Greenland: meltwater is a threat
Greenland: meltwater is a threat to the ocean circulation that sustains the Gulf Stream © Getty Images

Among many potential planetary “tipping points” — sudden and irreversible changes threatened by global warming — one of the more alarming is the possible collapse of the ocean circulation that sustains the Gulf Stream and, therefore, the mild climate of western Europe.

The Atlantic Meridional Overturning Circulation (Amoc) is like an immense marine conveyor that carries warm equatorial water northwards in the ocean’s upper layers while cooler water flows southward at greater depths. The heat transfer is equivalent to 10 Hiroshima-sized atomic bombs exploding every second, says Andrew Meijers, deputy science leader of the British Antarctic Survey’s polar oceans team.

Climate models have long predicted that Amoc will weaken as the world warms — and observations confirm that this is happening. Direct evidence has only been available since 2004. Then, a UK-US project called Rapid started continuous monitoring with an array of instruments across the Atlantic from Morocco to Florida, which measured temperature, salinity and current velocities from the surface to the sea floor.

But indirect observations, ranging from shipping logs to environmental evidence such as ocean sediments, take the history of Amoc much further back in time. These suggest that Amoc was fairly steady for many centuries until the mid- to late-1800s. The best estimate is that anthropogenic global warming has reduced the mass flow of Amoc by 15 per cent, says Meijers.

What is the Gulf Stream?

Stefan Rahmstorf, head of earth system analysis at the Potsdam Institute for Climate Impact Research in Germany, says: “Ocean and climate modelling shows further weakening during the 21st century, some by a little and some by a lot. There will be some variability. I would not be surprised if Amoc recovers a bit over the next five to 10 years.”

The currents are driven by differences in ocean temperature and salt concentration, which affect the water’s density. As warm Atlantic water flows northwards, the surface layers cool as some evaporation occurs, increasing salinity. Both effects make the water denser. It sinks deeper into the ocean and slowly moves southwards. Eventually, it returns to the surface and warms in “upwelling” regions many thousands of kilometres further south.

Global warming tends to weaken Amoc by increasing the surface temperature and by decreasing salinity as freshwater from melting Greenland and Arctic ice enters the ocean. There may also be a positive feedback loop as a weakening Gulf Stream reduces the northward supply of salty water.

Climate models find it difficult to handle the complexities in this process, leading to much uncertainty about what is likely to happen next.

“The latest generation of climate models suggest further weakening of 34 to 45 per cent by 2100 as a result of global warming,” Rahmstorf says. “This could bring us close to the tipping point at which the flow becomes unstable.” Then, Amoc could shut down entirely.

The latest technical report by the Intergovernmental Panel on Climate Change, released in August, concludes: “While there is medium confidence that the projected decline in Amoc will not involve an abrupt collapse before 2100, such a collapse might be triggered by an unexpected meltwater influx from the Greenland ice sheet.

“If an Amoc collapse were to occur, it would very likely cause abrupt shifts in the weather patterns and water cycle, such as a southward shift in the tropical rain belt, and could result in weakening of the African and Asian monsoons and strengthening of Southern Hemisphere monsoons.” Previous IPCC reports had expressed “high” rather than “medium” confidence that Amoc would not shut down. Richard Wood, head of the climate, cryosphere and oceans group at the UK Met Office Hadley Centre, warns: “Though . . . collapse probably will not happen, the impact will be high if it does.”

Models show that, as well as shifting rain belts in the tropics, a partial or complete collapse of Amoc would cool temperate regions across the northern hemisphere while intensifying heating in the south. In Europe, winters would become colder and more stormy while summers become drier.

Meijers also warns of the effects of changes in Amoc for the Southern Ocean around Antarctica, which absorbs most of the extra heat and carbon dioxide generated by human activities. “It sweeps most of the consequences of climate change under the carpet,” he says. “The question is how much longer it will continue to do so.”

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