Irrigation of biomass plantations may globally increase water stress more than climate change

The Earth system faces multiple environmental pressures, including climate change and water scarcity, while ensuring food and water security for a growing population remains crucial. Negative emission (NE) technologies, such as BECCS, are gaining interest to achieve the 1.5°C target without compromising sustainable development goals (SDGs), particularly water security. However, large-scale biomass production for BECCS likely requires irrigation, increasing pressure on freshwater resources. This study investigates the trade-off between the water stress from irrigation for bioenergy production and the avoided climate change impacts. It aims to quantitatively analyze the global-scale effects of BECCS on water stress compared to a scenario without significant BECCS deployment, considering the role of climate change, land use change, and the potential benefits of sustainable water management (SWM). The water stress is defined using the ratio of total human water withdrawals to available discharge.

Previous research highlighted the potential for increased water stress in mitigation scenarios based on irrigated bioenergy, particularly in regional studies focusing on the United States. This study expands upon these findings by providing a global-scale analysis. Other studies have focused on BECCS water demand but did not always use transient land use projections consistent with future greenhouse gas emission pathways and socio-economic development. This study addresses these limitations by using data from the Representative Concentration Pathways (RCPs) and Shared Socioeconomic Pathways (SSPs) within the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP2b) framework to ensure internal consistency across climate change, BECCS deployment, and land use trajectories.

The study uses the process-based global vegetation and water balance model LPJmL, driven by climate change scenarios from four General Circulation Models (GCMs) within the ISIMIP2b project (HadGEM2-ES, MIROC5, GFDL-ESM2M, IPSL-CM5A-LR). Two scenarios are compared: a strong mitigation scenario with widespread BECCS (scenario BECCS), and a climate change scenario with only marginal BECCS (scenario CC). A third scenario (BECCS+SWM) incorporates sustainable water management practices, including environmental flow requirements (EFRs) and improved on-field water management. The irrigation fraction for biomass plantations is determined through sensitivity analysis, aiming to reproduce the biomass harvests initially assumed in the ISIMIP2b scenarios. The water stress index (WSI) is calculated as the ratio of human water withdrawals to available discharge, with high stress defined as WSI > 40%. The study analyzes both global aggregates and spatial distributions of water stress, attributing differences between scenarios to climate, land use, and bioenergy irrigation using factorial simulations. The model incorporates various factors such as competition for water between crops and bioenergy plants, and the possibility of transferring water from neighboring cells. Agricultural areas are dynamically simulated considering rainfed or irrigated conditions based on different irrigation techniques. The model accounts for environmental flow requirements to safeguard river ecosystems.

By the end of the 21st century, the global area and population under high water stress are projected to increase significantly across all scenarios without sustainable water management, compared to the present day. The BECCS scenario shows a substantially higher increase in both area and population under high water stress compared to the CC scenario. Specifically, the global area under high water stress increases from approximately 1023 Mha today to 1580 Mha in the CC scenario and 1928 Mha in the BECCS scenario. The corresponding numbers for population under high water stress are 2.28 billion today, 4.15 billion in the CC scenario, and 4.58 billion in the BECCS scenario. The BECCS scenario extends high water stress to regions currently unaffected, including parts of Brazil and Sub-Saharan Africa, where large-scale biomass plantations are assumed. Factorial simulations demonstrate that irrigated biomass plantations are the primary driver for increased water stress in the BECCS scenario, although differences in land use or climate contribute in certain regions. Notably, the BECCS+SWM scenario shows a significant reduction in both the area and population under high water stress, reducing the global area under increased water stress from 72% to 37%, compared to the CC scenario. This highlights the mitigating potential of sustainable water management.

The findings show that climate mitigation via irrigated BECCS, at the global level, may exert similar or even higher water stress than unabated climate change. This challenges the assumption that BECCS is a straightforward solution for climate change mitigation. The study highlights a crucial trade-off between mitigating climate change and maintaining water security. While avoiding a 3°C warming by employing BECCS is beneficial for climate, the additional water demand from irrigated biomass plantations could significantly exacerbate water stress globally and regionally. The increased water stress in the BECCS scenario is primarily attributable to the additional water withdrawals needed for irrigation. However, the implementation of sustainable water management strategies, as demonstrated in the BECCS+SWM scenario, shows significant potential for mitigating the negative impacts on water resources. This demonstrates the importance of integrating sustainable water management practices into BECCS deployment strategies.

This study demonstrates that while BECCS can effectively mitigate climate change, its large-scale implementation using irrigation could significantly increase global water stress, even surpassing the impacts of climate change alone. This highlights the critical need to incorporate water availability constraints in integrated assessment models. Implementing sustainable water management practices is crucial for mitigating this negative effect and ensuring the sustainable deployment of BECCS. Future research should focus on exploring regional-specific irrigation thresholds and further integrating water management considerations into integrated assessment models.

The study relies on a single dynamic global vegetation model (LPJmL), although the results are largely influenced by externally provided climate and land-use data. The model does not fully account for all potential indirect effects of irrigation on the water cycle, such as changes in evapotranspiration or atmospheric moisture recycling. The assumed irrigation levels are based on sensitivity analysis and assumptions about technological improvements in crop productivity. Additionally, the study uses a specific set of climate and land-use scenarios, and therefore the results may vary with different scenarios or model setups.