By Paul Homewood

h/t Ian Magness

 Somebody’s been watching 2012 again!

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 “conveyor 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, said study author Niklas Boers, a researcher at the Potsdam Institute for Climate Impact Research in Germany…

The AMOC is the product of a gigantic, ocean-wide balancing act. It starts in the tropics, where high temperatures not only warm up the seawater but also increase its proportion of salt by boosting evaporation. This warm, salty water flows northeast from the U.S. coastline toward Europe — creating the current we know as the Gulf Stream.

But as the current gains latitude it cools, adding density to waters already laden with salt. By the time it hits Greenland, it is dense enough to sink deep beneath the surface. It pushes other submerged water south toward Antarctica, where it mixes with other ocean currents as part of a global system known as the “thermohaline circulation.”

So scientists have not actually directly observed the AMOC slowing down, but give me some more grant money and I will keep researching it!

The whole concept is frankly ludicrous anyway. The world has been much warmer than now for most of the time since the end of the ice age, particularly the Arctic regions. Yet  the AMOC never collapsed then.

They say it has happened before, and reference the sudden collapse of Lake Agassiz 8000 years ago, which sent unbelievably large amounts of freshwater into the Atlantic, triggering the Younger Dryas. But this catastrophic event cannot be compared with the minor oceanic changes taking place now.

The AMOC is a fundamental element of the Atlantic Multidecadal Oscillation, AMO. When the overturning circulation decreases, the North Atlantic temperatures become cooler, and vice versa.

It is suggested that the AMOC is running out of steam, but this reflects the cyclical nature of the AMO. During its warm phase, warm, tropical waters are carried towards the Arctic, where they sink due to their saltiness. Eventually the excess ocean heat in the tropics is exhausted, and the AMOC slows down, leading to a colder Atlantic.

This is also complemented by freshwater entering the Arctic. During the warm phase of the AMO, precipitation tends to increase in the Arctic. In particular, when this happens much more freshwater enters the Arctic Ocean from Siberian rivers. Freshwater freezes at a higher temperature than saltwater, so sea ice extent increases.

Indeed, this was exactly what happened during the 20thC in the Arctic. What was known as the The Warming in the North between 1920 and 1960, the sharp rise in temperatures around the Arctic (to similar levels as today), was triggered  by a large rise in oceanic salinity in the far north, as warmer Atlantic waters flowed north. (For more, see here).

Conversely, this was followed by The Great Salinity Anomaly between the 1960s and 80s, when the AMOC slowed down and freshwater increased in the Arctic Ocean. In time, as we know, the cycle reversed again, as it does every 25 years or so.

If the AMOC really is “running out of steam”, that would suggest that the AMO will soon turn cold again, leading to a slowdown or even reversal of global warming for the next three decades.


August 6, 2021