Comparing different generations of idealized solar geoengineering simulations in the Geoengineering Model Intercomparison Project (GeoMIP)

Ben Kravitz,Daniele Visioni,Pierre Nabat,Thibaut Lurton,Simone Tilmes,Olivier Boucher,Andy Jones,Alan Robock,Jason N S Cole,Jim Haywood,Ulrike Niemeier,Douglas G Macmartin,Roland Séférian

doi:10.5194/acp-21-4231-2021

Abstract

Abstract. Solar geoengineering has been receiving increased attention in recent years as a potential temporary solution to offset global warming. One method of approximating global-scale solar geoengineering in climate models is via solar reduction experiments. Two generations of models in the Geoengineering Model Intercomparison Project (GeoMIP) have now simulated offsetting a quadrupling of the CO2 concentration with solar reduction. This simulation is idealized and designed to elicit large responses in the models. Here, we show that energetics, temperature, and hydrological cycle changes in this experiment are statistically indistinguishable between the two ensembles. Of the variables analyzed here, the only major differences involve highly parameterized and uncertain processes, such as cloud forcing or terrestrial net primary productivity. We conclude that despite numerous structural differences and uncertainties in models over the past two generations of models, including an increase in climate sensitivity in the latest generation of models, the models are consistent in their aggregate climate response to global solar dimming.

Highlights

Solar geoengineering describes a set of technologies designed to temporarily, deliberately reduce some of the effects of climate change by changing the radiative balance of the planet, often by reflecting sunlight back to space (NRC, 2015)
Neglecting some outliers, for each flux except shortwave cloud forcing, the median model in one ensemble is within the interquartile range of the other ensemble. This indicates that there are no major differences between the ensembles in how the models handle energy balance and energetics, with the exception of clouds, which is consistent with findings about CMIP6 (Zelinka et al, 2020)
It appears that most of the major differences in shortwave cloud forcing are due to outliers in each ensemble, which are positive for CMIP5 and negative for CMIP6

Summary

Introduction

Solar geoengineering describes a set of technologies designed to (ideally) temporarily, deliberately reduce some of the effects of climate change by changing the radiative balance of the planet, often by reflecting sunlight back to space (NRC, 2015). GeoMIP has entered a new phase, concurrent with CMIP6 (CMIP6; Eyring et al, 2016), and with it are new solar geoengineering simulations with new and updated versions of the participating Earth system models (Kravitz et al, 2015). As such, this is an opportunity to revisit some central questions in solar geoengineering.

Simulations and participating models

18 IPSL-CM6A-LR

Energetics

Temperature

Hydrological and other integrative changes

Discussion and conclusions