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Abstract We compare abrupt CO2‐quadrupling (abrupt‐4xCO2) and ‐doubling (abrupt‐2xCO2) simulations across 10 CMIP6 models. Two models (CESM2 and MRI‐ESM2‐0) warm substantially more than twice as much under abrupt‐4xCO2 than abrupt‐2xCO2, which cannot be explained by the non‐logarithmic scaling of CO2forcing. Using an energy balance model, we show that increased warming rates within these two models are driven by both less‐negative radiative feedbacks and smaller global effective heat capacity under abrupt‐4xCO2. These differences are caused by a decrease in low cloud cover andshallower ocean heat storage, respectively; both are linked to smaller fractional declines in the Atlantic Meridional Overturning Circulation (AMOC) under abrupt‐4xCO2 (relative to abrupt‐2xCO2). On a global scale, higher climate sensitivity under larger forcing can be explained by a feedback‐temperature dependence; however, we find that forcing‐dependent spatial warming patterns due to AMOC decline are an important physical mechanism which reduces warming in a way that is not captured by a global‐mean framework.
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This content will become publicly available on January 7, 2026
The effect of CO 2 ramping rate on the transient weakening of the Atlantic Meridional Overturning Circulation
The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate that is projected to weaken under future anthropogenic climate change. While many studies have investigated the AMOC’s response to different levels and types of forcing in climate models, relatively little attention has been paid to the AMOC’s sensitivity to the rate of forcing change, despite it also being highly uncertain in future emissions scenarios. In this study, I isolate the AMOC’s response to different rates of CO2increase in a state-of-the-art global climate model and find that the AMOC undergoes more severe weakening under faster rates of CO2change, even when the magnitude of CO2change is the same. I then propose an AMOC-ocean heat transport-sea ice feedback that enhances the decline of the circulation and explains the dependence on the rate of forcing change. The AMOC’s rate-sensitive behavior leads to qualitatively different climates (including differing Arctic sea ice evolution) at the same CO2concentration, highlighting how the rate of forcing change is itself a key driver of global climatic change.
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- Award ID(s):
- 2303486
- PAR ID:
- 10612616
- Publisher / Repository:
- PNAS
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 122
- Issue:
- 1
- ISSN:
- 0027-8424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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