Scenarios for modeling solar radiation modification

Abstract

Making informed future decisions about solar radiation modification (SRM; also known as solar geoengineering)-approaches such as stratospheric aerosol injection (SAI) that would cool the climate by reflecting sunlight-requires projections of the climate response and associated human and ecosystem impacts. These projections, in turn, will rely on simulations with global climate models. As with climate-change projections, these simulations need to adequately span a range of possible futures, describing different choices, such as start date and temperature target, as well as risks, such as termination or interruptions. SRM modeling simulations to date typically consider only a single scenario, often with some unrealistic or arbitrarily chosen elements (such as starting deployment in 2020), and have often been chosen based on scientific rather than policy-relevant considerations (e.g., choosing quite substantial cooling specifically to achieve a bigger response). This limits the ability to compare risks both between SRM and non-SRM scenarios and between different SRM scenarios. To address this gap, we begin by outlining some general considerations on scenario design for SRM. We then describe a specific set of scenarios to capture a range of possible policy choices and uncertainties and present corresponding SAI simulations intended for broad community use.

Keywords: climate engineering; climate intervention; scenarios; solar geoengineering; solar radiation modification.

Fig. 1.

There are multiple dimensions to future scenarios for SRM: the emission scenario, temperature target, start date, deployment strategy (for SAI, the injection latitudes, seasons, altitude, and material), and other events that might occur, providing a combinatorial challenge. The proposed scenarios are illustrated here and graphically in Fig. 2.

Fig. 2.

Graphical illustration of scenarios described in Section 3. Simulation results for historical (through 2014) and SSP2-45 (2015 on) are from the CESM2(WACCM6) model, as described in Section 4 (three ensemble members; mean shown in thicker lines); simulation data for the SRM scenarios here are shown in Figs. 3 and 4.

Fig. 3.

High-level results from simulations involving different temperature targets: global mean temperature; SO₂ injection rates; land average precipitation minus evaporation P-E; Arctic September sea-ice extent; total column ozone in southern hemisphere (SH), 60 to 90 ${}^{°}$S in October (in Dobson Units, DU); Global Stratospheric Optical Depth; AMOC; and upper ocean heat content (indicative of thermosteric sea-level rise).

Fig. 4.

High-level results from simulations of different events—termination, interruptions, or a deliberate gradual phaseout. As in Fig. 3.