Leave it to CNN to jump the shark with scary claims Excerpts in italics with my bolds.

A crucial system of ocean currents is heading for a collapse that ‘would affect every person on the planet’

A new study published Tuesday in the journal Nature, found that the Atlantic Meridional Overturning Current – of which the Gulf Stream is a part – could collapse around the middle of the century, or even as early as 2025.

Scientists uninvolved with this study told CNN the exact tipping point for the critical system is uncertain, and that measurements of the currents have so far showed little trend or change. But they agreed these results are alarming and provide new evidence that the tipping point could occur sooner than previously thought.

Yikes!  Shades of Day After Tomorrow

Scientists Admonish Against Going Over the Top

Fortunately knowledgable experts in the area have weighed with context and perspective at Science Media Centre Expert reaction to paper warning of a collapse of the Atlantic meridional overturning circulation. 

Of course some commented as cheerleaders, but many cautioned against exaggeration and speculation. The RAPID programme (see diagram at top) measures daily flows of water at several depths between the North Atlantic and Arctic oceans, and its scientific coordinator, Prof Meric Srokosz, National Oceanography Centre, University of Southampton, said:

While the possible collapse of the AMOC with significant climatic impacts is a concern, providing a warning of its collapse is problematic as a long set of observations is required. In this paper the warning depends on using proxy AMOC data (here based on sea surface temperature, SST) as direct continuous AMOC measurements are only available since 2004. The warning comes from applying statistical techniques to a long time series (over a century) of proxy AMOC data, but the warning is only as good as the proxy data are in representing the true AMOC. So, this warning needs to be treated with caution as there is no consensus as to which proxies can accurately capture the behaviour of the AMOC over the long term.”

Prof Penny Holliday, Head of Marine Physics and Ocean Circulation at the National Oceanography Centre, and Principal Investigator for OSNAP, an international programme researching AMOC processes, variability and impacts, said:

“Confidence in the validity of the conclusions are undermined by our knowledge that sea surface temperature of the North Atlantic subpolar gyre is not a clear indicator of the state of the AMOC, and that there is no evidence that the AMOC has dramatically weakened in the past 50-75 years. A collapse of the AMOC would profoundly impact every person on Earth but this study overstates the certainly in the likelihood of it taking place within the next few years.”

Does the press release accurately reflect the science?

“On the whole it does – the title of the paper is more sensational than the actual statements within it, and the press release does make that clear. However there are two statements that are not accurate as follows:

‘The strength of the AMOC has only been monitored continuously since 2004 and these observations have shown AMOC to be weakening’

“This is stated in the paper but it is not correct information. The observations since 2004 show that the AMOC goes through fluctuations of being in a stronger or weaker state that last for about 10 years. The observations since 2004 show the subtropical AMOC getting slower from 2004 to 2012, but gradually becoming stronger since then. The only data from AMOC observations shown in the paper are from 5 sparse ship surveys and are used out of context – the authors use them to argue for a severe decline in the AMOC, but that interpretation has long been discredited in the scientific literature (including in the reference they cite for it).

‘The authors found early warning signals of a critical transition of the AMOC system and suggest that it could shut down or collapse as early as 2025 and no later than 2095.’

“This is not quite as the paper states. In the paper the time period of potential collapse depends on choices they have made in how they construct the time series of sea surface temperature which they use as evidence for change. They present three versions of the temperature records, and the three resulting model predictions suggests a collapse is ‘likely’ at any time from 2024 to 2180. The 2025-2095 is the period of time their statistical model predicts that a full or partial collapse is most likely.

How does this work fit with the existing evidence?

“The conclusions are different to the consensus derived from climate projections as described by the IPCC AR6 assessment. The averaged AMOC projections from climate models under all the IPCC emissions scenarios all show an AMOC decline, but not a collapse (a “high confidence” conclusion). Some individual climate model runs do show a future collapse in the AMOC, so the possibility cannot be entirely ruled out.

Are there any important limitations to be aware of?

