Irreversible change to the Gulf Stream

 From NBC News

The Atlantic Ocean’s current system, an engine of the Northern Hemsiphere’s climate, could be weakening to such an extent that it could soon bring big changes to the world’s weather, a scientific study said on Thursday.

The Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean currents that transports warm water from the tropics northwards into the North Atlantic.

As the atmosphere warms due to increased greenhouse gas emissions, the surface ocean beneath retains more of heat. A potential collapse of the system could have severe consequences for the world’s weather systems.

Climate models have shown that the AMOC is at its weakest in more than a 1,000 years. However, it has not been known whether the weakening is due to a change in circulation or it is to do with the loss of stability.

The study, published in the journal Nature Climate Change, said the difference is crucial.

“The loss of dynamical stability would imply that the AMOC has approached its critical threshold, beyond which a substantial and in practice likely irreversible transition to the weak mode could occur,” said Niklas Boers at the Potstdam Insitute for Climate Impact Research and author of the study.

By analyzing the sea-surface temperature and salinity patterns of the Atlantic Ocean, the study said the weakening of the last century is likely to be associated with a loss of stability.

“The findings support the assessment that the AMOC decline is not just a fluctuation or a linear response to increasing temperatures but likely means the approaching of a critical threshold beyond which the circulation system could collapse,” Boers said.

If the AMOC collapsed, it would increase cooling of the Northern Hemisphere, sea level rise in the Atlantic, an overall fall in precipitation over Europe and North America and a shift in monsoons in South America and Afrcia, Britain’s Met Office said.

Other climate models have said the AMOC will weaken over the coming century but that a collapse before 2100 is unlikely.

Here's the abstract for the Nature Climate Change article:

The Atlantic Meridional Overturning Circulation (AMOC), a major ocean current system transporting warm surface waters toward the northern Atlantic, has been suggested to exhibit two distinct modes of operation. A collapse from the currently attained strong to the weak mode would have severe impacts on the global climate system and further multi-stable Earth system components. Observations and recently suggested fingerprints of AMOC variability indicate a gradual weakening during the last decades, but estimates of the critical transition point remain uncertain. Here, a robust and general early-warning indicator for forthcoming critical transitions is introduced. Significant early-warning signals are found in eight independent AMOC indices, based on observational sea-surface temperature and salinity data from across the Atlantic Ocean basin. These results reveal spatially consistent empirical evidence that, in the course of the last century, the AMOC may have evolved from relatively stable conditions to a point close to a critical transition.

Here's an article about AMOC  from Carbon Brief, which is also the source of the chart.

Schematic of the AMOC. The red pathways show warmer water nearer the surface, while the purple pathways show colder, more dense water moving at depth. Credit: Met Office

We are clearly in a climate emergency.  We need to move fast to cut emissions.  And we are still dithering.