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.
Oceans are darkening all over the planet – what’s going on? Data now being collected by the Attenborough research ship is core to that plan. 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.
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