Edward  Glaeser writes for Boston.com about the environmental impacts of urban agriculture. While making many solid arguments about inputs for food production in northern vs. southern climates and economies of scale, he then sites this as his major focus:

But the most important environmental cost of metropolitan agriculture is that lower density levels mean more driving. Today, about 250 million Americans live on the 60 million acres of this country that are urban — which is about four people per acre. By contrast, America uses 442 million acres for cropland and 587 million acres for pastureland, which is about 1.4 and 1.9 acres per person respectively. If we allocated just 7.2 percent of this agricultural land into metropolitan area, we would halve metropolitan area densities.

The National Highway Travel Survey teaches us that when densities drops in half, holding fixed location within the metropolitan area, households buy about 107 gallons more gas per year. If halving densities also doubled distance to the metropolitan area center, this would add an extra 44 gallons of gas annually. Together, the increased gas consumption from moving less than a tenth of agricultural farmland into metropolitan areas would generate an extra 1.77 tons of carbon dioxide per year, which is 1.77 times the greenhouse gases produced by all food transportation and almost four and a half times the carbon emissions associated with food delivery.

First of all, I don’t think there are many who would reasonably assume that we can meet all of our food needs within a city’s limits certainly not to the point of reducing density by half. Instead, what the local food advocates I work with propose is higher densities, creating less sprawl, preserving existing agricultural land and freeing up more urban land for agriculture and other public use while supporting rural economies. Secondly, and let’s be serious for a moment, urban agriculture is hardly a major land-use concern especially as we continue to do things like this:

Charlotte’s new NASCAR Hall of Fame by Pei Cobb Freed & Partners. Image credit: Paul Warchol.

Here we have a monument to what may very well be the single most mindless waste of natural resources conceived by humankind: NASCAR. Only war can compare to the pure and senseless squander involved in this “sport” – at least you could argue for a just war. I would be, by any measure, far more interested in an analysis of the various benefits, agricultural or otherwise, we could achieve from the reallocation of resources currently squandered on this mind-numbingly short-sited spectacle than any critique of a non-existent large-scale urban agriculture schema. While we are at it let’s focus our ire on another existing density increasing, resource wasting urban land hog: golf. With about 17 000 course in the USA alone surely we can think of far better targets for our agricultural development arguments?

Consider: 1. By 2050 we can project that about 70 percent of the world's population will live in urban centres, and the majority of urban growth will occur in developing nations. [1. Population Reference Bureau, Human Population: Urbanism.]

2. By 2050, to feed a global population projected at nine billion, "we will need to produce as much food in the next 40 years as we have in the last 8,000". [2. Jason Clay of the World Wildlife Fund at the annual meeting of the American Association for the Advancement of Science. Quoted here.]

6.3 billion urban dwellers, mostly in poorer nations, within the next 40 years. Urban growth and sustainability depends not only on rural sustainability, but on imbrication of producer and consumer systems – the two must be considered as a whole. Initiatives like the New City Market are the beginnings of a re-imagining of our food systems based on an understanding that there can be no urban/rural divide in a strong food economy. Indeed, that this divide, which assumes consumption as separate or even as wholly divorced from production, is the root of our failing food systems. The competition for resources and land between cities and farms only exacerbates this.

Forward-looking urban policy must understand and incorporate food systems as a primary and foundational precondition to any and all growth. This change can not be limited to just land-use issues, but institutional food procurement policies, tax and fee incentives, waste management, urban food production, transportation infrastructure, regulatory change, downstream/storm water pollution abatement and so on. An urban system based on an understanding of, and integration with, its surrounding food systems can work as an engine of food security instead of an ecological and economic liability.

We not only need to think of the food economy as an endangered ecosystem that exists right on the borders and shores of our cities and act as if a city's continued prosperity depends on the survival of that system (it does), we also need to act with the understanding that cities are a part of and can strengthen that system as well.

UPDATE: This timely video from the WWF illustrates the above mentioned situation with stunning efficacy.

More videos here.