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Reporting on climate

The AMOC is slowing, it’s stable, it’s slowing, no, yes, …

26 Jan 2025 by Stefan

There’s been a bit of media whiplash on the issue of AMOC slowing lately – ranging from the AMOC being “on the brink of collapse” to it being “more stable than previously thought”. AMOC, of course, refers to the Atlantic Meridional Overturning Circulation, one of the worlds major ocean circulation systems which keeps the northern Atlantic region (including Europe) exceptionally warm for its latitude. So what is this whiplash about?

As is often the case with such media whiplash, there isn’t much scientific substance behind it, except for the usual small incremental steps in the search for improved understanding. It is rare that one single paper overthrows our thinking, though media reports unfortunately often give that impression. Real science is more like a huge jigsaw puzzle, where each new piece adds a little bit.

The latest new piece is a new reconstruction of how the AMOC has changed over the past 60 years, by Jens Terhaar and colleagues. The background to this discussion is familiar to our regular readers (else just enter ‘AMOC’ in the RealClimate search field): proper measurements of the AMOC flow are only available since 2004 in the RAPID project, thus for earlier times we need to use indirect clues. One of these is the sea surface temperature ‘finger print’ of AMOC changes as discussed in our paper Caesar et al. 2018 (Fig. 1). There we used the cold blob temperature anomaly (Nov-May) as an index for AMOC strength. Other studies have used other sea surface temperature or salinity patterns as well as paleoclimatic proxy data (e.g. sediment grain sizes), and generally found an AMOC decline since the 19th Century superimposed by some decadal variability. The new paper critices our (i.e. Caesar et al) reconstruction and suggests a new method using surface heat fluxes from reanalysis data as an indicator of AMOC strength.

Figure 1 The ‘cold blob’ and the warm stretch along the Gulf Stream path (red arrows). This ‘finger print’ of an AMOC slowdown is physically understood (Zhang 2008) and visible not just for the satellite era shown above, but also the reanalysis data (Fig. 3a below) and observed sea surface temperature trends since the year 1870 (Caesar et al. 2018). Map: Ruijian Gou.

Here’s three questions about it.

1. Does the ‘cold blob’ work well as AMOC indicator?

We had tested that in the historic runs of 15 different CMIP5 climate models in Caesar et al. 2018 (our Fig. 5) and found it works very well, except for two outlier models which were known to not produce a realistic AMOC. Now Terhaar et al. redid this test with the new CMIP6 model generation und found it works less well, i.e. the uncertainty is larger (although for future simulations where the AMOC shows a significant decline in the models, our AMOC index also works well in their analysis).

Which raises the question: which models are better for this purpose: CMIP5 or CMIP6? One might think that newer models are better – but this does not seem to be the case for CMIP6. Irrespective of the AMOC, the CMIP6 models created substantial controversy when their results came out: the climate sensitivity of a subset of ‘hot models’ was far too high, these models did not reproduce past temperature evolution well (compared to observed data), and IPCC made the unprecedented move of not presenting future projections as straightforward model average plus/minus model spread, but instead used the new concept of “assessed global warming” where models are weighted according to how well they reproduce observational data.

In the North Atlantic, the historic runs of CMIP6 models on average do not reproduce the ‘cold blob’ despite this being such a striking feature of the observational data, as shown clearly in the Summary for Policy Makers of the IPCC AR6 (see Fig. 2 below). Of the 24 CMIP6 models, a full 23 underestimate the sea surface cooling in the ‘cold blob’. And most of the CMIP6 models even show a strengthening of the AMOC in the historic period, which past studies have shown to be linked to strong aerosol forcing in many of these models (e.g. Menary et al. 2020, Robson et al. 2022). The historic Northern Hemisphere temperature evolution in the models with a strong aerosol effect “is not consistent with observations” and they “simulate the wrong sign of subpolar North Atlantic surface salinity trends”, as Robson et al. write. Thus I consider CMIP6 models as less suited to test how well the ‘cold blob’ works as AMOC indicator than the CMIP5 models.

