Mid-latitude climate response

Stratospheric aerosol injection (SAI) could change weather patterns across the mid-latitudes, the band of the globe that includes most of North America, Europe, and Asia, in ways that differ from greenhouse gas warming. Two mechanisms drive this. First, different SAI strategies change the temperature difference between the equator and the poles, which shapes the strength and position of the jet stream, the fast-moving band of air that impacts much of mid-latitude weather. Second, injecting sulfur into the stratosphere heats that layer of the atmosphere directly, especially when injected near the equator (Jones et al., 2021); this heating triggers changes in air circulation that have been linked to surface weather effects in the mid-latitudes, particularly in winter.

These mechanisms are fairly well understood on their own, but it is still unclear how strongly SAI would actually affect mid-latitude weather extremes and circulation in practice. Part of the uncertainty comes from limited testing: few climate models have simulated the high-latitude and off-equator injection scenarios that are increasingly relevant as potential deployment strategies. Running more Earth System Models (ESMs) across a wider range of these scenarios would sharpen these predictions and supply the regional climate data decision makers need. The other driver of uncertainty is more fundamental: today's models don't fully match real-world wind patterns—they show significant errors in the position and behavior of the jet stream and the stratospheric polar vortex (the band of winds circling each pole)—and scientists don't yet agree on how these would shift under global warming alone, let alone under SAI.

This research activity funds research over multiple years to develop mechanistic understanding of how SAI affects mid-latitude surface climate and to create tools that enable rapid assessment of these impacts across Earth system models and injection scenarios. This work includes the following components:

1. Mechanistic understanding of mid-latitude drivers — improving the mechanistic understanding of the drivers of residual changes in mid-latitude climate relative to the intended climate outcome under different SAI deployment scenarios. This phase will identify which physical mechanisms drive mid-latitude responses and how sensitive these responses are to injection strategy, background climate state, model mean state biases, and model representation of variability.
2. Detection and attribution tools for SAI impacts on extremes — developing tools that operationalize the ability to detect and attribute SAI's impacts on extremes, building on recent advances in warming attribution (e.g., Philip et al., 2022). Successful operationalization would allow near-real-time statements about how much a given weather extreme (e.g., a heatwave) would have differed in intensity or likelihood with or without SAI. Research groups will develop frameworks, methodologies, and potentially software tools that make these attribution analyses feasible across multiple models and scenarios, creating infrastructure for future impact assessments.

This area of study will be ongoing, with multiple impact studies over time. This research activity is focused on the initial scope of work needed to meaningfully advance scientific understanding of these processes and make future research faster and more robust. Reflective has funded the following projects in this area of study:

• Global Drought and Extreme Precipitation Responses to SAI Strategies. Dr. Samantha Stevenson & Dr. Se-Yong Song, University of California, Santa Barbara
• European climate extremes under G6-1.5K-SAI and G6-1.5K-HiLLA [internal project]. Francis Osei Tutu Afrifa & Alistair Duffey, Reflective