Comparison of Equilibrium Climate Sensitivity Estimates From Slab Ocean, 150‐Year, and Longer Simulations

Abstract

We compare equilibrium climate sensitivity (ECS) estimates from pairs of long (≥800‐year) control and abruptly quadrupled CO2 simulations with shorter (150‐ and 300‐year) coupled atmosphere‐ocean simulations and slab ocean models (SOMs). Consistent with previous work, ECS estimates from shorter coupled simulations based on annual averages for years 1–150 underestimate those from SOM (−8% ± 13%) and long (−14% ± 8%) simulations. Analysis of only years 21–150 improved agreement with SOM (−2% ± 14%) and long (−8% ± 10%) estimates. Use of pentadal averages for years 51–150 results in improved agreement with long simulations (−4% ± 11%). While ECS estimates from current generation U.S. models based on SOM and coupled annual averages of years 1–150 range from 2.6°C to 5.3°C, estimates based longer simulations of the same models range from 3.2°C to 7.0°C. Such variations between methods argues for caution in comparison and interpretation of ECS estimates across models.

GISS‐E2.1: Configurations and Climatology

“…The summary of various metrics of climate sensitivity (along with the comparison to the previous models) is seen in Table 6. We note that the effective climate sensitivity as calculated by the Gregory method (Gregory et al., 2004) almost always underestimates the true long‐term ECS by 10% to 20% (Dunne et al., 2020). The perhaps more relevant TCR is slightly larger in the E2.1 models than previously, consistent with a smaller rate of mixing of heat into the ocean (and slightly smaller present‐day overall radiative imbalance (Table 2).…”

“…Better‐calibrated lower resolution models and more sophisticated higher resolution models here can play a significant role in expanding that diversity and avoiding the potential danger of similar, and perhaps problematic, new assumptions being adopted by all model groups as they jointly improve such features as cloud and aerosol microphysics (Andrews et al., 2019; Gettelman et al., 2019; Golaz et al., 2019). The apparent increase in climate sensitivity to doubled CO 2 in some of the next‐generation models (Dunne et al., 2020; Forster et al., 2019; Zelinka et al., 2020) whether realistic or not, is very concerning. If this is a reflection of the real world, climate impacts are likely to be greater than we have up to now anticipated, and if it is not, then it raises serious questions about model independence and underlines the importance of true structural diversity.…”

Slow feedbacks resulting from strongly enhanced atmospheric methane mixing ratios in a chemistry–climate model with mixed-layer ocean

“…The Brewer-Dobson circulation (BDC) is expected to accelerate in a warming climate (Rind et al, 1990;Butchart and Scaife, 2001;Garcia and Randel, 2008;Butchart, 2014;Eichinger et al, 2019). Feedbacks on the chemical composition of the atmosphere, especially of the stratosphere, which result from changes in the BDC are of particular interest in this study as they will modify the mainly chemically induced changes discussed by Winterstein et al (2019).…”

“…Furthermore, mixing with air out of the upper tropical troposphere could also contribute to the warming patches in the subtropical and extratropical lower stratosphere. This region is particularly affected by mixing (Dietmüller et al, 2018;Eichinger et al, 2019), and mixing itself can also be influenced by climate change (Eichinger et al, 2019).…”

CO2 Increase Experiments Using the CESM: Relationship to Climate Sensitivity and Comparison of CESM1 to CESM2

“…We note, however, that the Gregory et al (2004) method suffers from the same pitfalls when applied to SOM  (ΔT) results as it does when applied to full ESM results; that is, rapid initial adjustment can affect the regression estimate of  . As with full ESM results, better estimates of ECS may be obtained if initial rapid adjustment in  (ΔT) is excluded (Dunne et al, 2020).…”