Tropical Climate Change Under Solar Geoengineering

PI: Malte Jansen
Team: Allison Hogikyan, Postdoctoral Scholar, Geophysical Sciences

This project will study how the tropical climate responds to increased carbon dioxide when warming is offset by solar geoengineering, specifically stratospheric aerosol injection (SAI). While SAI could reduce global temperatures, it would not remove impacts linked to the direct effect of CO2, such as the changes in atmospheric circulation caused by higher CO2 levels that occur even without overall warming.

The direct effect of CO2 is often defined as the climate response to increased CO2 when sea surface temperatures (SSTs) are held fixed. However, even if average temperatures do not rise, interactions between the tropical atmosphere and ocean can still produce changes in SST patterns. These patterns can be linked to widespread fire, flood, and drought hazards, as well as disruptions to global atmospheric circulation and the Earth’s energy balance. Understanding the fully coupled atmosphere-ocean response to SAI-offset CO2 forcing is therefore critical for accurately characterizing climate change under solar geoengineering.

The research will use simulations from the GeoMIP and CMIP, which rely on coupled atmosphere-ocean climate models to simulate a CO2 increase offset by SAI. The team proposes that climate change in these simulations can be understood as an “effective direct effect of CO2”: the coupled response to higher CO2 that allows changes in SST patterns while keeping the global mean SST constant.

If the traditional, uncoupled “direct effect” and the coupled “effective direct effect” are similar, researchers can build on existing knowledge to better interpret climate dynamics and impacts under solar geoengineering. If they differ, the study will provide new insight into how the tropical atmosphere-ocean system responds to radiative forcing and what the climate of a solar-geoengineered world might look like.