Water vapor response

Stratospheric water vapor is a potent greenhouse gas that traps heat and significantly affects Earth's energy balance. Stratospheric aerosol injection (SAI) warms the upper atmosphere, as sulfate aerosols absorb near-infrared radiation — sunlight just beyond the visible range — depending on the size distribution. Warming the tropical tropopause layer allows more water vapor to enter the stratosphere. This additional water vapor could have substantial climate effects in two ways: directly, by trapping additional heat like other greenhouse gases, and indirectly, by affecting the properties of injected aerosol particles themselves. However, current climate models represent stratospheric water vapor poorly, and there is limited understanding of how the heating from SAI translates into changes in upper-atmosphere moisture and the resulting climate impacts. Understanding this connection is critical because increased stratospheric water vapor may cause injected particles to grow larger, sink faster, and remain aloft for shorter periods—all of which reduce SAI's cooling effectiveness. Accurately predicting how much SAI cools the planet therefore requires understanding how stratospheric water vapor changes in response to SAI.

This activity involves two complementary modeling components:

1. Lagrangian trajectory analysis — estimating how stratospheric water vapor will change in response to SAI-induced warming. This work will employ three-hourly ERA5 reanalysis wind fields at half-degree spatial resolution to track air parcel trajectories for approximately 30-day periods, identifying when parcels cross the stratospheric cold point (where water vapor becomes saturated) and quantifying in-situ temperatures. By applying the Clausius-Clapeyron equation to estimate moisture capacity changes resulting from SAI-induced temperature perturbations, the analysis will determine stratospheric water vapor increases across multiple scenarios. This component will require 1–5 years of continuous trajectory simulations to capture seasonal variability and establish robust statistics.
2. Radiative transfer calculations — quantifying how increased stratospheric water vapor affects radiative forcing, including how large the greenhouse effect is and how the increased water vapor affects the local heating induced by the aerosols. This analysis will neglect aerosol property changes; relative humidity can be varied separately in microphysics models and chamber experiments, which can help constrain those particular effects of increasing water vapor. The final deliverable will include quantitative estimates of stratospheric water vapor increases under different SAI scenarios and their net radiative consequences, likely resulting in 1–2 peer-reviewed publications.

This work could be accelerated with a team of two full-time researchers as opposed to one.