Beyond the Headlines: Salient Scientists Decode the AMOC
We caught the recent coverage by CNN , The Washington Post , and The Guardian on the Atlantic Meridional Overturning Circulation research (aka the AMOC). Stories here, here, and here. Climate change coverage like this is essential – shoutouts to authors Laura Paddison , Sarah Kaplan, and Jonathan Watts , respectively.
Getting the details right is essential for good science. Salient’s cofounder and chief scientist, Dr. Sam Levang, weighs in on the research profiled:
"This is one of many studies that demonstrate instability in the AMOC under specific modeling assumptions, typically by adding a lot of freshwater to the North Atlantic. The drama of an AMOC collapse scenario is stimulating for academics and media alike."
However, the AMOC is very complicated, and there is still a lot of uncertainty around how it will evolve in the coming years. When you weigh the evidence from the global water cycle as a whole in our currently warming world, the doomsday collapse scenario looks less likely.
Salient’s president and co-founder, oceanographer Dr. Ray Schmitt explains further: "There is no doubt that abrupt climate change happened thousands of years ago when massive freshwater discharges from melting ice sheets hit the North Atlantic, but none of those large reservoirs remain today.” This is related to several key issues with the paper:
- “The researchers add too much freshwater to the North Atlantic and compensate for it in the South Atlantic. There is an AMOC in the Atlantic because it is saltier than the other oceans. And the reason it is saltier is because of strong moisture transport to the tropical Pacific across Central America. As the water cycle intensifies with global warming, this transport increases, so the Atlantic gets saltier. The author’s artificial forcing scheme has no net salinity change.”
- "Greenland glaciers are melting at a modest rate, but at 80x less than the assumption in the paper. The slow melting is not enough to compensate for salinification from stronger evaporation in the subtropics and the increased transport of moisture from Atlantic to Pacific."
Papers that Drs. Levang and Schmitt co-wrote address pieces of this issue (linked below), using a collection of CMIP5 models of the type used in the recent paper. They found almost no effect of salinity changes on the AMOC because of a strong salinification in the subtropics. The studies did find a weakening of the AMOC but it was because of warming, not freshening.
More observations are necessary – there is a short record of estimated AMOC strength and the variance is such that no real trend can be identified.
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Modeling studies like the profiled AMOC research provide fascinating insight into the workings of the climate system. But confirming that such models are useful projections for the real world isn’t possible for at least decades. The S2S timescale that Salient operates on allows us the luxury of robustly backtesting theories and models.
Dive deeper
Read the papers coauthored by Salient’s Dr. Levang and Dr. Schmitt on this topic:
- Samuel Levang and Raymond Schmitt, Centennial Changes of the Global Water Cycle in CMIP5 Models, Journal of Climate 28, 16. 2015.
- Samuel Levang and Raymond Schmitt, What Causes the AMOC to Weaken in CMIP5?, Journal of Climate 33, 4. 2020.
And note that yesterday's post on the Salient blog describes how we achieve reliability and proper scoring while backtesting.
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3moThanks Salient for an insightful post. For some useful references to the amoc and a lite spin on the drivers of tipping points through a double pendulum analogue... https://drmarkosb.wixsite.com/dr-mark-osborne/post/climate-choas-tipping-points-and-the-dance-of-the-double-pendulum
Professor at University of Gothenburg
5moOn the same topic: https://a.tellusjournals.se/ articles/10.16993/tellusa.679
Retired Process Engineering & Energy Team leader
5moFailing to mention the difference between CMIP5 and CMIP6 is a major fail. The latter accounts for climate sensitivity far better than the former in as much as cloud feedbacks are improved markedly such that observed temperatures align with modelled whereas they don't with the latter. Take the time to listen to George Tselioudis , who was one of the authors along with James E. Hansen of, Global warming in the Pipeline, explain with illustrated graphics in the following at about 20 min in https://youtu.be/NXDWpBlPCY8?si=mScaKrRecPrs9IJ7
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5moYour "debunking" is easily debunked: While the high level of freshwater forcing used in the model is not representative of current conditions ("80x"), it serves the purpose of enhancing our understanding of the AMOC's dynamics and potential tipping points: - The primary aim of the study was to identify tipping points and early warning signs for the AMOC collapse. By using a high level of freshwater forcing, the researchers could more easily induce and observe these tipping points within the model. - High freshwater forcing allows for stress-testing the AMOC within the model to understand its response under extreme conditions. - Quantum Computing is not really available yet, so using a higher level of forcing makes it feasible to observe the effects within a reasonable time-frame and computational resource limit. The approach was chosen to better understand the processes involved in AMOC changes rather than to predict a specific future scenario based on current melting rates.
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5moFeeling grateful for national climate coverage (thank you, Laura Paddison, Sarah Kaplan, and Jonathan Watts) and for rigorous science (thank you, Sam Levang and Ray Schmitt).