Figure 2 Comparison of observed and simulated annual mean surface temperature change for 1°C global warming (IPCC, 2021, Figure SPM.5). The models on average do not reproduce the observed cold blob.

2. Is the new AMOC reconstruction method, based on the surface heat loss, better?

In the CMIP6 models it looks like that, and the link between AMOC heat transport and surface heat loss to the north makes physical sense. However, in the models the surface heat loss is perfectly known. In the real ocean that is not an observed quantity. It has to be taken from model simulations, the so-called reanalysis. While these simulations assimilate observational data, over most of the ocean surface these are basically sea surface temperatures, but surface heat loss depends also on air temperature, wind speed, humidity, radiation and cloud cover in complex ways, all of which are not accurately known. Therefore these surface heat loss data are much less accurate than sea surface temperature data and in my view not well suited to reconstruct the AMOC time evolution. 

That is supported by the fact that two different reanalysis data sets were used, leading to quite different AMOC reconstructions. Also the AMOC time evolution they found differs from other reconstruction methods for the same time period (see point 3 below).

And there is another issue: we’ve previously looked at ERA5 surface heat flux trend, as shown here from my article in Oceanography 2024:

Figure 3 Sea surface temperature trend (left) and surface heat flux trend (right) 1940-2022 from the ERA5 reanalysis data also used in Terhaar et al. Source: Oceanography 2024.

You see in both figures (in temperature as well as surface heat flux) the AMOC slowdown ‘fingerprint’ which includes both the ‘cold blob’ and a warming along the American coast due to a northward Gulf Stream shift, which is also a symptom of AMOC weakening. However, Terhaar et al. integrate over the whole northern Atlantic north of 26 °N so that the red area of increasing heat loss largely compensates for the blue area of decreasing heat loss. So in their analysis these two things cancel, while in the established concept of the ‘fingerprint’ (see Zhang 2008: Coherent surface-subsurface fingerprint of the Atlantic meridional overturning circulation) these two things both reinforce the evidence for an AMOC weakening.

3. How do these new reconstructions compare to others?

Here is how the Terhaar reconstructions (bottom two) compare:

Figure 4 Several AMOC reconstructions, with the RAPID measurements on top. The reconstruction by Frajka-Williams et al. 2015 used surface height data from satellite, and the Worthington et al 2021 reconstruction uses a water mass regression based on RAPID data. Graph: Levke Caesar.

The reconstruction at the bottom using a reanalysis product from Japan doesn’t resemble anything, while the blue one using the European ERA5 reanalysis at least has the 1980s minimum and early 2000s maximum in common with other data, albeit with much smaller amplitude; it is a lot smoother. Thus it also misses the strong AMOC decline 2004-2010 and subsequent partial recovery seen in the RAPID measurements as well as the Caesar and Worthington reconstructions. A main reason for the lack of significant trend in the Terhaar reconstructions further is the time interval they used; for the same time span the Caesar reconstruction also does not show an even remotely significant trend (p-value is only 0.5), so in this respect our reconstructions actually agree for the period they overlap. The fact that ours shows a significant AMOC decline is because of the stable AMOC we find during 1900-1960, which is stronger than in the following sixty years. Here our reconstruction method shows its advantage in that reliable and accurate sea surface temperature data exist so far back in time.

Hence, I do not believe that the new attempt to reconstruct the AMOC is more reliable than earlier methods based on temperature or salinity patterns, on density changes in the ‘cold blob’ region, or on various paleoclimatic proxy data, which have concluded there is a weakening. But since we don’t have direct current measurements going far enough back in time, some uncertainty about that remains. The new study however does not change my assessment of AMOC weakening in any way.

And all agree that the AMOC will weaken in response to global warming in future and that this poses a serious risk, whether this weakening has already emerged from natural variability in the limited observational data we have, or not. Hence the open letter of 44 experts presented in October at the Arctic Circle Assembly (see video of my plenary presentation there), which says:

We, the undersigned, are scientists working in the field of climate research and feel it is urgent to draw the attention of the Nordic Council of Ministers to the serious risk of a major ocean circulation change in the Atlantic. A string of scientific studies in the past few years suggests that this risk has so far been greatly underestimated. Such an ocean circulation change would have devastating and irreversible impacts especially for Nordic countries, but also for other parts of the world.

