Changes in clouds and thermodynamics under solar geoengineering and implications for required solar reduction

Abstract

<p><strong>Abstract.</strong> The amount of solar constant reduction required to offset the global warming from an increase in atmospheric CO<sub>2</sub> concentration is an interesting question with implications for assessing the feasibility of solar geoengineering scenarios and for improving our theoretical understanding of Earth's climate response to greenhouse gas and solar forcings. This study investigates this question by analyzing the results of 11 coupled atmosphere-ocean global climate models running Experiment G1 of the Geoengineering Model Intercomparison Project, in which CO<sub>2</sub> concentrations are abruptly quadrupled and the solar constant is simultaneously reduced by an amount tuned to maintain top of atmosphere energy balance and preindustrial global mean temperature. The required solar constant reduction in G1 is between 3.2 % and 5.0 %, depending on the model, and is uncorrelated with the models' equilibrium climate sensitivity, while a formula from the experiment specifications based on the models' effective CO<sub>2</sub> forcing and planetary albedo is well-correlated with but consistently underpredicts the required solar reduction. We propose an alternative theory for the required solar reduction based on CO<sub>2</sub> instantaneous forcing and the sum of radiative adjustments to the combined CO<sub>2</sub> and solar forcings. We quantify these radiative adjustments in G1 using established methods and explore changes in atmospheric temperature, humidity and cloud fraction in order to understand the causes of these radiative adjustments. <br><br> The zonal mean temperature response in G1 exhibits cooling in the tropics and warming in high latitudes at the surface; greater cooling in the upper troposphere at all latitudes; and stratospheric cooling which is mainly due to the CO<sub>2</sub> increase. Tropospheric specific humidity decreases due to the temperature decrease, while stratospheric humidity may increase or decrease depending on the model's temperature change in the tropical tropopause layer. Low cloud fraction decreases in all models in G1, an effect that is robust and widespread across ocean and vegetated land areas. We attribute this to a reduction in boundary layer inversion strength over the ocean, and a reduction in the release of water from plants due to the increased CO<sub>2</sub>. High cloud fraction increases in the global mean in most models. The low cloud fraction reduction and atmospheric temperature decrease have strong warming effects on the planet, due to reduced reflection of shortwave radiation and reduced emission of longwave radiation, respectively. About 50 % to 75 % of the temperature effect is caused by the stratospheric cooling, while the reduction in atmospheric humidity results in increased outgoing longwave radiation that roughly offsets the tropospheric temperature effect. Taken together, the sum of the diagnosed radiative adjustments and the CO<sub>2</sub> instantaneous forcing predicts the required solar forcing in G1 to within about 6 %. The cloud fraction response to the G1 experiment raises interesting questions about cloud rapid adjustments and feedbacks under solar versus greenhouse forcings, which would be best explored in a model intercomparison framework with a solar-forcing-only experiment.</p>

Is Turning Down the Sun a Good Proxy for Stratospheric Sulfate Geoengineering?

“…Due to the uncertainties in our understanding of stratospheric sulfate microphysics and interaction with radiation, and to the lack, in some models, of a proper representation of stratospheric circulation, this simplification has also allowed more climate models to perform similar simulations (Kravitz, Caldeira, et al., 2013). Many studies have thus used a uniform reduction of the solar constant (solar dimming, SD) as a proxy to simulate the effects of stratospheric sulfate geoengineering, looking at its consequences on surface processes, for instance on the hydrological cycle (Guo et al., 2018; Irvine et al., 2019; Ji et al., 2018; Russotto & Ackerman, 2018a, 2018b; Smyth et al., 2017) and vegetation (Dagon & Schrag, 2019; Glienke et al., 2015). Some recent studies aiming to generally evaluate Solar Radiation Management (SRM) techniques in the framework of Integrated Assessment Modeling have also used SD climate simulations as a proxy for any SRM method (Harding et al., 2020; Low & Schfer, 2019; Oschlies et al., 2017; Tavoni et al., 2017).…”

Weakening of the Extratropical Storm Tracks in Solar Geoengineering Scenarios

“…Weakening of the extratropical storm tracks would be expected to, for example, reduce wind extremes in midlatitudes but also possibly lead to less efficient ventilation of air pollution from the boundary layer (Leibensperger et al, 2008). A weakening of the storm tracks may also contribute to the decrease in low cloud fraction over the storm‐track regions (Russotto & Ackerman, 2018b) and weakened poleward energy transport (Russotto & Ackerman, 2018a) identified previously in the G1 experiment.…”

“…Because many climate models do not include the relevant processes to reliably simulate more realistic approaches, an idealized scenario with reduced solar constant, known as sunshade geoengineering, is often studied in climate models as a simpler proxy for stratospheric aerosol injection (Kravitz et al, 2013). Reducing the solar constant does not offset the radiative forcing of increased CO 2 at each latitude separately, and thus, there are residual changes in temperature at different latitudes (Kravitz et al, 2013; Moore et al, 2014; Russotto & Ackerman, 2018a), which have the potential to affect the general circulation.…”