New research on when AMOC may go amok

There has been much research and speculation about whether the Atlantic Meridional Overturning Circulation (AMOC, or more popularly the Gulf Stream) may collapse and what the consequences might be.

Now there is new 2023 research which firms up predictions as to when it might happen. The bad news – it might be earlier than previously estimated.

A March 2021 PNAS paper, had said: “the recently discovered AMOC decline during the last decades is not just a fluctuation related to low-frequency climate variability or a linear response to increasing temperatures. Rather, the presented findings suggest that this decline may be associated with an almost complete loss of stability of the AMOC over the course of the last century, and that the AMOC could be close to a critical transition to its weak circulation mode.”

What this might mean was addressed in another 2021 Nature (2021) article which said: “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.”

The paper focussed on early-warning indicators stating that “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.”

A Nature geoscience paper, Current Atlantic Meridional Overturning Circulation weakest in last millennium, published in February this year firmed up the conclusions by using a variety of published proxy records to reconstruct the evolution of the AMOC since about 400CE.

Now the direct measurement of the flow since 2004 and these proxy measures have provided significant additional data for a new paper.

The paper, in the 25 July 2023 Nature Communications, by Peter and Suzanne Ditlevsen of the University of Copenhagen is based on the clear warming trend over the 1870 to 2020 period and firms up the likelihood of AMOC collapse suggesting it could come sooner than earlier estimates have suggested.

They report that a forthcoming collapse of AMOC is a major concern as it is one of the most important tipping points in Earth’s climate system.

They point out that earlier assessments and modelling by the IPCC (Intergovernmental Panel on Climate Change) suggest that a full collapse was unlikely within the 21st Century.

Yet the Ditlevsens, using updated data and models, estimate at 95% confidence that the transition of the AMOC is most likely to occur between 2025 and 2095 with a possibility of a mid-century reversal.

They say: “The AMOC has only been monitored continuously since 2004 through combined measurements from moored instruments, induced electrical currents in submarine cables and satellite surface measurements….However longer records are needed to assess the significance. For that careful fingerprinting techniques have been applied to longer records of sea surface temperature, which, backed by a survey of a large ensemble of climate model simulation significance …to contain an optimal fingerprint of the strength of the AMOC.”

What would be the consequences? We don’t know, although a new Ice Age in the northern hemisphere would be possible and the extent to which it will be tempered by rising temperatures due to climate change are unknown at present. To put it mildly, whatever the consequences they won’t be pretty.

One thing is certain though, the grandchildren and great grandchildren of the current mob of climate denialists will be having a very unpleasant time. Sadly, they won’t be around to see it. Their descendants are also unlikely to be inheriting any of the beachfront mansions the current generation own.

The author has written a bit about AMOC over the past few years. Two of the pieces can be found at the links below.  They, and the Duitlevsen paper were brought to the author’s attention by his friend John Spitzer.

AMOC amok | Noel Turnbull

It may be hard to imagine but it could have been worse | Noel Turnbull