Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling
17 July 2020

On the Evolution of Climate Feedbacks and Implied Climate Sensitivity Over Time in Earth System Model Simulations

The radiative damping of warming weakens over time in response to quadrupled CO2 in Earth System Models, leading to increasing implied climate sensitivity. Reasons for this are detailed across two generations of ESMs.

Science

Scientists at Lawrence Livermore National Laboratory in collaboration with colleagues from the University of Washington, Nanjing University, and the Met Office Hadley Centre have detailed the evolution of radiative feedbacks and implied climate sensitivity across two generations of Earth System Models that have performed abrupt CO2 quadrupling experiments as part of phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6). As the planet warms, radiative damping weakens in both ensembles, but this change is less dramatic in the latest CMIP6 models. Evolving surface warming patterns are found to modulate the strength of radiative damping over time, but models disagree on how these patterns evolve. In particular, the degree to which warming is concentrated in the West Pacific warm pool is important for explaining differences among CMIP5 models, but this region seems less important for driving differences among CMIP6 models. This may be due to the stronger influence of extratropical cloud feedbacks in CMIP6.

Impact

Understanding the reasons for differences in climate sensitivity among Earth System Models is an abiding goal of climate science. Recent work has noted that estimates of climate sensitivity inferred from observed trends are likely to underestimate climate sensitivity in response to doubled CO2 concentration. This is mostly because observed surface warming patterns tend to be highly heterogeneous and instigate stronger radiative damping than the relatively uniform warming expected in response to doubled CO2. In this work, these effects are quantified across a large collection of CMIP models to highlight where models agree and disagree in the strength of evolving feedbacks, how they relate to warming patterns, and how these effects have changed between CMIP5 and CMIP6.

Summary

Radiative feedbacks depend on the spatial patterns of sea-surface temperature (SST) and thus can change over time as SST patterns evolve – the so-called ‘pattern effect’. This study investigates inter-model differences in the magnitude of the pattern effect and how these differences contribute to the spread in effective equilibrium climate sensitivity (ECS) within CMIP5 and CMIP6 models. Effective ECS in CMIP5 estimated from 150-year-long abrupt4xCO2 simulations is on average 10% higher than that estimated from the early portion (first 50 years) of those simulations which serves as an analog for historical warming; this difference is reduced to 7% on average in CMIP6. The (negative) net radiative feedback weakens over the course of the abrupt4xCO2 simulations in the vast majority of CMIP5 and CMIP6 models, but this weakening is less dramatic on average in CMIP6. For both ensembles, the total variance in the effective ECS is found to be dominated by the spread in radiative response on fast timescales, rather than the spread in feedback changes. Using Green’s functions derived from two AGCMs shows that the spread in feedbacks on fast timescales may be primarily due to differences in atmospheric model physics, whereas the spread in feedback evolution is primarily governed by differences in SST patterns. Inter-model spread in feedback evolution is well explained by differences in the relative warming in the West Pacific warm-pool regions for the CMIP5 models, but this relation fails to explain differences across the CMIP6 models, suggesting that a stronger sensitivity of extratropical clouds to surface warming may also contribute to feedback changes in CMIP6.

Contact
Mark Zelinka
Lawrence Livermore National Laboratory
Publications
Dong, Y, K Armour, M Zelinka, C Proistosescu, D Battisti, C Zhou, and T Andrews.  2020.  "Inter-Model Spread in the Pattern Effect and its Contribution to Climate Sensitivity in CMIP5 and CMIP6 Models."  Journal of Climate.  https://doi.org/10.1175/JCLI-D-19-1011.1.