Abstract
Limiting mean global warming to well below 2 °C will probably require substantial negative emissions (NEs) within the 21st century. To achieve these, bioenergy plantations with subsequent carbon capture and storage (BECCS) may have to be implemented at a large scale. Irrigation of these plantations might be necessary to increase the yield, which is likely to put further pressure on already stressed freshwater systems. Conversely, the potential of bioenergy plantations (BPs) dedicated to achieving NEs through CO2 assimilation may be limited in regions with low freshwater availability. This paper provides a first-order quantification of the biophysical potentials of BECCS as a negative emission technology contribution to reaching the 1.5 °C warming target, as constrained by associated water availabilities and requirements. Using a global biosphere model, we analyze the availability of freshwater for irrigation of BPs designed to meet the projected NEs to fulfill the 1.5 °C target, spatially explicitly on areas not reserved for ecosystem conservation or agriculture. We take account of the simultaneous water demands for agriculture, industries, and households and also account for environmental flow requirements (EFRs) needed to safeguard aquatic ecosystems. Furthermore, we assess to what extent different forms of improved water management on the suggested BPs and on cropland may help to reduce the freshwater abstractions. Results indicate that global water withdrawals for irrigation of BPs range between ∼400 and ∼3000 km3 yr−1, depending on the scenario and the conversion efficiency of the carbon capture and storage process. Consideration of EFRs reduces the NE potential significantly, but can partly be compensated for by improved on-field water management.
Highlights
With the Paris Agreement (UNFCCC 2015), the international community has agreed to aim for a global mean temperature (GMT) increase of well below 2 °C compared to preindustrial levels, and pursue efforts to limit it to 1.5 °C
Scenarios We compare the water requirements associated with the two sequestration demands for four different water use scenarios: rainfed only (RF), unconstrained irrigation withdrawals (IRR), availability-constrained irrigation respecting environmental flow requirements (EFR), and the latter combined with improved crop water management (WM)
Note that despite non-negligible withdrawals for bioenergy plantations (BPs) in the order of 400 km3 yr−1 in scenario WM of basic and TechUp setups, the total withdrawals may even fall below those of the respective RF scenario (3011 km3 yr−1), because EFRs are taken into account for withdrawals of agricultural irrigation and because water is assumed to be more effectively managed on cropland
Summary
With the Paris Agreement (UNFCCC 2015), the international community has agreed to aim for a global mean temperature (GMT) (see table 3 for a full list of abbreviations) increase of well below 2 °C compared to preindustrial levels, and pursue efforts to limit it to 1.5 °C. The NET most widely used in projections for the 21st century is bioenergy plantations (BPs) with subsequent carbon capture and storage (BECCS) (Fuss et al 2014, Schleussner et al 2016). BECCS could potentially provide large amounts of negative emissions (NEs), but in turn competes with agriculture and other uses such as ecosystem conservation for land requirements. Different (portfolios of) NETs (Minasny et al 2017, Werner et al 2018) or alternative mitigation pathways (van Vuuren et al 2018) are receiving more and more attention, but bioenergy utilization will likely be significant during the 21st century (Masson-Delmotte et al 2018), since it is relatively cheap, compared to direct-air-capture and more land-effective than afforestation (Smith et al 2016). Our study provides additional value in support of making deployment decisions based on economic, and eco-hydrologic reasoning
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