOC has collapsed previously. In fact, the thermohaline circulation has only been operating in its present mode since the last ice age.
Evidence for some 25 jumps during the last glacial period between the AMOC’s present and a collapsed state have been detected in ice cores from Greenland.
Each of these shifts saw extreme changes in the Earth’s climate — some 18–27F (10–15C) over the course of just a decade.
The full findings of the study were published in the journal Nature Communications.
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