Rooftop farms and gardens that take advantage of underutilized roof space are a popular option, providing new food resources while simultaneously cooling the surrounding area, increasing building insulation, and improving air quality.A team led by Dr. Sarabeth Buckley, now at the University of Cambridge, theorized that repurposing the CO2 from building exhaust as a kind of fertilizer might help counter some of these challenges.Rooftop vegetable gardensCorn and spinach were chosen because they are common edible plants and because they use different pathways to photosynthesize, one of which (C3, used by spinach but not corn) is more sensitive to elevated CO2 levels and should benefit more from the exhaust vents' CO2 content."With the CO2 measurements, high concentrations were found both inside classrooms and at rooftop exhaust vents when people were in the building," Buckley said.A quadruple helping of spinachPlants were monitored throughout growth for size, number of leaves, and then after harvesting for wet and dry biomass, with striking results: spinach grown next to the exhaust vents had four times the biomass of spinach grown next to a control fan."