Prof. Stefan RahmstorfNov 20, 2024
We must thus expect a faster AMOC decline than hashtag#IPCC has predicted: "It is very likely (90–100% probability) that the AMOC will weaken faster than CMIP6 projections if meltwater forcing is considered." That's bad news; it increases the risk of passing the AMOC tipping point.
If you want to know more about this topic, watch my short talk: https://youtu.be/mm_YZ2juQL4?si=1W9RAjWSQu7YSwzX
Is the AMOC Shutting Down?

YouTube
Show threadanlomedad

@rahmstorf

Does anyone here know how or when AMOC is restarted after a shutdown?
How it used to get restarted in past climate changes. And how that restart would play out under our unusual changed factors?

I was thinking that maybe, a restart requires all Milankovic cycles to sufficiently favour Northern Hemisphere. And the process would be that sufficiently warmer summer temperatures over the Northern North Atlantic manage to evaporate enough ocean water for it to become saltier – and that would kickstart convection in place, with strictly local conditions as the first engine.

All 3 Milankovic cycles sufficiently favouring Northern Hemisphere, that's in about 120,000 years....

#AMOC #Ocean #AMOCShutdown #HeinrichEvent #ClimateChange #paleoclimate #Holocene #Anthropocene

Nov 20, 2024 at 6:13pmWeb
Show threadWaNeNov 20, 2024

@anlomedad @rahmstorf

This study presents novel insight into the mechanisms of Atlantic Meridional Overturning Circulation (AMOC) reduction and its recovery under a warmer climate scenario.

https://www.nature.com/articles/s41598-023-43143-5

AMOC decline and recovery in a warmer climate - Scientific Reports

This study presents novel insight into the mechanisms of Atlantic Meridional Overturning Circulation (AMOC) reduction and its recovery under a warmer climate scenario. An one-thousand-year-long numerical simulation of a global coupled ocean–ice–atmosphere climate model, subjected to a stationary atmospheric radiative forcing, depict a coherent picture of the Arctic sea ice melting as a trigger for the initial AMOC reduction, along with decreases in the northward fluxes of salt and heat. Further atmospheric-driven ocean processes contribute to an erosion of the stable stratification of the fresher, yet colder waters in the surface layers of the North Atlantic, contributing to the recovery of a permanently altered AMOC.

Nature
Show threadanlomedadNov 20, 2024

@WaNe @rahmstorf

Very interesting. You're a star!

That's for a 4x CO2 experiment.
We'll have less than 2x CO2 (because fossil fuel supply chains collapse with everything else due to feedbacks in our human systems when #AMOC tips and halts).
Far less forcing than in the experiment.

That wind-driven element, "Ekman pumping" is intriguing. It would require a strengthening of the East/NorthEast surface wind across the North Atlantic. But the NorthEast direction is cut off, I imagine, due to the slowdown of the #SubpolarGyre as a result of AMOC shutdown.
Ah, no. The temperature gradient for the #jetstream to careen from warm Washington toward cold Scandinavia in the NorthEast is going to give Mr. Ekman a push as well.
How the jetstream across the Atlantic might behave in summers under such conditions ... hm.

P.S. I'll reduce the 120,000 years I assumed above, to 65,000 years, when all 3 Milankovic cycles [sufficiently] favour Northern Hemisphere again. See chart showing the M-cycles, CO2 and sea level for the past 450ky and 99ky into the future.

Show threadProf. Stefan RahmstorfNov 21, 2024
@anlomedad @WaNe That’s only a 50 % AMOC weakening, so not passed the tipping point.