Abstract
Abstract. The polar amplification of warming and the ability of the Intertropical Convergence Zone (ITCZ) to shift to the north or south are two very important problems in climate science. Examining these behaviors in global climate models (GCMs) running solar geoengineering experiments is helpful not only for predicting the effects of solar geoengineering but also for understanding how these processes work under increased carbon dioxide (CO2). Both polar amplification and ITCZ shifts are closely related to the meridional transport of moist static energy (MSE) by the atmosphere. This study examines changes in MSE transport in 10 fully coupled GCMs in experiment G1 of the Geoengineering Model Intercomparison Project (GeoMIP), in which the solar constant is reduced to compensate for the radiative forcing from abruptly quadrupled CO2 concentrations. In G1, poleward MSE transport decreases relative to preindustrial conditions in all models, in contrast to the Coupled Model Intercomparison Project phase 5 (CMIP5) abrupt4xCO2 experiment, in which poleward MSE transport increases. We show that since poleward energy transport decreases rather than increases, and local feedbacks cannot change the sign of an initial temperature change, the residual polar amplification in the G1 experiment must be due to the net positive forcing in the polar regions and net negative forcing in the tropics, which arise from the different spatial patterns of the simultaneously imposed solar and CO2 forcings. However, the reduction in poleward energy transport likely plays a role in limiting the polar warming in G1. An attribution study with a moist energy balance model shows that cloud feedbacks are the largest source of uncertainty regarding changes in poleward energy transport in midlatitudes in G1, as well as for changes in cross-equatorial energy transport, which are anticorrelated with ITCZ shifts.
Highlights
As solar geoengineering, or the artificial cooling of Earth by reflecting sunlight, increasingly gains attention as part of a possible strategy to deal with the effects of climate change, two important issues are whether the polar amplification of CO2-induced warming can be fully counteracted, and whether regional precipitation patterns will shift, exacerbating flooding in some areas and drought in others (e.g., Irvine et al, 2010)
Bischoff and Schneider (2014) developed a theory for the relationship between cross-equatorial energy transport and the energy flux Equator, assuming the latter was correlated with the Intertropical Convergence Zone (ITCZ) position, and argued that while the energy flux Equator is proportional to the crossequatorial energy flux, the constant of proportionality is governed by the net energy input into the tropical atmosphere, which can allow the energy flux Equator to move while crossequatorial energy transport does not change
Our analysis of the Geoengineering Model Intercomparison Project (GeoMIP) G1 ensemble shows that, when CO2 concentrations are increased and the solar constant is reduced to compensate, poleward atmospheric energy transport decreases (Fig. 1a, d). This is because of an increase in polar temperatures and decrease in tropical temperatures, or “residual polar amplification”, that results from the different spatial patterns of the opposing solar and CO2 forcings
Summary
The artificial cooling of Earth by reflecting sunlight, increasingly gains attention as part of a possible strategy to deal with the effects of climate change, two important issues are whether the polar amplification of CO2-induced warming can be fully counteracted, and whether regional precipitation patterns will shift, exacerbating flooding in some areas and drought in others (e.g., Irvine et al, 2010). The Geoengineering Model Intercomparison Project (GeoMIP) provides an opportunity to use similar methods to investigate how atmospheric energy transport may change under solar geoengineering conditions, which can help us understand the reasons for residual polar amplification and tropical precipitation shifts. Solar constant reduction experiments underestimate the precipitation reduction compared to model runs that increase the concentration of sulfate aerosols in the stratosphere (Niemeier et al, 2013; Ferraro and Griffiths, 2016) This is because sulfate aerosols absorb longwave radiation, which reduces the net atmospheric radiative cooling rate and allows for less precipitation, since the latent heat release from precipitation formation must be balanced by net radiative cooling.
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