How will growing cities eat?

Food security should not be ignored when assessing the future of our cities (Nature 467, issue 7318; 2010). Urban populations already comprise more than half of humanity and are expanding. This demographic shift will leave fewer farmers to cultivate the food on which cities depend, exacerbating the 20% decline in the number of farmers over the past 40–50 years.

Food for cities will either have to be sourced from remote locations across the globe, or cities will have to incorporate their own food-production facilities by such developments as peri-urban farming.

A scientific question is how cities and the land areas needed to feed them scale in relation to city population density. Scaling would be expected to be positively nonlinear because of the highly variable biological productivity of terrestrial and marine ecosystems that produce this food. In other words, larger cities will sequester proportionately larger and more marginal, low-productivity areas of land in order to be fed. Local urban and peri-urban food production tends to increase the yield per unit area and may partially counteract this trend.

The nonlinear scaling of food provision with city population size contradicts the idea that cities are proportionately more efficient per inhabitant, with respect to their use of infrastructure, carbon emissions and other services (Nature 467, 912–913; 2010). Food security of dense megacities depends on large, low-population land areas that exist elsewhere and yield disproportionately high food surpluses.