Designing Marine Cloud Brightening to meet climate objectives

2440352 (MacMartin). Climate change is already bringing damages to the US and elsewhere, and these will continue to get worse with continued emissions of CO2 and other greenhouse gases. Even with efforts to reduce emissions, there will still be significant impacts, both economic and human, resulting from future climate change. An additional option that could potentially reduce many climate damages is to cool the planet by reflecting a tiny bit more sunlight back to space. However, there is not yet sufficient information to support informed decisions. The two ideas most often suggested are Marine Cloud Brightening (MCB, the main focus of this work) and Stratospheric Aerosol Intervention (SAI). MCB would involve spraying sea salt aerosols into low clouds over the ocean to increase (force) cloud reflectivity, providing local and global cooling. The regional climate effects and associated impacts of MCB would depend quite strongly on where and how much forcing is applied, suggesting the potential to design an MCB approach to achieve multiple climate objectives. While there has been similar research conducted for SAI, there has been much more limited work treating MCB as a design problem. This project first aims to fill this gap, designing an MCB strategy to simultaneously manage multiple climate objectives by tuning the amount of forcing in different regions. This is an essential step towards assessing the climate impacts of MCB, and thus evaluating what role it might play in managing future climate risks. Furthermore, the combination of SAI and MCB may be able to reduce climate impacts better than either alone; the second goal is thus to assess this potential. Finally, this project will support integration of expertise between climate scientists and engineering, strengthening both disciplines. Given the risks of climate change, it is essential to understand to what extent different solar geoengineering approaches might contribute to an overall portfolio of response options. This project will introduce systematic design principles into Marine Cloud Brightening (MCB) research, and will carefully assess how SAI and MCB might complement each other. Existing MCB simulations have often simply introduced some specified perturbation and evaluated what happens, rather than starting with desired outcomes and determining both where to perturb, and how much forcing to apply in different regions, in order to achieve those outcomes. The broader MCB research community is now exploring how forcing in different regions affects the climate differently. This project will leverage this momentum, and build on the experience gained with SAI, by using optimization tools to systematically evaluate MCB as a design problem and develop multi-degree-of-freedom feedback algorithms to achieve desired climate outcomes in the presence of uncertainty. The approach thus introduces new tools that will be broadly useful for the research community going forward, and defines an approach for systematically evaluating the benefits and risks of MCB. Furthermore, this approach will enable a simultaneous optimization combining SAI and MCB to assess whether their combination could be “better” than either alone. Simulations will be conducted in climate models for different regions, and for combinations of regions. The final simulations conducted as part of this research will be made available to the climate impacts modeling community to better assess the role of MCB in managing future climate risks. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.