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How unstable is the pattern of ocean circulation?

David Thornalley advises caution over long-term trends

How unstable is the pattern of ocean circulation?

Following publication in February of research co-authored by UCL Geography’s Dr David Thornallay, highlighting the weakening of the Atlantic Meridional Overturning Circulation (AMOC), he was invited by both The Guardian and the Daily Mail on-line in early August to comment on further evidence published by Nature Climate Change.

The research, by Niklas Boers, examines datasets of temperature and salinity going back 150 years, showing that global heating is not just changing the Circulation’s flow pattern, but also increasing the instability of its currents. During the last century, the AMOC may thus have evolved from relatively stable conditions to a point close to a critical transition.

In David’s view, “These signs of decreasing stability are concerning. But we still don’t know if a collapse will occur, or how close we might be to it.” (The Guardian on-line, 5th August).

Elaborating in the Daily Mail on-line:

'The study might be showing increasing instability in a feature of ocean circulation (which is a concern in itself), but it might not be AMOC.

Scientists should be cautious about the quality of the observational data from the early part of the record, as direct ocean measurements were sparse.

This could mean the variability in the first part of the record is not accurately captured, which could alter the assessment of long term trends in variability and stability.


Boers, N. (2021) ‘Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation’ Nature Climate Change, 11, 680–688.

L. Caesar, G. D. McCarthy, D. J. R. Thornalley, N. Cahill, S. Rahmstorf (2020): Current Atlantic Meridional Overturning Circulation weakest in last millenniumNature Geoscience, 25 February 2021, [DOI: 10.1038/s41561-021-00699-z]

The Guardian: t

Daily Mail:

See also:

Earth’s Gulf Stream System at its weakest in over a millennium:


The Gulf Stream System is at its weakest in over a millennium. © Natalie Renier/Woods Hole Oceanographic Institution)

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