Potential impacts of stratospheric aerosol injection on drought risk managements over major river basins in Africa

Abstract

Most socio-economic activities in Africa depend on the continent’s river basins, but effectively managing drought risks over the basins in response to climate change remains a big challenge. While studies have shown that the stratospheric aerosol injection (SAI) intervention could mitigate temperature-related climate change impacts over Africa, there is a dearth of information on how the SAI intervention could influence drought characteristics and drought risk managements over the river basins. The present study thus examines the potential impacts of climate change and the SAI intervention on droughts and drought management over the major river basins in Africa. Multi-ensemble climate simulation datasets from the Stratospheric Aerosol Geoengineering Large Ensemble (GLENS) Project were analysed for the study. The Standardized Precipitation Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPI) were used to characterize the upper and lower limits of future drought severity, respectively, over the basins. The SPEI is a function of rainfall and potential evapotranspiration, whereas the SPI is only a function of rainfall, so the difference between the two indices is influenced by atmospheric evaporative demand. The results of the study show that, while the SAI intervention, as simulated in GLENS, may offset the impacts of climate change on temperature and atmospheric evaporative demand, the level of SAI that compensates for temperature change would overcompensate for the impacts on precipitation and therefore impose a climate water balance deficit in the tropics. SAI would narrow the gaps between SPEI and SPI projections over the basins by reducing SPEI drought frequency through reduced temperature and atmospheric evaporative demand while increasing SPI drought frequency through reduced rainfall. The narrowing of this gap lowers the level of uncertainty regarding future changes in drought frequency, but nonetheless has implications for future drought management in the basins, because while SAI lowers the upper limit of the future drought stress, it also raises the lower limit of the drought stress.

Comparative Analysis of Meteorological Drought based on the SPI and SPEI Indices

“…For example, at the end of the time series of the Matam station, SPEI-12 and SPEI-24 indicate a wet period while SPI-12 and SPI-24 indicate a continuation of the drought (Figure 4). The analysis of the SPI and SPEI results supports numerous studies in Senegal [16,17,19,28,36,37] which concluded that there has been a rainfall deficit since the 1970s. Similar results were also obtained by Ali and Lebel (2009) [38] who showed the persistence of drought during the 1970s throughout West Africa, especially the Sahel.…”

Different types of drought under climate change or geoengineering: Systematic review of societal implications

“…Different regions of Africa have different projections for SPI (rainfall) droughts and SPEI (potential evapotranspiration) droughts under different scenarios of climate change and geoengineering. Abiodun et al (2021) examined projections of climate change using a single climate model, the Community Earth System Model (CESM1) with 20 ensemble members under the RCP 8.5 scenario, and compared this with a scenario of solar geoengineering from the Stratospheric Aerosol Geoengineering Large Ensemble (GLENS) Project. In this experiment the stratospheric injection rate is calculated to maintain the global mean temperature as well as the inter-hemispheric and equatorto-pole near-surface temperatures at the 2020 level until the end of the century while keeping other forcing as in the RCP 8.5 scenario.…”

“…To contextualize the analysis of societal outcomes, we reproduce here the results from Abiodun et al (2021), which depict the geographic distribution of changes to SPI and SPEI across Africa under a strong climate change scenario and a specific scenario solar geoengineering that fully offsets global temperature increases due to climate change. When we refer to "solar geoengineering, " we are referring specifically to stratospheric aerosol injection.…”

“…Given the complexity of these projections, the implications for rainfall and temperature under a future with climate change or with solar geoengineering are complex, involving a spatiallyvarying mix of changes to temperature and precipitation. For Africa, Abiodun et al (2021) found that climate change is projected to decrease water availability due to increasing evaporation, while geoengineering would mitigate the increase in evaporation but also decrease total precipitation in many regions. However, it is not clear what these results mean for human systems, for example, to know whether existing water management plans will be affected by the two different scenarios.…”

“…When compared to a preindustrial climate, projections of climate change in Africa tend to include increases in extreme droughts as defined by SPEI, because of the inclusion of increasing temperatures which drive increases in PET. On the other hand projections of the impact of solar geoengineering tend to include more extreme droughts as defined by SPI, which only uses rainfall (Abiodun et al, 2021).…”