The paradox of the darkening planet and the Earth’s climate sensitivity

Studies of the observed record of global warming suggest that the Earth’s climate sensitivity is at the lower end of the range produced by the CMIP6 Earth System Models (Jimenez and Mauritsen, 2019; Nijsse et al., 2020; Tokarska et al., 2020). However, studies based on top-of-the-atmosphere fluxes often suggest the opposite (Brown et al., 2017; Sherwood et al., 2020).

Earthshine estimates (Goode et al., 2021) and satellite measurements of the planetary albedo from CERES (Loeb et al., 2018) both indicate that the Earth has darkened significantly over the past two decades. Planetary darkening is also simulated in CMIP6 historical simulations , but the models with the highest climate sensitivities tend to fit the observed decline in planetary albedo much better. Observed planetary darkening therefore favours higher climate sensitivities, but constraints based on ground-based global warming records favour lower climate senstivities.

We explore this apparent paradox by calculating the contributions to changes in global warming that arise from diagnosable changes in planetary albedo and effective global emissivity, in both models and observational records. Differences between low and high sensitivity models are found to be predominantly due to the rate at which the modelled planetary albedo declines, which can in principle be due to a combination of forcing and feedbacks. However, planetary darkening in higher sensitivity models is primarily due to reductions in cloud cover, which results in a positive SW cloud feedback.

By contrast, the planetary darkening seen in the CERES satellite record is driven not by reductions in cloud cover, but instead by the darkening of clouds, and to a lesser extent by the darkening of clear skies. This suggest that darkening in CERES is driven by reductions in aerosols, which leads to reductions in negative aerosol forcing.  Planetary darkening in CERES therefore seems to be due primarily to changes in aerosol forcing.

Our proposed resolution to ‘The paradox of the darkening planet and the Earth’s climate sensitivity’ is therefore that climate sensitivity is indeed towards the lower end of the CMIP6 model range (as suggested by observed records of global warming), and that higher sensitivity models get the rate of planetary darkening ‘right’ but by the wrong mechanism (i.e. as a cloud forcing rather than as an aerosol feedback).  We will back this up by comparing the spatial patterns of planetary albedo change from models and the CERES satellite data, and finish by discussing possible implications for the time-varying aerosol precursor fields that are used to drive the CMIP6 simulations.