Diffuse radiation impact on vegetation under Geoengineering Model Intercomparison Project G6 experiments

Abstract:

Stratospheric aerosol intervention (SAI) has been suggested as a climate intervention technique that would reduce total solar radiation reaching the surface by reflecting the sunlight back to the space. The aerosol layer in the stratosphere would also forward scatter sunlight and increase surface diffuse radiation. The multiple changes in downward solar radiation under SAI would impact vegetation in various ways. Reduction of total solar radiation would limit the available photosynthetically active radiation for plant photosynthesis, while increased diffuse radiation would benefit plant growth by promoting canopy photosynthesis under the top layer, and by reducing leaf temperatures. The total influence of radiation change on vegetation would depend on the amount of solar radiation reduction, the percentage of diffuse radiation, and other climate changes from SAI. We use G6 experiments from the Geoengineering Model Intercomparison Project to study diffuse radiation impact on vegetation. In G6, a global warming scenario SSP5-8.5 is the reference case and there are two SAI simulations: (1) G6sulfur, which injects sulfate aerosol to return net anthropogenic radiative forcing to a medium forcing scenario SSP2-4.5; and (2) G6solar, which reduces the solar constant to reach the same radiative target as in G6sulfur. We analyze output from 7 Earth system models. All models show an increasing trend of net primary productivity (NPP) in all scenarios as a result of increased CO₂ concentration. With the cooling effect and changed precipitation, models show different NPP responses under the same SAI scenario due to different land model configurations. For example, CESM simulates a reduction of NPP under SAI compared to SSP5-8.5, because the release of nitrogen from organic matter decomposition slows down in a cooler environment, and therefore NPP is reduced under SAI by nitrogen limitation. With more diffuse radiation in G6sulfur, global average NPP is higher than in G6solar. However, there are significant regional differences, as regional climate changes of temperature, precipitation and cloud coverage are heterogeneous, and in addition, different vegetation types respond to diffuse radiation changes differently. Detailed results from a range of climate models participating in G6 will be presented in the meeting.