Beyond climate velocity: Indicators for ecological impacts of late, sudden SRM

Abstract Stratospheric Aerosol Injection (SAI) may be able to cool the Earth and (imperfectly) counter effects of greenhouse-gas-related warming. Increasing efforts are spent on researching SAI’s effects on the physical climate, but the impact on ecological systems is harder to project. As a rough approximation, it is often assumed that fast changes are particularly detrimental. One common measure is the climate velocity, that is the velocity with which relevant iso-lines (e.g. isotherms) move; an ecological system unable to migrate at this speed cannot track its climate niche. We argue that additional measures are needed to capture the climate impacts of various SAI scenarios. Using three climate scenarios—RCP8.5 without SAI, with SAI starting from 2020, or with SAI starting from 2080—we show that delayed SAI can roughly restore relevant climate variables in most parts of the world, but the delay leads to long periods of high temperature overshoot and rapid cooling, both of which may damage ecological systems. It is unclear a priori whether a rapid reversal of prior climate trends is as detrimental as a rapid continuation. This makes it necessary to also consider sign changes of climate velocities. In addition, measures for the magnitude and duration of overshoot are needed. These additional indicators should be included in risk assessments of SAI strategies. However, given the severe political conundrum of (early) SAI deployment, our findings can only be one element in a holistic assessment.