February 25, 2021

expert reaction to study of the Atlantic Meridional Overturning Circulation (AMOC)

A study published in Nature Geoscience suggests that the Atlantic Meridional Overturning Circulation (AMOC), also known as the Gulf Stream System, is the weakest it’s been in the last millennium.

Dr Laura Jackson, a Met Office scientist specialising in AMOC, said:

“This paper adds to evidence that the AMOC has been weakening over the historical period, but there are still uncertainties associated with using these indirect observations. It is very likely the AMOC will weaken with increasing greenhouse gases, but unlikely we would see a collapse. However, a collapse or a slowdown would be extremely impactful, so AMOC research remain a high priority.”

Dr David Ferreira, Associate Professor in Meteorology at the University of Reading:

“Despite satellites, Argo floats and many other measurement devices deployed in the ocean today, we only have a sketchy view of the present-day AMOC, except in two locations in the North Atlantic where arrays have been deployed. It is important to keep this in mind when considering this study.

“Here, it is also crucial to make the distinction between the Gulf Stream and the AMOC. The Gulf Stream is driven by the atmospheric winds which do not show strong trends. The AMOC, however, is associated with convection and the formation of cold dense water around Greenland. Although the two circulations muddle up along the coast of the US, a healthy Gulf Stream can remain in place even with a weak AMOC.”

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

“Linking AMOC variability to the Gulf Stream, as stated in the press release, is somewhat misleading. The Gulf Stream is primarily a wind-driven ocean circulation, and there is no evidence that I am aware of that the Gulf Stream is itself weakening. The extent to which AMOC variability affects European weather is a matter of great uncertainty, as there are many other drivers for variations in European weather, including atmospheric chaos. Overall, this is an interesting study and one that needs continued investigation. However, it shouldn’t be over-interpreted.”

Dr Andrew Meijers, Deputy Science Leader of Polar Oceans at British Antarctic Survey, said:

“This is a very novel study that has produced an exciting result.  While proxy based data always needs to be interpreted with caution, the authors have used a wide range of independent sources for their study, and so it is very telling that almost all point to a significant slowdown in the AMOC over the last century.  Other studies have suggested such a decline over recent decades, but this work provides new long term context and reveals that prior to the era of human induced climate change the Atlantic overturning circulation was relatively stable and stronger than it is now.  This indicates that the slowdown is likely not a natural change, but the result of human influence.  The AMOC has a profound influence on global climate, and particularly in North America and Europe, so this evidence of an ongoing weakening of the circulation is critical new evidence for the interpretation of future projections of regional and global climate.  Additionally, the AMOC is frequently modelled as having a tipping point below some circulation strength, a point at which the relatively stable overturning circulation becomes unstable or even collapses.  The ongoing weaking of the overturning means we risk finding that point, which would have profound and likely irreversible impacts on climate.”

Dr Karsten Haustein, Climate Service Center Germany, said:

“The paper expands on the lead author’s earlier paper (Nature, 2018), where they provided solid evidence that the AMOC strength has indeed declined since the late 19th century. In the 2018 paper, they were also able to demonstrate that the warming hole in the North Atlantic ocean is the emerging signal associated with human-induced global warming. These findings are corroborated by climate models as well as statistical analysis, as shown, for example, in our paper¹.

“Here, the authors expand the AMOC timeseries from 2018 further back in time, using published proxy records up until 400 AD. Those proxy data draw a consistent picture of the past evolution of the overturning circulation. They find that it only started to decline in the early 19th century, having reached its weakest state in the past decade. While the AMOC won’t ‘collapse’ anytime soon, the authors warn that the current could become unstable by the end of this century if warming continues unabated. It has already been increasing the risk for stronger hurricanes at the US East coast due to warmer ocean waters, as well as potentially altering circulation patterns over Western Europe. In my view, the results are robust and in line with theoretical expectations.”

¹ https://journals.ametsoc.org/view/journals/clim/32/16/jcli-d-18-0555.1.xml

Dr Jon Robson, Principal Research Scientist, University of Reading, said:

“This study builds on previous research to highlight that “proxies”, or indirect measures, of the ocean circulation consistently indicate that there has been a large change in the North Atlantic since 1900. One worrisome interpretation is that a significant weakening of the Atlantic Overturning Circulation has occurred, which inevitably raises questions of whether human emissions of CO2 have been to blame and what the implications might be. For example, these findings might suggest that the Atlantic Ocean circulation is more sensitive to climate change than has previously been thought. However, that is only one interpretation, and without direct observations, we remain unsure of how to interpret these changes. As the authors note, there is a pressing need to bring all the tools available to us to interpret these changes across the North Atlantic so that we can better predict what may happen next and to understand the wider implications for society.”

‘Current Atlantic Meridional Overturning Circulation weakest in last millennium’ by L. Caesar et al. was published in Nature Geoscience at 4pm UK TIME on Thursday 25 February 2021.

DOI: https://doi.org/10.1038/s41561-021-00699-z

Declared interests

None received.