Exploring the Effects of Solar Radiation Management on Water Cycling in a Coupled Land–Atmosphere Model*

Abstract

Solar radiation management (SRM) has been proposed as a form of geoengineering to reduce the climate effects of anthropogenic greenhouse gas emissions. Modeling studies have concluded that SRM, through a reduction in total solar irradiance by approximately 2%, roughly compensates for global mean temperature changes from a doubling of carbon dioxide concentrations. This paper examines the impact of SRM on the terrestrial hydrologic cycle using the Community Land Model, version 4, coupled to the Community Atmosphere Model, version 4, with reductions in solar radiation relative to simulations with present-day and elevated CO2 concentrations. There are significant global and regional impacts due to vegetation–climate interactions that are not compensated when reductions in total solar irradiance of 1%, 2%, and 3% are imposed on top of a doubling of present-day CO2 concentrations. Water cycling slows down under SRM, including decreases in global mean precipitation and evapotranspiration. Changes in runoff and soil moisture are spatially and temporally variable, with implications for local water availability. In the tropics, evapotranspiration decreases because of increases in vegetation water use efficiency. In northern midlatitudes, soil moisture increases when evapotranspiration decreases, with some exceptions during boreal summer. Changes in soil evaporation influence water cycling in the southern subtropics, rather than changes in transpiration. The hydrologic response to SRM is nonlinear, with global mean decreases greater than expected. These results imply that SRM may not compensate for higher greenhouse gas concentrations when one considers land–atmosphere interactions.

Regional Climate Variability Under Model Simulations of Solar Geoengineering

“…Previous climate modeling studies have shown that globally averaged surface warming from increases in atmospheric CO 2 concentration can be roughly compensated by spatially uniform reductions in the solar constant (Caldeira & Wood, ; Kravitz, Caldeira, et al, ). Similar studies have shown that this compensation also leads to a decrease in global annual mean precipitation (e.g., Bala et al, ; Dagon & Schrag, ; Kravitz, Rasch, et al, ; McCusker et al, ; Niemeier et al, ; Ricke et al, ; Tilmes et al, ). This phenomenon has been observed following large volcanic eruptions, when aerosol surface cooling reduces the latent heat flux of water vapor to the atmosphere and weakens the global hydrologic cycle (Gillett et al, ; Iles et al, ; Trenberth & Dai, ).…”

“…Changes in shortwave forcing such as solar variability or volcanic eruptions have also been shown to be more effective in driving precipitation changes than the equivalent CO 2 forcing (Allen & Ingram, ; O'Gorman et al, ). Evaporation is expected to decrease under solar geoengineering because latent heat flux will respond more strongly to changes in shortwave radiation than changes in longwave radiation (Andrews et al, ; Bala et al, ; Cao et al, ; Curry et al, ; Dagon & Schrag, ; Kleidon & Renner, ; Tilmes et al, ).…”

“…Changes in transpiration alter plant water use efficiency (Swann et al, ), defined as the ratio of photosynthesis to evapotranspiration (ET). Dagon and Schrag () demonstrate that solar geoengineering has similar global and regional impacts on terrestrial water cycling. When solar geoengineering is modeled as a decrease in incoming solar radiation to compensate surface temperature changes from increased CO 2 , the CO 2 ‐induced stomatal closure is augmented by the decrease in solar radiation, leading to further decreases in ET (Dagon & Schrag, ).…”

Dimming in Iran since the 2000s and the potential underlying causes

“…Solar radiation that reaches the Earth surface (R s ) (also known as global solar radiation) plays a key role in various natural processes such as the hydrological cycle (Wild et al, 2005;Xia et al, 2006;Wild, 2009;Dagon and Schrag, 2016;Manara et al, 2016;Jahani et al, 2017). For example, R s provides the energy for photosynthesis, evaporation, evapotranspiration, cloud formation, snow melt, etc.…”

“…For example, R s provides the energy for photosynthesis, evaporation, evapotranspiration, cloud formation, snow melt, etc. (Kun et al, 2006;Wild et al, 2015a;Dagon and Schrag, 2016;Huber et al, 2016). Therefore, it is considered as one of the most important inputs of hydrological, irrigation scheduling and crop growth models (Aladenola and Madramootoo, 2014;Jahani et al, 2016;Jahani et al, 2017;Quej et al, 2017;Wang et al, 2017).…”