Mastodawn

anlomedad Jul 29, 2024

" Also, we find that rate-induced effects tend to allow a stabilization of the #AMOC in cases where the peak of the West Antarctica Ice Sheet meltwater flux occurs before the peak of the Greenland Ice Sheet meltwater flux."

Ah great. So we should pray for the #WAIS reaching its tipping point before #Greenland 's ice shield melts down to its point of no return? 😁
https://esd.copernicus.org/articles/15/859/2024/esd-15-859-2024.html Sacha #Sinet et al, 2024

It's not what they mean, I know. What I actually wanted to highlight is the circumstance that AMOC is also sensitive to the rate or speed in which freshwater is added, not only to the final amount of it. And this was also found in their study.

I first came across it in a 2021 paper by #Lohmann #Ditlevsen "Abrupt climate change as a rate-dependent cascading tipping point" https://esd.copernicus.org/articles/12/819/2021/esd-12-819-2021.html ,
Peter Ditlevsen being the brother in the sibling duo who in 2023 found a cool way to analyse noisy data in search for impending tipping behaviour and said, AMOC tips 2025-2097.

AMOC stability amid tipping ice sheets: the crucial role of rate and noise

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) has recently been categorized as a core tipping element as, under climate change, it is believed to be prone to critical transition implying drastic consequences on a planetary scale. Moreover, the AMOC is strongly coupled to polar ice sheets via meltwater fluxes. On the one hand, most studies agree on the fact that a collapse of the Greenland Ice Sheet would result in a weakening of AMOC. On the other hand, the consequences of a collapse of the West Antarctica Ice Sheet are less well understood. However, some studies suggest that meltwater originating from the Southern Hemisphere is able to stabilize the AMOC. Using a conceptual model of the AMOC and a minimal parameterization of ice sheet collapse, we investigate the origin and relevance of this stabilization effect in both the deterministic and stochastic cases. While a substantial stabilization is found in both cases, we find that rate- and noise-induced effects have substantial impact on the AMOC stability, as those imply that leaving the AMOC bistable regime is neither necessary nor sufficient for the AMOC to tip. Also, we find that rate-induced effects tend to allow a stabilization of the AMOC in cases where the peak of the West Antarctica Ice Sheet meltwater flux occurs before the peak of the Greenland Ice Sheet meltwater flux.

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anlomedad Jul 29, 2024

Another team asked whether #AMOC collapse is at all possible in #IPCC #climate models bc those models are apparently known to have strong bias toward keeping a stable AMOC. (I can relate😁 )
The team ran an experiment in which they took a pre-industrial Earth without climate change,
and every model year added a little bit more freshwater to it. This was just to prove that even those unrealistically stable model types can produce a tipped and collapsed AMOC with a certain AMOUNT of freshwater. (As opposed to making AMOC tipsy by the speed of adding smaller amounts of freshwater. See above post.)

#vanWesten etal 2024 "Physics-based early warning signal shows that AMOC is on tipping course"
https://www.science.org/doi/abs/10.1126/sciadv.adk1189
The paper also describes in vivid colour the impacts a tipped AMOC has (in a pre-industrial world). Immediately. Even tho the time from tipping to collapse or OFF might be 100 years: dramatic changes all over the world start from the first decade of a tipped system.

Have a look at all the diagrams and maps in the paper and in the supplement to feed your nightmares with gory details.

In a bi-monthly rhythm, this team of 4 scientists feeds us more details on their findings.

In this preprint, they project the tipping schedule: "The collapse time is estimated between 2037-2064 (10-90% CI) with a mean of 2050 and the probability of an AMOC collapse before the year 2050 is estimated to be 59 +/- 17%." https://arxiv.org/abs/2406.11738 #Smolders 2024 "Probability Estimates of a 21st Century AMOC Collapse"

Don't confuse it with #Ditlevsen 2023 who also put that year in our calendar, but arriving at it using a different method .

I don't know what they mean with "collapse". Do they mean the moment of tipping, the period of unstoppable slowdown? Or the actual off-state, the collapsED AMOC?

Anyway. What the study also looks at are locations of Early Warning Signals, EWS. And unfortunately, the 2 regions where "we" are actively monitoring the AMOC strength are not early-warning-regions at all. Nor is the Cold Blob.
Bummer. (The installations are called RAPID and OSNAP if you want to look them up.)

Where we should have set up monitoring installations 20 years ago was near South Africa's coast in a depth of 500m, where salinity transport changes before tipping occurs 4000km further North...

I do recall reading just this in their already published paper, too. Latitude 34°S is where all the action is, according to the team in Utrecht, Netherlands.

In the next post I'll show a heatmap of salinity at 32 to 36°S in depths of 150 to 700m. Brace yourselves...

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anlomedad Jul 30, 2024

I'll be posting the promised chart later. I realized I need to include more depths and 1 more latitude band to really cover the #AMOC Canary proposed by the Dutch team.
When I first plotted salinity at 34°S in February, I couldn't discern from the peer-reviewed paper which coordinates and depths to use. This is only now clear with their new preprint:
depths 500m-2000m and latitudes ~30 to 36°S, and all across the #Atlantic, not only near South Africa.

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anlomedad Aug 2, 2024

The promised plots.
I'll show data since 2003 only because only then did real measurements really kick off. Before that, measurements were so spotty with 1 in January in one year, then 3 years nothing, then 5 measurements in for example August and so on.
Data source for measurements is #NOAA https://www.ncei.noaa.gov/access/world-ocean-database-select/dbsearch.html

The other plot is from a #ReAnalysis called EN4.2 https://www.metoffice.gov.uk/hadobs/en4/download-en4-2-2.html#c14_analyses
It is based on the same NOAA data, and goes back to 1900. EN4.2 has better quality assurance than my download. And EN4.2 is able to calculate the likely values in adjacent coordinates and adjacent months.

Among other Reanalyses, EN4.2 has also been used in the newest preprint by the Dutch team around #vanWesten. It sees AMOC tip in 2065 in RCP8.5 and in 2085 in RCP4.5. But it is based on biased Reanalyses and known-to-be too stable #CMIP6: https://arxiv.org/pdf/2407.19909

Whereas the first new preprint by van Westen's team, which tips AMOC 2037-2064, uses only real measurements of the 3 dedicated monitoring arrays in the North, tropical and South #Atlantic. https://arxiv.org/pdf/2406.11738
Here, the caveat is: very short data series.

With measurements being so spotty, I only show monthly salinity 2003ff in down to 10m depth. And only from the area in the South Atlantic. Averaged on a 2 by 2 degree grid from Argentina's coast to longitude -48, and latitudes 40-48 South.

Sorry, not sorry: neither dataset shows a smoking gun. 😁
#FridaysForFuture #ChartsForFuture

World Ocean Database Select

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anlomedad

The preprint with #AMOC tipping years 2037-2064 by team #vanWesten has now been peer-reviewed and CNN has a good explainer article on it. Also quotes @rahmstorf https://amp.cnn.com/cnn/2024/08/02/climate/atlantic-circulation-collapse-timing

The post above shows the Southern #Atlantic surface salinity measurements they based their prediction on. (I was too lazy to plot other ocean depths and latitude bands where they also found Early Warning Signals – the data is just too spotty.)

A critical system of Atlantic Ocean currents could collapse as early as the 2030s, new research suggests

It uses state-of-the-art models to estimate the shutdown could happen between 2037 and 2064, and that it’s more likely than not to collapse by 2050.

CNN