In densely populated buildings, CO2 emissions from human respiration is surprisingly high, and growing rooftop spinach in CO2 recycled from building ventilation can quadruple growth. The majority of urban dwellers spend their time indoors, so cities have a potentially huge, steady supply of untapped CO2. Rooftops typically take up 20-30% of urban space, mostly unused. Surveying available rooftop area for growing food in Boston, researchers found that if we grew vegetables there, adding CO2 building exhaust to the mix, the city could produce enough food to satisfy 290% of its own vegetable demand (assuming that crops of different varieties would benefit similarly).
Climate Crisis Blog
