Agriculture is mankind's single largest usage of water, comprising 70% of all water usage. Optimizing water usage in agriculture is therefore crucial to ensuring global water security. A greenhouse is quantitatively modeled as a bioreactor and it is shown that the bulk of the water supplied to a conventionally aspirated greenhouse is lost in the form of humidity. This implies that evaporative losses in agriculture comprise a clear majority of mankind's total water consumption. Inlet CO2 enrichment using existing membrane materials can minimize the air feed rate required to supply adequate CO2 for photosynthesis, thereby mitigating evaporative losses. (c) 2018 American Institute of Chemical Engineers AIChE J, 64: 2324-2328, 2018