Climate Speeds Help Frame Relative Ecological Risk in Future Climate Change and Stratospheric Aerosol Injection Scenarios

Anthropogenic climate change is already a major source of global ecosystem risk in the present day, with worsening effects anticipated in all future Shared Socioeconomic Pathway scenarios. Risks such as these motivate the study of potential methods of climate intervention to reduce climate impacts alongside decarbonization. Stratospheric aerosol injection (SAI) is a proposed hypothetical method to emit aerosols into the stratosphere, where they would reflect a small portion of sunlight and thereby cool the planet. Two new Earth system modeling experiments depict policy-relevant SAI scenarios with similar temperature targets (near 1.5C above preindustrial), but with deployment delayed by 10 years between experiments. We show using climate speeds (a measure of the movement of thermal niches) that this relatively short delay leads to highly distinct profiles of ecological risk between the two SAI scenarios and relative to a baseline of no-SAI climate change consistent with present policy. Climate speeds when global temperature is maintained with SAI are indistinguishable on a planetary scale from those experienced under preindustrial conditions. In contrast, the delayed SAI deployment produces very large climate speeds far beyond natural variability and robustly greater than no-SAI climate change with present policy. We demonstrate that examining the global area exposed to threshold climate speeds facilitates evaluation of relative ecological risk among future scenarios with varying degrees of climate mitigation and intervention strategies, as well as comparison of model performance against reanalysis-estimated conditions. Our work identifies a key tradeoff between rapid cooling and ecological impacts in future climate intervention scenario design, while supporting broad discussion of relative ecological risk across a range of scenarios.