August 6, 2021

expert reaction to study looking at early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation

A study published in Nature Climate Change looks at observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation (AMOC).

Prof Tim Palmer, Royal Society Research Professor at the University of Oxford, said:

“There are a number of caveats which need to be mentioned in relation to Boers’ Nature Climate Change article on the Atlantic Meridional Overturning Circulation (AMOC). This is a three-dimensional circulation that spans the entire Atlantic and South Atlantic regions and, unlike the Gulf Stream, extend down into the deep ocean. 

“Firstly, it does not imply the Gulf Stream will collapse. The Gulf Stream is forced by atmospheric winds and these will continue to blow. If the AMOC does shut down, the Gulf Stream will flow a little further south than where it flows now. This will lead to cooler temperatures over the North Atlantic and hence over Northern Europe. This may help offset the effects of climate change in these regions (and potentially help stabilise Greenland ice loss – which would be a good thing).

“On the other hand, if AMOC shuts down, this could negatively impact the climate further afield, such as the West African monsoon system and the moisture flow into the Amazon.

“Deep ocean direct observations of AMOC do not lend strong support to Boers’ claim that the  AMOC is  close to collapse. His study is based on indirect observations  (of sea surface temperature and salinity). On the other hand, we do not have many years of direct observations. It is vital that direct deep-ocean observations of AMOC continue.

“Although simple idealised models suggest AMOC has tipping point behaviour, this is not confirmed in most contemporary comprehensive climate models. These latter models, by contrast, merely suggest a slowdown in AMOC due to  our carbon emissions. On the other hand, it is possible that the contemporary climate models are not representing the dynamics of AMOC well due to poor horizontal and vertical resolution. Increased computer resources are needed to have more confidence in whether or not AMOC exhibits tipping-point behaviour. I personally have written about the need for a “CERN  for climate change” to pool resources to provide the supercomputing needed to simulate climate change – and AMOC in particular – more accurately.

“In conclusion, whilst Boers’ study suggests the possibility or risk of an immediate AMOC shutdown, it is not clear how likely this is in reality. And whatever happens to AMOC, the Gulf Stream will continue to flow.”

‘Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation’ by Niklas Boers was published in Nature Climate Change on Thursday 5^th August.

DOI: 10.1038/s41558-021-01097-4

Declared interests

None received.