“There are some questionable assertions and decisions in the methods as follows. The authors state confidently that the sea surface temperature (SST) of the North Atlantic subpolar gyre can be used as a proxy for the strength of the AMOC. The validity of an SST proxy for AMOC strength is a matter of ongoing scientific debate however, because it is based on model behaviour and is not proven using real-world data.

There is solid evidence that there is no such clear relationship,
especially on timescales of less than 30 years.

“I believe the authors have overstated the pattern of subpolar North Atlantic SST change by subtracting two (and three) times the global mean surface temperature trend. This is not the usual approach for highlighting North Atlantic regional temperature trend (instead it is more usual to subtract just 1 x the global trend). The choice means that some of the SST data they use in the statistical model has exaggerated decline since the 1970s when the global SST began to sharply rise. In the version of the statistical model for which the global mean SST trend is removed, the predicted likely time of a partial or complete collapse becomes later and over a wider window of time.

“As mentioned above, the actual observations of AMOC since 2004 have long-since discredited the evidence that the authors are using to validate their modified SST temperature record. The 5 data points they show in the paper were collected several years apart by ship surveys, and it is well known and well established that they give a highly misleading impression of AMOC decline. All the observational evidence we have shows no evidence of dramatic decline in the AMOC over the past 50-75 years.

How uncertain are the uncertainties?

“The authors say that the model’s 95% confidence interval is 2025-2095. This is a measure of statistical uncertainty and they state in the discussion that they cannot rule out slowing rather than a collapse, as well as listing other reservations and caveats. Because of the limitations of their use of modified SST as a proxy for AMOC, the uncertainty in the stated message in the title and abstract is high.

What are the implications in the real world?

“The potential for the AMOC system of currents to collapse under global warming is a high impact, low likelihood scenario, and policymakers and planners do need to be aware of it. NOC and international partners are investing in ongoing observations of the AMOC in order to determine how closely changes in AMOC contribute to changes in SST and consequential climate and social and economic impacts on people. The strength of the out-of-sight ocean currents of the AMOC has surprisingly direct impacts on food, water and energy security, infrastructure risk, biodiversity, and human health. The paper demonstrates that decades of observations are needed to be able to detect a major tipping point in the AMOC, and the authors call for continued measurements of these great Atlantic ocean currents for long enough to do so.

Prof. Dr. Jochem Marotzke, Director of the Department Ocean in the Earth System, Max-Planck-Institute for Meteorology, Hamburg, Germany, said:

“The work provides no reason to change the assessment of the 6th IPCC Assessment Report: ‘There is medium confidence that there will be no abrupt collapse before 2100′. The statement so confidently made in this paper that collapse will occur in the 21st century has feet of clay. The maths are solid, but the starting point is highly dubious: the essential equation – marked with (1) in the paper – relies on the simplified models representing bifurcation – i.e. AMOC collapse – also being correct. But the more comprehensive models do not show this very bifurcation. In this respect, the paper does not live up to its self-imposed claim: ‘The strategy is to infer the evolution of the AMOC solely on observed changes in mean, variance and autocorrelation.’

The interpretation relies to an enormous extent on the authors’ theoretical
understanding being correct, and there are huge doubts about that.

“It must be added that there is considerable doubt as to whether surface temperature measurements are a valid proxy for the AMOC. Again, the paper addresses these uncertainties inadequately.

“When reporting about this study, it is important to include the key aspects in which this paper fails to include the scientific uncertainties.

Prof Niklas Boers, Professor of Earth System Modelling at the Technical University of Munich, said:

“I do not agree with the outcome of this study. While the qualitative statement that the AMOC has been losing stability in the course of the last century is true and supported by the data, uncertainties are too high to reliably estimate a time of tipping. In particular, the uncertainties in the heavily oversimplified model assumptions by the authors are too high. Moreover, the uncertainties in the underlying datasets are huge and would make the extrapolation carried out by the authors far too uncertain to actually report a year or even a decade for the AMOC tipping.”

Background Post 2019 AMOC Update: Oceans Moderate Climate Threat