1K: Effects of Sulfate Geoengineering on Earth’s Photochemistry and Photobiology

One proposed geoengineering approach to reducing global warming involves massive injections of sulfur dioxide (SO2) into the stratosphere, forming sulfate aerosols that would scatter some incoming solar radiation back to space and so decrease heating rates below. However, beyond simple heating, solar photons at visible and UV wavelengths drive photochemical and photobiological processes that are the basis of all life on Earth. Any long-term systematic changes in visible or UV irradiances incident on the biosphere are of great concern.

We used results from the Community Earth System Model version 1 (CESM1) with the Whole Atmosphere Community Climate Model (WACCM) as its atmospheric component, to compare current (year 2020) and future (year 2080) visible and UV radiation impingent on the troposphere and biosphere, assuming greenhouse gases continue to be emitted according to the RCP8.5 scenario, with and without geoengineering. The ground-level irradiances and actinic fluxes were computed with the Tropospheric Ultraviolet-Visible (TUV) model. The figure shows that without geoengineering, PAR and NO2 photolysis coefficients (jNO2) will remain similar to today’s values, while the UV Index, DNA damaging irradiance (iDNA), and the photolysis coefficient for production of excited oxygen atoms from ozone (jO1D), will be reduced because of the recovery of stratospheric ozone over the next half century.  With geoengineering, scattering by sulfate aerosols causes additional reductions in all of these quantities.  The reductions are not constant, but depend on the solar angle and therefore on season and time of day, e.g., with PAR reductions of 8-16% at 30 N and 21-78% at 70N.  Reductions in photosynthesis could lower agricultural yields and generally slow primary productivity, fundamentally slowing the biogeochemical cycling of carbon, nitrogen, and other nutrients.

Decreases in the photolysis coefficients (right panels of the figure) will have mixed effects on air quality, as they slow both the production and the destruction of air pollutants such as ground-level ozone. Overall, urban areas may benefit from slower ozone production chemistry, while wider geographic regions would experience an ozone increase due to slower loss chemistry.  Diurnal patterns (e.g. the ratio of jO1D and jNO2 over the course of a day) will also change and will need to be reconsidered in air quality models.

Changes in UV and visible radiation.
Figure 1. Changes in UV and visible radiation reaching Earth’s surface, in 2080 with and without sulfur geoengineering, relative to 2020.  Left panels show photo-biologically relative quantities Photosythetically Active Radiation (PAR), UV Index, and DNA-damaging irradiance. Right panels show photo-chemically relevant quantities. Click for larger image.

Reference

Madronich, S., S. Tilmes, B. Kravitz, D. G. MacMartin, and J. H. Richter, Response of surface ultraviolet and visible radiation to stratospheric SO2 injections, Atmosphere, 9, 432, doi:10.3390/atmos9110432, 2018.