James' World 2

Hmmmm …. Atlantic Meridional Overturning Circulation (AMOC)

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On a sunny evening in July, a research ship called the Sir David Attenborough was working near the Sorgenfri glacier in south-eastern Greenland when a kilometre-long wall of craggy, bulging ice collapsed into the fjord. The scientists on deck watched in stunned silence as an estimated 18 million tonnes of ice slowly flipped over, sending up vast plumes of spray.

Iceberg calving on this scale was once relatively rare – but it has become commonplace in recent years as Earth warms. And that acceleration could have catastrophic impacts that extend far beyond Greenland. It is weakening a system of ocean currents in the North Atlantic that is vital to societies in Europe and around the world. If the Atlantic Meridional Overturning Circulation (AMOC) were to shut down, the continent would plunge into “ice age” winters with year-round long droughts, and agriculture would become close to impossible.

Yet whether the AMOC really will fail, when that might happen and how quickly – these points all remain highly uncertain. And because of that, the threat remains a distant one in policy-makers’ minds. That is why an £81 million plan is now afoot to build an early warning system that could alert humanity if AMOC is nearing a dangerous tipping point.

Data now being collected by the Attenborough research ship is core to that plan. “We might be committed to centuries of change without knowing it,” said ocean scientist Paul Holland at the British Antarctic Survey (BAS) as he prepared to set sail from the port of Harwich, UK, some weeks earlier. “We might be passing these tipping points without realising.”

So how worried should we be about the AMOC collapsing, what will it take to build an early warning system, and will the alarm bells persuade governments to pull back from the point of no return – or at least to prepare for the consequences?

The AMOC is a vast current system that circulates in the ocean between North America and Europe, driven by temperature and salinity. Around the Gulf of Mexico, the harsh sun heats the ocean and drives evaporation. The resulting warm, salty water flows towards the seas between Greenland and the UK in a part of the AMOC called the Gulf Stream. As the circulation pushes north, the atmosphere cools the salty water, so that it becomes denser and cascades towards the ocean floor like a subsurface waterfall. From there, water flows southward along the seafloor back towards the Americas. To replace the water sinking in the North Atlantic, more warm water must then flow from the Gulf of Mexico, powering the circulation.

The AMOC is a crucial section of an undulating but well-established conveyor belt of ocean currents that flow around the globe. Traversing the entire belt takes a typical “parcel” (about 1 cubic metre) of water about 1000 years.

Like the rest of this conveyor belt, the AMOC, which circulates between North America and Europe, is driven by temperature and salinity. Around the Gulf of Mexico, the harsh sun heats the ocean and drives evaporation. The resulting warm, salty water flows towards the seas between Greenland and the UK in a part of the AMOC called the Gulf Stream. As the circulation pushes north, the atmosphere cools the salty water, so that it becomes denser and cascades towards the ocean floor like a subsurface waterfall. From there, water flows southward along the seafloor back towards the Americas. To replace the water sinking in the North Atlantic, more warm water must then flow from the Gulf of Mexico, powering the circulation.

Without the AMOC, models predict that sea ice would cover the North Sea down to Norfolk in the UK and Friesland in the Netherlands. London would experience an average winter temperature of 2°C (36°F), with cold snaps of -20°C (-4°F). Similar cold would descend on Amsterdam, Brussels, Dublin and Paris. Oslo would see deep freezes of -48°C (-54°F) or below at least every 10 years.

“The winter is like being in the ice age, and in Scotland, it’s in the ice age for half the year,” says Earth system scientist Tim Lenton at the University of Exeter, UK.

Yet this vital circulation is also the section of the global conveyor belt most likely to slow down, or shut down completely, in the coming decades or centuries due to climate change. This possibility was first suggested in a 1987 paper by geochemist Wallace Broecker at Columbia University in New York. Fluctuations in Earth’s climate aren’t always gradual, he wrote, and human-made “global warming”– a term he popularised – could disrupt the feedbacks fuelling the AMOC.

At the time, evidence was mounting that AMOC had shut down in the past because of natural changes in the planet’s temperature. About 12,900 years ago, as the last glacial period was ending, the northern hemisphere suddenly slipped back into near-glacial conditions for 1300 years – a period called the Younger Dryas, after an Arctic flower that spread through Europe then. Shortly after Broecker’s paper was published, geochemists studying the composition of shells trapped in marine sediments found that the AMOC was sending less surface water to the seafloor during the Younger Dryas.

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Doug Chayka

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