Developing an Observation-Based Transfer Function for Climate Sensitivity

Mark Zelinka | 19-FS-022


Equilibrium climate sensitivity (ECS), the equilibrated global surface temperature response to a doubling of atmospheric carbon dioxide concentration, represents a key unknown in current climate research. Attempts to infer ECS from shorter timescale climate fluctuations are hampered by the fact that the radiative damping of surface temperature, which largely determines ECS, varies substantially in time. These variations are often attributed to variations in warming patterns, but observational evidence for and constraints on this effect are lacking.

Our research used satellite-observed radiative fluxes at the top of the atmosphere to confirm that temperature-mediated changes in the planet’s energy balance (the radiative damping rate) vary considerably depending on the time period analyzed. Variations are mostly driven by radiative anomalies induced by changes in tropical low-level cloud coverage. The response of these clouds to warming is strongly modulated by low-level stability changes. When the inversion at the top of the marine boundary layer is strengthened, low cloud coverage—and, hence, planetary radiative cooling—increases. A stronger inversion is favored when surface warming is concentrated in already warm regions of the tropics. The study also provided observation-based evidence that the overall radiative damping rate and implied ECS are strong functions of a "warm-get-warmer" pattern. Results of this work strongly suggest the feasibility of an observationally-based transfer function between ECS inferred from observed trends and "true" ECS in response to a doubling of carbon dioxide. We made substantial progress in developing ways to better constrain climate sensitivity using satellite observations.

Impact on Mission

This work addressed Lawrence Livermore National Laboratory's initiative for energy and climate security, which focuses on understanding energy-climate impacts, risks, and possible mitigation strategies. The research is closely aligned with the Livermore's earth and atmospheric science core competencies and represents improved Laboratory expertise in groundbreaking analyses of the contributing factors to climate change and potential consequences.