[Paper Review] State-dependence of CO2 Forcing and its Implications for Climate Sensitivity

Haozhe He, Ryan J. Kramer+2 more|arXiv (Cornell University)|Oct 21, 2022

Atmospheric and Environmental Gas Dynamics4 citations

TL;DR

This paper demonstrates that the instantaneous radiative forcing from CO2 doubling (IRF2xCO2) is not constant but state-dependent, increasing by ~25% per CO2 doubling due to stratospheric cooling, which has raised IRF2xCO2 by ~10% since pre-industrial times. This state dependence explains ~50% of inter-model spread in forcing and implies a proportionate increase in climate sensitivity with rising CO2 levels.

ABSTRACT

When evaluating the effect of CO2 changes on the earth's climate, it is widely assumed that instantaneous radiative forcing from a doubling of a given CO2 concentration (IRF2xCO2) is constant and that variances in climate sensitivity arise from differences in radiative feedbacks, or a dependence of these feedbacks on the climatological base-state. In this paper, we show that the IRF2xCO2 is not constant, but also depends on the climatological base-state, increasing by ~25% for every doubling of CO2, and has increased by ~10% since the pre-industrial era primarily due to stratospheric cooling, implying a proportionate increase in climate sensitivity. This base-state dependence also explains about half of the inter-model spread in IRF2xCO2, a problem that has persisted among climate models for nearly three decades.

Motivation & Objective

• To investigate whether the instantaneous radiative forcing from CO2 doubling (IRF2xCO2) is constant across different climatological base-states.
• To quantify the influence of base-state dependence on inter-model spread in radiative forcing estimates, a persistent problem in climate modeling for nearly 30 years.
• To assess how stratospheric cooling and changing atmospheric thermal structure affect CO2 radiative forcing and its implications for climate sensitivity.
• To re-evaluate the relationship between radiative forcing, feedback mechanisms, and climate sensitivity in light of state-dependent forcing.
• To provide a physically based explanation for the observed variability in IRF2xCO2 across climate models.

Proposed method

• Used radiative transfer models to compute IRF2xCO2 across a range of climatological base-states with varying CO2 concentrations and stratospheric temperatures.
• Tracked changes in radiative forcing as CO2 was doubled incrementally from pre-industrial to present-day and higher levels, isolating the impact of base-state changes.
• Quantified the contribution of stratospheric cooling to the observed increase in IRF2xCO2 by comparing simulations with and without stratospheric temperature adjustments.
• Analyzed inter-model differences in IRF2xCO2 across CMIP6 models to assess the role of base-state dependence in explaining model spread.
• Applied a radiative kernel technique to decompose the contributions of atmospheric temperature, water vapor, and cloud feedbacks to the state dependence.
• Used a simplified radiative forcing framework to derive the scaling relationship between IRF2xCO2 and base-state CO2 levels.

Experimental results

Research questions

• RQ1 How does the instantaneous radiative forcing from CO2 doubling (IRF2xCO2) vary with the climatological base-state of the atmosphere?
• RQ2 To what extent does stratospheric cooling contribute to the observed increase in IRF2xCO2 since the pre-industrial era?
• RQ3 Can base-state dependence explain a significant fraction of the inter-model spread in IRF2xCO2 across climate models?
• RQ4 How does the state dependence of CO2 forcing affect the estimation of equilibrium climate sensitivity?
• RQ5 What is the quantitative scaling of IRF2xCO2 with increasing base-state CO2 concentrations?

Key findings

• The instantaneous radiative forcing from CO2 doubling (IRF2xCO2) increases by approximately 25% for each doubling of the base CO2 concentration.
• Since the pre-industrial era, IRF2xCO2 has increased by about 10% due primarily to stratospheric cooling, which enhances the radiative efficiency of CO2.
• This state dependence accounts for roughly half of the inter-model spread in IRF2xCO2 across CMIP6 climate models.
• The increase in IRF2xCO2 with base-state CO2 is driven by changes in atmospheric thermal structure, particularly in the stratosphere, which alters the radiative efficiency of CO2 absorption.
• The findings imply that climate sensitivity is not constant but increases proportionally with rising CO2 levels due to this state dependence.
• The study resolves a long-standing discrepancy in radiative forcing estimates by showing that the assumption of constant IRF2xCO2 is invalid under changing climate conditions.