Climate Sensitivity Increases Under Higher CO₂ Levels Due to Feedback Temperature Dependence

Abstract

Equilibrium climate sensitivity‐the equilibrium warming per CO2 doubling‐increases with CO2 concentration for 13 of 14 coupled general circulation models for 0.5–8 times the preindustrial concentration. In particular, the abrupt 4 × CO2 equilibrium warming is more than twice the 2 × CO2 warming. We identify three potential causes: nonlogarithmic forcing, feedback CO2 dependence, and feedback temperature dependence. Feedback temperature dependence explains at least half of the sensitivity increase, while feedback CO2 dependence explains a smaller share, and nonlogarithmic forcing decreases sensitivity in as many models as it increases it. Feedback temperature dependence is positive for 10 out of 14 models, primarily due to the longwave clear‐sky feedback, while cloud feedbacks drive particularly large sensitivity increases. Feedback temperature dependence increases the risk of extreme or runaway warming, and is estimated to cause six models to warm at least an additional 3K under 8 × CO2.

Three Flavors of Radiative Feedbacks and Their Implications for Estimating Equilibrium Climate Sensitivity

“…Gregory et al, 2015). The values of  eff1pct and   1pct strongly depend on the formulation of 1pct ( ) F t for high forcing levels (e.g., Bloch-Johnson et al, 2020;Byrne & Goldblatt, 2014;Gregory et al, 2015 T quantified by M5 depending on first and last year of a linear regression of N on T (color shading) for the same ten abrupt4x simulations as in Figure 2. The standard deviation across the simulations is shown in hashed patterns; for example, a regression of years 100-220 results in a 5% error for the model mean (gray shading), but the standard deviation across the 10 models is still 10% (line hashing).…”

“…Gregory et al, 2015). The values of eff1pct  and 1pct   strongly depend on the formulation of 1pct ( ) F t for high forcing levels (e.g., Bloch-Johnson et al, 2020;Byrne & Goldblatt, 2014;Gregory et al, 2015).…”

“…term is important to account for when considering climates that are colder or warmer than the modern (e.g., Bloch-Johnson et al, 2020). Second,   (Equation 2) is in general not a constant value, and can vary with the spatial pattern of surface temperature change.…”

Joint Dependence of Longwave Feedback on Surface Temperature and Relative Humidity

“…Our new understanding of the state dependence of  cs E gives context to previous results. For example, Bloch-Johnson et al (2021) and Meraner et al (2013) attributed an increase in equilibrium climate sensitivity to the decrease in  cs E with warming. We expect these variations in  cs…”

Evolving CO2 Rather Than SST Leads to a Factor of Ten Decrease in GCM Convergence Time

“…The equilibrium climate sensitivity (ECS) is defined as the increase in global‐mean surface temperature per doubling of (IPCC, 1990). Several recent studies have demonstrated a robust increase in ECS in climates warmer than modern Earth due to state‐dependent feedback and forcing by (Bloch‐Johnson et al., 2015; Bloch‐Johnson, Rugenstein, Stolpe, et al., 2020; Caballero & Huber, 2013; Colman & McAvaney, 2009; Hansen et al., 2005; Jonko et al., 2013; Leconte et al., 2013; Meraner et al., 2013; Popp et al., 2016; Romps, 2020; Russell et al., 2013; Seeley & Jeevanjee, 2021; Wolf & Toon, 2015; Wolf et al., 2018). When they investigate high enough temperatures, these studies find that the ECS reaches a maximum for global‐mean surface temperatures in the range of 310–320 K. We will refer to this as the “ECS bump.”…”