Changes in clouds and thermodynamics under solar geoengineering and implications for required solar reduction
Authors: Russotto Rick D. , Ackerman Thomas P. 
Published: August 2018

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.