Dynamic climate emulators for solar geoengineering

Douglas G Macmartin,Ben Kravitz

doi:10.5194/acp-16-15789-2016

Abstract

Abstract. Climate emulators trained on existing simulations can be used to project project the climate effects that result from different possible future pathways of anthropogenic forcing, without further relying on general circulation model (GCM) simulations. We extend this idea to include different amounts of solar geoengineering in addition to different pathways of greenhouse gas concentrations, by training emulators from a multi-model ensemble of simulations from the Geoengineering Model Intercomparison Project (GeoMIP). The emulator is trained on the abrupt 4 × CO2 and a compensating solar reduction simulation (G1), and evaluated by comparing predictions against a simulated 1 % per year CO2 increase and a similarly smaller solar reduction (G2). We find reasonable agreement in most models for predicting changes in temperature and precipitation (including regional effects), and annual-mean Northern Hemisphere sea ice extent, with the difference between simulation and prediction typically being smaller than natural variability. This verifies that the linearity assumption used in constructing the emulator is sufficient for these variables over the range of forcing considered. Annual-minimum Northern Hemisphere sea ice extent is less well predicted, indicating a limit to the linearity assumption.

Highlights

Climate emulators have been used extensively to provide projections of climate change for different anthropogenic forcing trajectories. These are trained based on a limited number of simulations with general circulation models (GCMs) and allow for prediction of climate response for a much broader set of trajectories, trading the fidelity of a GCM simulation for computational efficiency
To validate the emulation of solar reduction, we use the Geoengineering Model Intercomparison Project (GeoMIP) G2 scenario, in which CO2 levels increase at 1 % per year, and for the first 50 years, the solar reduction is gradually increased to balance this forcing
After 50 years, the solar reduction is returned to zero so that only the radiative forcing from the CO2 remains

Summary

Introduction

Climate emulators have been used extensively to provide projections of climate change for different anthropogenic forcing trajectories. The spatial patterns of the responses to solar and greenhouse gas forcing will not be the same, leading to regional differences in outcomes (Ricke et al, 2010; Kravitz et al, 2014, 2015), and the precipitation responses are not the same (Bala et al, 2010; Andrews et al, 2010), nor necessarily the time evolution of the responses (Cao et al, 2015) All of these factors are important to capture if the emulator is to be useful in understanding climate effects of strategies that include solar geoengineering.

Impulse response

Spatial analysis

Results and validation

Discussion