An ocean circulation system that lowers temperatures in the tropics and keeps some areas in northern latitudes warmer could collapse as a result of climate change, researchers found.
Unless efforts are made to reduce emissions, the research suggests that the Atlantic meridional overturning circulation (Amoc) could collapse about the middle of this century – or even as early as 2025.
Written by two researchers in Denmark and published in Nature Communications, the new study predicts "severe impacts on the climate in the North Atlantic region" should the collapse occur.
"The main take-home message is that it may actually happen this century," Prof Susanne Ditlevsen, of the department of mathematical sciences at the University of Copenhagen, told The National.
Without the Amoc, Prof Ditlevsen said that Scandinavia could end up with a climate similar to that of Alaska, which is not warmed by an equivalent circulation in the Pacific Ocean.
In some of the hottest parts of the world, the effect of the loss of the Amoc would be to amplify the existing effects of climate change – with potentially severe implications.
"The tropical areas will heat up because they don’t get rid of the heat," Prof Ditlevsen said. "That means an increase in the tropical areas on top of global warming."
There would also be greater risks of storms, more extreme weather and changed precipitation patterns, all of which would have "huge implications" for how societies are organised, she added.
The analysis is based on a business-as-usual scenario in which greenhouse gas emissions continue to increase, which is "one of the worst scenarios" in reports by the Intergovernmental Panel on Climate Change (IPCC).
Under this framework, there is a high likelihood that the Amoc could cease to function some time between 2025 and 2095, the new study finds, with the middle ground estimate being about 2050.
"We don’t know how big a reduction in CO2 emissions or greenhouse gas emissions is needed to stop this, but it’s definitely something we would strongly recommend," Prof Ditlevsen said.
"We have to take serious measures to reduce our carbon footprint. We don’t say we have already passed the tipping point. We hope it would be possible to stop it."
The UK’s Met Office states in an online briefing document that Amoc is driven seawater density, which is affected by temperature and salt content.
At the moment, warm water from the tropics flows northwards into the North Atlantic, cools and experiences some evaporation, which, the organisation states, increases the salt content.
This makes the water denser and causes it to sink before it moves southwards, well below the ocean surface, and comes back to the surface.
Scientists had already forecast that climate change would weaken the Amoc because waters would be warmer (because of increased temperatures) and would have lower salt content (because of increased rainfall). These two factors would work to reduce the strength of the circulation. Until the latest study, however, the collapse this century of the Amoc had been thought unlikely.
Gulf stream will 'largely be unaffected'
While the collapse of the Amoc would have significant implications, Prof Ditlevsen said that the Gulf Stream, a major current in the Atlantic, would largely be unaffected.
The Gulf Stream, which is is a much larger circulation pattern than the Amoc and involves greater quantities of water and heat, is largely driven by winds and the rotation of the Earth.
Prof Ditlevsen acknowledged "so many uncertainties in this study", but said that, given the long timescales on which changes to such systems happen, the analysis had produced a "quite narrow" time band during which collapse may take place.
There is limited direct observational data of the Amoc, with records stretching back less than two decades, so the study used information on sea surface temperatures, for which there are more than 150 years’ worth of data, as a proxy for the Amoc.
Over the coming years, as more data becomes available, it should be possible to produce better predictions, Prof Ditlevsen said.
The Amoc has collapsed and reformed several times as a result of ice ages coming and going before human civilisation developed.
The study, "Warning of a forthcoming collapse of the Atlantic meridional overturning circulation," is also authored by Prof Peter Ditlevsen, of the Niels Bohr Institute at the University of Copenhagen.