Post script

Since I’m sometimes asked about that: last year a data study by Volkov et al. revised the slowing trend of the Florida current as well as the AMOC. Contrary to ‘climate skeptics’ claims, it has no impact on our long-term estimate of ~3 Sv slowing since 1950, i.e. -0.4 Sv/decade (Caesar et al. 2018). Both the original and the revised trend estimates for the RAPID section data (see Figure) suggest the recent AMOC weakening since 2004 is steeper than the long-term trend we estimated.

Filed Under: Climate Science, Featured Story, Instrumental Record, IPCC, Model-Obs Comparisons, Oceans, Reporting on climate Tagged With: AMOC, Atlantic Meridional Overturning Circulation, CMIP5, CMIP6, North Atlantic

2023 appears to follow an upward trend in the North Atlantic/Caribbean named tropical cyclone count

17 Dec 2023 by rasmus

This year’s (2023) tropical cyclone season in the North Atlantic and Caribbean witnessed a relatively high number of named tropical cyclones: 20. In spite of the current El Niño, which tends to give lower numbers. But it appears to follow a historical trend for named tropical cyclones with an increasing number over time.

The number of North Atlantic/Caribbean named tropical cyclones in and the estimates based on the area with sea surface temperature above 25.6°C <span id="cite_ITEM-25351-0" name="citation"><a href="#ITEM-25351-0">"(Benestad,</a></span>. Model was calibrated on the 1900-1960 (grey shaded) period.
The number of North Atlantic/Caribbean named tropical cyclones in and the estimates based on the area with sea surface temperature above 25.6°C (Benestad, 2009).

The curve presented above is an update of the analysis presented in 2020 and posted here on RealClimate.

[Read more…] about 2023 appears to follow an upward trend in the North Atlantic/Caribbean named tropical cyclone count

References

  1. R.E. Benestad, "On tropical cyclone frequency and the warm pool area", Natural Hazards and Earth System Sciences, vol. 9, pp. 635-645, 2009. http://dx.doi.org/10.5194/nhess-9-635-2009

Filed Under: Climate impacts, Climate Science, Featured Story, Hurricanes, Reporting on climate

Old habits

26 Sep 2023 by rasmus

Media awareness about global warming and climate change has grown fairly steadily since 2004. My impression is that journalists today tend to possess a higher climate literacy than before. This increasing awareness and improved knowledge is encouraging, but there are also some common interpretations which could be more nuanced. Here are two examples, polar amplification and extreme rainfall.

[Read more…] about Old habits

Filed Under: Arctic and Antarctic, Climate Science, Communicating Climate, Featured Story, hydrological cycle, Reporting on climate

Deciphering the ‘SPM AR6 WG1’ code

12 Aug 2021 by rasmus

I followed with great interest the launch of the sixth assessment report Working Group 1 (The Physical Science Basis) from the Intergovernmental Panel on Climate Change (IPCC) on August 9th. 

The main report is quite impressive (see earlier posts here, here, here, and here) but the press conference didn’t come across as being focused and well-prepared. In my opinion the press conference on 9 August 2021 didn’t do justice to the vast effort that went into it.

[Read more…] about Deciphering the ‘SPM AR6 WG1’ code

Filed Under: Communicating Climate, Reporting on climate

The best case for worst case scenarios

26 Feb 2019 by Gavin

The “end of the world” or “good for you” are the two least likely among the spectrum of potential outcomes.

Stephen Schneider

Scientists have been looking at best, middling and worst case scenarios for anthropogenic climate change for decades. For instance, Stephen Schneider himself took a turn back in 2009. And others have postulated both far more rosy and far more catastrophic possibilities as well (with somewhat variable evidentiary bases).

