Background/Question/Methods Climate intervention would have complex and varying effects on ecosystems. Much of this variation arises from the fact that these interventions have physical effects (e.g., surface shortwave radiation reduction, cooling) that are unique and distinct from those caused by greenhouse gases (e.g., warming, ocean acidification, photosynthesis enhancement). Thus, an intervention such as stratospheric aerosol injection (SAI) would affect some of the same ecological processes that are responding to anthropogenic climate change, but the nature of the responses is likely to differ. Here we focus on how ecosystem processes and biogeochemistry (e.g., alterations to productivity, the water cycle, nutrient dynamics) are likely to be affected differently than organisms and communities. We hypothesize that the differences will arise due to differential effects of SAI and greenhouse gases on carbon dioxide (CO2) concentrations in the atmosphere, which directly influences ecosystem carbon, water and nutrient fluxes. Differences will also arise from the fact that greenhouse gases alter the climate continuously, while sunlight reductions by SAI will have effects primarily during the day, and more in summer than in winter. Results/Conclusions Much of the uncertainty about effects of SAI on ecosystem processes is rooted in the disconnect between temperature and CO2 that SAI would produce. Much of the extra CO2 humans have added to the atmosphere has been absorbed by the land and ocean, primarily through uptake by ecological systems. Cooler temperatures could reduce photosynthetic carbon uptake if warming leads to higher productivity; or cooler temperatures could increase uptake if heat or other stress on ecosystems is reduced. Interactions between temperature, precipitation, and CO2 levels in the atmosphere also affect the ability of ecosystems to absorb other greenhouse gases (methane, nitrous oxide) in complex ways that are difficult to predict under SAI. The disconnect between temperature and CO2 that could be induced by SAI would also have substantial effects on the hydrologic cycle. A combination of elevated atmospheric CO2, which increases plant water use efficiency and SAI-induced cooling might synergistically reduce biological water use. Together, these factors could reduce transpiration, leaving more water in the soil and in streams draining terrestrial ecosystems. Consequent changes to runoff and streamflow could affect aquatic habitats, interactions between terrestrial and aquatic ecosystems, and biogeochemical processes that regulate nutrient export from watersheds.
