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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
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Research article 16 Apr 2018

Research article | 16 Apr 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

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

Rick D. Russotto1 and Thomas P. Ackerman1,2 Rick D. Russotto and Thomas P. Ackerman
  • 1Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
  • 2Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington, USA

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 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 CO2 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 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. Taken together, the sum of the diagnosed radiative adjustments and the CO2 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.

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Rick D. Russotto and Thomas P. Ackerman
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Rick D. Russotto and Thomas P. Ackerman
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Short summary
In simulations with different climate models in which the strength of the sun is reduced to cancel the surface warming from a quadrupling of atmospheric carbon dioxide, low cloud cover decreases, high cloud cover increases, the upper troposphere and stratosphere cool, and water vapor concentration decreases. We propose a new formula that predicts the required solar reduction well. The stratospheric cooling and low cloud reduction lead to more sunlight reduction being needed.
In simulations with different climate models in which the strength of the sun is reduced to...