Negative emissions and energy demand of macronutrient fertilization

To stay below the global temperature rise proposed in Kyoto Protocol and the Paris Agreement (2015), it seems that negative emission of CO₂ to the atmosphere will be required (Cheah et al., 2016; Pires, 2017). Marine biological capture uses solar energy to sequester CO₂ below the permanent thermocline and this makes it more attractive than chemical strategies such as carbon capture and storage (CCS) (Williamson et al., 2012; De Silva et al., 2015). The bio-sequestration of CO₂ by macronutrient fertilization can increase CO₂ uptake by the phytoplankton into biomass via photosynthesis effect. A disadvantage of marine macronutrient fertilization is that it consumes oxygen in the ocean. Macronutrient fertilization requires only 3 GJ per tonne of new primary production. Most other low opportunity cost sequestration options have a higher energy demand. Lawrence (2013) found the efficiency of sequestration for 100 years in terms of new primary production was 75%. However there are undesirable collateral impacts of oxygen depletion. Using the same global biogeochemical model as Jones and Caldeira (2003), the oxygen demand was computed for nourishment by macronutrients between ± 40° latitude. The model neglected the production of NO₂. The temporal and spatial variations of oxygen depletion over a 100 year of fertilization were simulated. Various scenarios were investigated for oxygen change. Collateral benefit of increase food supply for pelagic fish may somewhat offset the negative impact of oxygen reduction in the ocean.