Asymmetric and Non-monotonic Response of the Climate System to Idealized CO₂ Forcing

• Main Author: Mitevski, Ivan
• Format: Thesis
• Language: English
• Published: 2023
• Subjects: Atmosphere; Atmospheric carbon dioxide; Meridional overturning circulation; Ocean currents; Ocean temperature; Climatology; Earth temperature; North Atlantic; Sea ice
• Online Access: https://doi.org/10.7916/18qg-2y74

In this thesis, I explore the climate system's response to symmetric abrupt and transient CO₂ forcing across a range of concentrations, from ⅛ ⨉ to 8⨉CO₂, relative to pre-industrial levels. I use two CMIP6 class models: the CESM Large Ensemble (CESM-LE) model configuration and the NASA Goddard Institute for Space Studies Model E2.1-G (GISS-E2.1-G). I use a hierarchy of (1) fully coupled atmosphere-ocean-sea-ice-land, (2) slab ocean, and (3) prescribed sea surface temperature simulations to analyze and support the findings. First, I find an asymmetric response in global mean surface air temperature (????????_s) and effective climate sensitivity (EffCS) between colder and warmer experiments. The ????????_s response at 8⨉CO₂ is more than a third larger than the corresponding cooling at ⅛⨉CO₂. I attribute this assymetry primarily due to the non-logarithmic CO₂ forcing, not to changes in the radiative feedbacks. Second, I identify a non-monotonic response of EffCS in the warmer scenarios, with a minimum occurring at 4⨉CO₂ (3⨉CO₂) in CESM-LE (GISS-E2.1-G). This minimum in the warming simulations is associated with a non-monotonicity in the radiative feedback. Similar non-monotonic responses in Northern Hemisphere sea-ice, precipitation, the latitude of zero precipitation-minus-evaporation, and the strength of the Hadley cell are also identified. Comparing the climate response over the same CO₂ range between fully coupled and slab-ocean versions of the same models, I demonstrate that the climate system’s non-monotonic response is linked to changes in ocean dynamics, associated with a collapse of the Atlantic Meridional Overturning Circulation (AMOC). Third, to establish the significance of North Atlantic cooling in driving the non-monotonic changes in the radiative feedback, I conducted additional atmosphere-only (AMIP) simulations using the same models but with prescribed sea surface temperatures (SSTs) restricted to different regions. Through these simulations, I uncovered that the minimum EffCS value, characterized by notably ...