A Critical Ocean System May Be Heading For Collapse Due to Climate Change

The Washington Post reports:

Human-caused warming has led to an 'almost complete loss of stability' in the system that drives Atlantic Ocean currents, a new study has found — raising the worrying prospect that this critical aquatic 'conveyer belt' could be close to collapse.

In recent years, scientists have warned about a weakening of the Atlantic Meridional Overturning Circulation (AMOC), which transports warm, salty water from the tropics to northern Europe and then sends colder water back south along the ocean floor. Researchers who study ancient climate change have also uncovered evidence that the AMOC can turn off abruptly, causing wild temperature swings and other dramatic shifts in global weather systems. Scientists haven't directly observed the AMOC slowing down. But the new analysis, published Thursday in the journal Nature Climate Change, draws on more than a century of ocean temperature and salinity data to show significant changes in eight indirect measures of the circulation's strength. These indicators suggest that the AMOC is running out of steam, making it more susceptible to disruptions that might knock it out of equilibrium, says study author Niklas Boers, a researcher at the Potsdam Institute for Climate Impact Science in Germany.

If the circulation shuts down, it could bring extreme cold to Europe and parts of North America, raise sea levels along the east coast of the United States and disrupt seasonal monsoons that provide water to much of the world.

'This is an increase in understanding... of how close to a tipping point the AMOC might already be,' said Levke Caesar, a climate physicist at Maynooth University who was not involved in the study. Boers' analysis doesn't suggest exactly when the switch might happen. But 'the mere possibility that the AMOC tipping point is close should be motivation enough for us to take countermeasures,' Caesar said. 'The consequences of a collapse would likely be far-reaching...' The new analysis suggests 'the critical threshold is most likely much closer than we would have expected,' Boers said...

[T]he apparent consequences of the AMOC slowing are already being felt. A persistent 'cold blob' in the ocean south of Greenland is thought to result from less warm water reaching that region. The lagging Gulf Stream has caused exceptionally high sea level rise along the east coast of the United States. Key fisheries have been upended by the rapid temperature swings, and beloved species are struggling to cope with the changes. If the AMOC does completely shut down, the change would be irreversible in human lifetimes, Boers said. The 'bi-stable' nature of the phenomenon means it will find new equilibrium in its 'off' state. Turning it back on would require a shift in the climate far greater than the changes that triggered the shutdown.

'It's one of those events that should not happen, and we should try all that we can to reduce greenhouse gas emissions as quickly as possible,' Boers said. 'This is a system we don't want to mess with.'

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