Kicking the Can Down the Road: Understanding the Effects of Delaying the Deployment of Stratospheric Aerosol Injection

TL;DR

This study examines the effects of delaying stratospheric aerosol injection deployment on climate mitigation, revealing that delays require increased sulfur dioxide emissions and may have limited short-term climate differences, but long-term impacts remain uncertain.

Contribution

It provides the first detailed simulation-based analysis of the consequences of delaying SAI deployment, highlighting the need for higher emissions and potential long-term risks.

Findings

Delayed SAI requires 20% more sulfur dioxide injections.

Short-term climate impacts are minimal between 2035 and 2045 start dates.

Longer-term effects and tipping points need further investigation.

Abstract

Climate change is a prevalent threat, and it is unlikely that current mitigation efforts will be enough to avoid unwanted impacts. One potential option to reduce climate change impacts is the use of stratospheric aerosol injection (SAI). Even if SAI is ultimately deployed, it might be initiated only after some temperature target is exceeded. The consequences of such a delay are assessed herein. This study compares two cases, with the same target global mean temperature of 1.5C above preindustrial, but start dates of 2035 or a delayed start in 2045. We make use of simulations in the Community Earth System Model version 2 with the Whole Atmosphere Coupled Chemistry Model version 6 (CESM2-WACCM6), using SAI under the SSP2-4.5 emissions pathway. We find that delaying the start of deployment (relative to the target temperature) necessitates lower net radiative forcing (-30%) and thus larger sulfur dioxide injection rates (+20%), even after surface temperatures converge, to compensate for the extra energy absorbed by the Earth system. However, many of the surface climate differences between the 2035 and 2045 start simulations appear to be small during the 10-25 years following the delayed SAI start, although longer simulations would be needed to assess any longer-term impacts in this model. In addition, irreversibilities and tipping points that might be triggered during the period of increased warming may not be adequately represented in the model but could change this conclusion in the real world.