[Read more…] about The best case for worst case scenarios

References

  1. S. Schneider, "The worst-case scenario", Nature, vol. 458, pp. 1104-1105, 2009. http://dx.doi.org/10.1038/4581104a

Filed Under: Climate impacts, Climate modelling, Climate Science, Communicating Climate, IPCC, Reporting on climate, Scientific practice

Musing about Losing Earth

4 Aug 2018 by Gavin

The NY Times Magazine has a special issue this weekend on climate change. The main article is “Losing the Earth” by Nathaniel Rich, is premised on the idea that in the period 1979 to 1989 when we basically knew everything we needed to know that climate change was a risk, and the politics had not yet been polarized, we missed our opportunity to act. Stated this way, it would probably be uncontroversial, but since the article puts the blame for this on “human nature”, rather than any actual humans, extensive Twitter discussion ensues…

[Read more…] about Musing about Losing Earth

Filed Under: Climate impacts, Climate Science, Communicating Climate, Reporting on climate

Snow Water Ice and Water and Adaptive Actions for a Changing Arctic

27 Apr 2017 by rasmus

The Arctic is changing fast, and the Arctic Council recently commissioned the Arctic Monitoring and Assessment Programme (AMAP) to write two new reports on the state of the Arctic cryosphere (snow, water, and ice) and how the people and the ecosystems in the Arctic can live with these changes.

The two reports have now just been published and are called Snow Water Ice and Permafrost in the Arctic Update (SWIPA-update) and Adaptive Actions for a Changing Arctic (AACA).

[Read more…] about Snow Water Ice and Water and Adaptive Actions for a Changing Arctic

Filed Under: Climate impacts, Climate modelling, Climate Science, Oceans, Reporting on climate

Fake news, hacked mail, alternative facts – that’s old hat for climate scientists

8 Feb 2017 by Stefan

Distortion? False information? Conspiracy theories? Hacked email? Climate scientists have known all this for decades. What can be learned from their rich experience with climate propaganda.

The world is slowly waking up. “Post-truth” was declared the word of the year 2016 by the Oxford Dictionaries. Finally, people start to widely appreciate how dangerous the epidemic of fake news is for democracy.

Stir up hate, destroy discourse, make insane claims until no one can distinguish the most bizarre absurdity from the truth any more.

Thus the Austrian author Robert Misik aptly describes the strategy of right-wing populists.

Some call it “alternative facts”. (Those are the convenient alternative to true facts.) Let’s simply call it propaganda. [Read more…] about Fake news, hacked mail, alternative facts – that’s old hat for climate scientists

Filed Under: Climate Science, Communicating Climate, In the News, Reporting on climate, skeptics

New report: Climate change, impacts and vulnerability in Europe 2016

6 Feb 2017 by rasmus

Another climate report is out – what’s new? Many of the previous reports have presented updated status on the climate and familiar topics such as temperature, precipitation, ice, snow, wind, and storm activities.

The latest report Climate change, impacts and vulnerability in Europe 2016 from the European Environment Agency (EEA) also includes an assessment of hail, a weather phenomenon that is often associated with lightening (a previous report from EASAC from 2013 also covers hail).

Usually, there has not been a lot of information about hail, but that is improving. Still, the jury is still out when it comes to hail and climate change:

Despite improvements in data availability, trends and projections of hail events are still uncertain.

[Read more…] about New report: Climate change, impacts and vulnerability in Europe 2016

Filed Under: Climate impacts, Climate Science, hydrological cycle, Reporting on climate

Climatology and meteorology are your friends

19 Dec 2016 by rasmus

The Norwegian Meteorological institute has celebrated its 150th anniversary this year. It was founded to provide weather data and tentative warnings to farmers, sailors, and fishermen. The inception of Norwegian climatology in the mid-1800s started with studies of geographical climatic variations to adapt important infrastructure to the ambient climate. The purpose of the meteorology and climatology was to protect lives and properties.

[Read more…] about Climatology and meteorology are your friends

Filed Under: Climate Science, Communicating Climate, Instrumental Record, RC Forum, Reporting on climate

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