Will the circle be unbroken? The climate mitigation and sustainable development given by a circular economy of carbon, nitrogen, phosphorus and water

Closing the loop in the flow of C, nutrients and water between agriculture, the human diet and sanitation services offers benefits for humanity across multiple platforms of public health, food security and climate mitigation. This study assesses these benefits by describing the hypothetical scenario of a global, ‘fully functional’ circular economy, in which 100% of C, N and P were recovered from human excreta and returned to agricultural soil. Crop nutrient demand is calculated and compared with that which could be recovered, and greenhouse (GHG) emissions from fertilizer production, fertilizer application and sanitation services are presented, as are freshwater availability and crop irrigation requirements. These are considered to analyse the broader effects of this circular economy that is driven by dietary nutrition demand on climate change, the provision of sanitation services and crop irrigation, in 2022 and with projections to 2030 and 2050. We find the capacity of the circular economy to deliver crop nutrients and mitigate GHG emissions varies by region. Some regions benefit from supplementing conventional mineral fertilizers with excreta-derived fertilizers, others from reducing GHG emissions from sanitation services through improved resource recovery rates. A hypothetical, fully functional circular economy that recovers all excreta nutrient C, N and P would reduce global GHG emissions from N and P mineral fertilizer production and application by 140 Tg CO2 equivalents (CO2 e) per year in 2022 (∼12% of total emissions from mineral fertilizer production and application) and provide a maximum of 104 Tg C per year for sequestration in global cropland (∼12% of estimated annual soil C sequestration potential). A portion of this sequestered C will return to the atmosphere via soil respiration, however, with co-benefits to other soil functions such as crop nutrient fertility. The maximum potential reduction in GHG emissions from sanitation services through these measures would bring reductions of 445 Tg CO2 e per year in 2022, rising to 562 Tg CO2 e in 2050. Our results provide evidence to guide specific regional policy on reducing GHG emissions, offsetting mineral fertilizer use and optimizing municipal water use using the circular economy.