PNNL

Abstract

The climate effects that would result from different possible future pathways of anthropogenic forcing can be predicted by training an emulator on existing simulations, avoiding the need for new general circulation model (GCM) simulations for every possible pathway. 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 4xCO2 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 simulation (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. 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 the limits of the linearity assumption. This emulator can be used to project climate changes from a variety of possible future choices and based on multiple climate models. Indeed, for future pathways involving relatively small forcing from solar geoengineering, the errors introduced from nonlinear effects may be smaller than the uncertainty due to natural variability, and the emulator prediction may be a more accurate estimate of the forced component of the models' response than an actual simulation would be.