Implications of non-constant feedbacks on climate sensitivity

February 20, 2019

Maria Rugenstein, Visiting from the Max Planck Institute for Meteorology

Hosted by Dave Thompson

Abstract

For decades scientists have used equilibrium climate sensitivity—the equilibrium surface warming associated with doubling the atmospheric CO2 concentration—to project future climate change, to test our understanding of climate feedbacks and paleo proxies, and as a benchmark in model intercomparisons. I will discuss the strengths and weaknesses of the concept of climate sensitivity and recent debates about its usefulness.

Computational costs have led to the widespread practice of extrapolating equilibrium conditions from transient simulations. This is shown to be problematic, because the assumption that radiative feedbacks are constant is invalid on many time scales. I demonstrate how the evolution of the pattern of surface warming and ocean heat uptake—moving from a more homogeneous toward a more high-latitude enhanced pattern—make local and global feedbacks more sensitive as the climate equilibrates. Thus, the ocean strongly affects atmospheric temperatures not only through heat uptake but also through its influence on atmospheric feedbacks. I will pose the hypothesis that ocean heat uptake and radiative feedback changes are linked and form a GAIA-like feedback. Lastly, I will also introduce a model intercomparison effort–Longrunmip–that aims to compare atmosphere-ocean general circulation models on centennial to millennial time scales. These simulations allow us to robustly quantify the strength of changing feedbacks, the actual equilibration temperature, and thereby test extrapolation methods. Our findings imply limited predictive power of transient simulations for the equilibrium and raise questions about the adequacy of commonly used energy balance frameworks.