Post AY6nRq2FXPvWEiwIvw by DrEvanGowan@fediscience.org
 (DIR) More posts by DrEvanGowan@fediscience.org
 (DIR) Post #AY6nRmsZGxOqRpO1vU by DrEvanGowan@fediscience.org
       2023-07-27T03:36:34Z
       
       0 likes, 0 repeats
       
       The paper by Ditlevsen and Ditlevsen about the possible imminent collapse of the Atlantic Meridional Overturning Circulation (#AMOC) is getting a lot of attention, and for a good reason. As #PaleoClimate scientists, we look back to the past for analogues. The AMOC regularly collapsed during the last glacial period (~25 times). Barker and Knorr published a hypothesis that there was a sweet spot of CO₂ concentrations and Laurentide Ice Sheet size that made this possible. https://www.nature.com/articles/s41467-021-22388-6
       
 (DIR) Post #AY6nRnl9zz4JB8vd1E by DrEvanGowan@fediscience.org
       2023-07-27T03:38:46Z
       
       0 likes, 0 repeats
       
       The size of the Laurentide Ice Sheet created feedbacks in terms of redirecting wind patterns and providing enough freshwater to cause AMOC to be unstable. The CO₂ levels changed in response to these changes, eventually pushing the climate to a state that would revive the AMOC.
       
 (DIR) Post #AY6nRoVFEdvpU4UQqm by DrEvanGowan@fediscience.org
       2023-07-27T03:41:41Z
       
       0 likes, 0 repeats
       
       Another hypothesis that furthers this was published by Wickert et al earlier this year. In their hypothesis, the intermediate sized Laurentide Ice Sheet would retreat, producing large proglacial lakes that drain towards the Gulf of Mexico, providing the freshwater to shut down the AMOC. When the AMOC shut down, the ice sheet would then advance again. After a while, the lack of freshwater input would eventually revive the AMOC. The cycle would repeat once again. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GL100391
       
 (DIR) Post #AY6nRpEGXFwbjhYO1Y by DrEvanGowan@fediscience.org
       2023-07-27T03:44:18Z
       
       0 likes, 0 repeats
       
       This is a really nice hypothesis, because the Marine Isotope Stage 3 (MIS3, 55-27 thousand years ago) was an extended stadial period when the Laurentide Ice Sheet remained in an intermediate state. The AMOC regularly collapsed. It did not collapse again until the Younger Dryas when the ice sheet configuration was similar to the MIS 3 configuration. The orbital conditions in MIS 3 were unique for the past million years, and it is unlikely we will find anything else like that in the paleo record.
       
 (DIR) Post #AY6nRq2FXPvWEiwIvw by DrEvanGowan@fediscience.org
       2023-07-27T03:47:55Z
       
       0 likes, 0 repeats
       
       Of course, as great as the paleoclimate record is, the rapid rise of CO₂ to levels exceeding 400 PPM has no past analogue. The mechanism for collapse will be completely different Although the indicators analyzed Ditlevsen and Ditlevsen point towards collapse, we may not know if the conditions are actually right for collapse until the collapse actually happens. If it does, past analogues suggest it can take well over 1000 years to recover. https://www.nature.com/articles/s41467-023-39810-w
       
 (DIR) Post #AY6nRqjUwcWOOrAqLQ by DrEvanGowan@fediscience.org
       2023-07-27T03:55:09Z
       
       0 likes, 0 repeats
       
       Modelling AMOC collapse in climate models has not been easy. There is currently a proposal (which I was involved in developing) to compare models in idealized experiments during MIS 3 to attempt to figure out the thresholds the collapse. You can read about it in this paper. https://cp.copernicus.org/articles/19/915/2023/