AMOC amok

One of the great mysteries of the possible global impacts of climate change is the fate of the Gulf Stream more properly known as the Atlantic Meridional Overturning Circulation (AMOC).

A new Nature paper (2 March 2022) by Laura C. Jackson, Arne Biastoch, Martha W. Buckley, Damien F. Desbruyeres, Eleanor Frajka-Williams, Ben Moat and Jon Robson on the evolution of AMOC since 1980 throws more light on what might be happening with AMOC and why.

Earlier research published in March 2021 and reported in the blog last year suggested the system could be breaking down.

These complex interactions of climate change and AMOC changes were highlighted by one event during Scott Morrison’s June 2021 visit to the UK for the G7. You may recall he took a side trip to see some relatives at a time when thousands of other Australians at the time would have loved to be able to do the same

On the trip he received a gift from Boris Johnson – a gift which was a tangible illustration of the complex interrelationship of current global warming and the possible consequences of an AMOC reversal.

It was a bottle of wine produced from grapes grown in the south east of England.

Due to climate change the south east of England is now producing a wide range of high quality sparkling wines. French producers are buying land and establishing vineyards there as rising temperatures are causing problems in some champagne growing areas and Jancis Robinson, among others, has been praising the quality of the English production.

However, according to that earlier research the English good fortune may not last as more evidence is emerging that the Gulf Stream, the Atlantic Meridional Overturning Circulation (AMOC), the key circulation system of the Atlantic Ocean, transporting warm masses northward from the tropics at the surface and southward at the bottom of the ocean from near Greenland may be breaking down.

The latest research published in Nature this year complicates matters even more finding evidence of strengthening and weakening while acknowledging the role of anthropogenic warming.

The authors say: “Decadal changes in the AMOC, whether through internal variability or anthropogenically forced weakening, therefore have wide-ranging impacts. In this Review, we synthesize the understanding of contemporary decadal variability in the AMOC, bringing together evidence from observations, ocean reanalyses, forced models and AMOC proxies.

“Since 1980, there is evidence for periods of strengthening and weakening, although the magnitudes of change (5–25%) are uncertain. In the subpolar North Atlantic, the AMOC strengthened until the mid-1990s and then weakened until the early 2010s, with some evidence of a strengthening thereafter; these changes are probably linked to buoyancy forcing related to the North Atlantic Oscillation.

“In the subtropics, there is some evidence of the AMOC strengthening from 2001 to 2005 and strong evidence of a weakening from 2005 to 2014. Such large interannual and decadal variability complicates the detection of ongoing long-term trends but does not preclude a weakening associated with anthropogenic warming. Research priorities include developing robust and sustainable solutions for the long-term monitoring of the AMOC, observation–modelling collaborations to improve the representation of processes in the North Atlantic and better ways to distinguish anthropogenic weakening from internal variability.”

However, the earlier Nature paper, plus another one published in February last year, suggested AMOC variations may be indicative of loss of stability in the system.

A number of scientists undertook research, published in a March 2021 PNAS paper, which 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.”

An earlier Nature geoscience paper, Current Atlantic Meridional Overturning Circulation weakest in last millennium, published in February this year firmed up the conclusions.

The authors compared a variety of published proxy records to reconstruct the evolution of the AMOC since about 400CE. The flow has only been directly measured since 2004 and these proxy measures provide significant additional data.

So, it will be interesting to see what is more powerful – climate change or AMOC destablisation and what strange outcomes that may bring.

It could be like the last ice age which enabled early humans to walk from what is now Europe across what is now the English Channel to what is now the UK.

It took a while and marine archaeology on the Dogger Bank shows they had many stops on the way.

If they were doing it in the future they may either be able to do the same or have to swim or jump on a boat. By then the boat people  would, at least, not be facing an unwelcoming Johnson Government intent on sending them off to Uganda.

The Nature articles were brought to the author’s attention by John Spitzer