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

Abstract

Abstract. The amount of solar constant reduction required to offset the global warming from an increase in atmospheric CO2 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 CO2 concentrations are abruptly quadrupled and the solar constant is simultaneously reduced by an amount tuned to maintain the top-of-atmosphere energy balance and pre-industrial 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 CO2 forcing and planetary albedo is well correlated with but consistently underpredicts the required solar reduction. We propose an explanation for the required solar reduction based on CO2 instantaneous forcing and the sum of radiative adjustments to the combined CO2 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. 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 CO2 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 CO2. 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. The longwave (LW) effect of the cloud changes is small in the global mean, despite the increase in high cloud fraction. Taken together, the sum of the diagnosed radiative adjustments and the CO2 instantaneous forcing explains 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.

Identifying the sources of uncertainty in climate model simulations of solar radiation modification with the G6sulfur and G6solar Geoengineering Model Intercomparison Project (GeoMIP) simulations

“…Globally, the changes are driven by the perturbation of the surface heat fluxes (Tilmes et al, 2013;Kravitz et al, 2013b;Niemeier et al, 2013) and changes in sea-land temperature contrast. Regionally, however, the modification of the baseline distribution of precipitation can be due to changes in the inter-tropical convergence zone (ITCZ; Russotto and Ackerman, 2018b;Cheng et al, 2019) produced by changes in the inter-hemispheric temperature gradient, general circulation changes produced by stratospheric heating (Simpson et al, 2019) and regional and seasonal changes in heat fluxes and temperature gradients (Jones et al, 2018;Visioni et al, 2020b). In the case of sulfate injections, these changes can be strongly dependent on latitudinal and temporal distribution of the aerosol cloud as well (Kravitz et al, 2019;Visioni et al, 2020b).…”

“…Two previous experiments in particular have been widely analyzed and discussed: G1, where the solar constant is reduced in order to offset the temperature increase produced by a 4× increase in CO 2 compared to pre-industrial concentrations (Kravitz et al, 2013b;Tilmes et al, 2013;Glienke et al, 2015;Russotto and Ackerman, 2018b;Kravitz et al, 2021), and G4, where a constant amount of SO 2 is injected into the equatorial stratosphere under emissions from the Representative Concentration Pathway 4.5 (RCP4.5) (Pitari et al, 2014;Kashimura et al, 2017;Visioni et al, 2017b;Plazzotta et al, 2019. However, previously performed GeoMIP experiments were not intended to be "realistic" deployments of geoengineering, either because they were performed under idealized conditions (such as 4×CO 2 concentrations) or because they considered a fixed, constant amount of injected SO 2 with an abrupt beginning and ending.…”

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.…”

On the Linearity of External Forcing Response in Solar Geoengineering Experiments

“…Given that ERF takes into account all radiative adjustments from temperature, surface albedo, water vapor, and clouds, it is insufficient as a stand alone metric to assess linearity between single versus combined forcing experiments. For both SOLAR and the 4xCO 2 experiments specifically, differences in both sign and magnitude of radiative adjustments have been linked to residual positive forcing in GeoMIP G1 experiments (Russotto & Ackerman, 2018). To understand what drives the nonlinearity in the response, we now present the decomposition of ERF into the IRF and its individual radiative adjustments.…”

“…The solar constant is then tuned iteratively are used to achieve approximate top of atmosphere (TOA) energy balance closure (Kravitz, Robock, Boucher, Schmidt, Taylor, Stenchikov, & Michael, 2011). The inter-model spread of the offset has been primarily attributed to rapid adjustments in the climate system as a response to both CO 2 increases and solar constant reductions (Russotto & Ackerman, 2018). Rapid responses in temperature, moisture, and clouds induce radiative perturbations alongside the direct Instantaneous Radiative Forcing (IRF) of a given forcing factor (e.g., CO 2 , aerosols, and solar) which taken together determine the ERF (Sherwood et al, 2015).…”