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.
Highlights
In light of the warming of Earth in response to anthropogenic greenhouse gas emissions (IPCC, 2013), and continued lack of progress in curbing those emissionsPublished by Copernicus Publications on behalf of the European Geosciences Union.R
This paper characterizes the physical responses of the atmosphere and surface to the Geoengineering Model Intercomparison Project (GeoMIP) G1 scenario and quantifies their radiative effects, with the goal of explaining what determines the solar constant reduction required to balance the CO2 increase
The tropospheric temperature effect is a reversal of the negative lapse rate feedback that happens in global warming simulations, in which the tropical upper troposphere warms more than the surface; in G1, because the tropics cool and the tropical temperature profile tends to follow a moist adiabat, the upper troposphere cools, which has a warming effect on the climate by reducing outgoing LW radiation (OLR)
Summary
In light of the warming of Earth in response to anthropogenic greenhouse gas emissions (IPCC, 2013), and continued lack of progress in curbing those emissions. We use 680 hPa as the boundary between low and middle clouds and 440 hPa as the boundary between middle and high clouds, following the standards for the International Satellite Cloud Climatology Project (ISCCP; see Fig. 2 of Rossow and Schiffer, 1999), or 3250 and 6500 m in the case of HadGEM2-ES, which roughly correspond to these pressure levels in the 1976 Standard Atmosphere (NOAA, 1976) These plots, and all subsequent multi-model mean maps, show stippling where fewer than all but two of the included models agree on the sign of the change, so that unstippled areas indicate robust changes. Besides changes in stability metrics, other factors that have been suggested as explaining changes in marine stratocumulus cloud fraction under global warming conditions in largeeddy simulation models include reduced LW radiative cooling from cloud tops due to increased CO2 and H2O concentrations, decreased subsidence above the boundary layer, and increased sea surface temperatures (Bretherton, 2015). Near-surface relative humidity increases in G1 in most areas, despite the reduction in evaporation (Smyth et al, 2017), implying that evaporation changes are not the reason for the low cloud changes there
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