In the post WW II sixty years what used to be "the city" has become a seemingly endless geographic sprawl of mostly low density land use. The most extensive urban space in the world extends from Portland Maine to Miami Florida, over 1,000 miles . The US Geological Survey found in 2002 that only 4 counties of two hundred were rural. The best method of grasping this new reality of urban is NASA satellite photos or with Google Earth. For better or for worse our world is increasingly becoming a Megapolis.
Sustainable development for the Megapolis is most commonly referred to as "Carbon Neutral". For our purpose, a more specific name is 'ZNNEI' (Zero Net Negative Ecological Impact). Sustainable development can be achieved by balancing urban air and water pollutants (carbon, nitrogen, methane, among others) with increased sequestering of these contaminants.
Clearly, the first step is to reduce overall pollution and this is already underway. The second step is to enhance the environment to capture much of the pollution and store in and below the soil. Agriculture within the Megapolis, or more precisely "Urban Agriculture" is the most well known method accomplish this sequestration. Urban Agriculture also is cost free and improves the local economies.
Urban agriculture will make roofs, fences, walls, parking lots, roadsides and vacant lots and abandoned sites productive. It will convert lawns to produce fruits and vegetables. Street trees will bear fruit. Waterways will produce fish and vegetables. Steep slopes will be terraced and produce vegetables and vineyards, as well as provide pastures for sheep and goats.
Urban Agriculture is not the total solution - but is an indispensible major element in a plan and program to achieve an urban society that is carbon neutral and does not further destroy our planet.