There should be no doubt about the value of producing food in the middle of the city, where most of the food has to be trucked in from very far away. Intensive urban gardening improves air quality, reduces heat island effects, and provides fresh local produce to urban dwellers. It is also easier to grow organic in a greenhouse, because the environment is more controlled and there is less need for herbicides and pesticides.
What is not quite as obvious, however, is that a green house attached to a residential space can be, if managed correctly, extremely energy efficient as well. Greenhouses naturally absorb heat all day long from sunlight, and that heat is trapped in the air inside the greenhouse. That is why a green house can grow plants through the cold seasons in most climates. That heat can also be used, however, to augment the heating system in an attached residential space during the cold months (a process called isolated gain), and can be an important part of a general passive heating system. In the summer, when there is waste heat in the residential spaces, they can be vented to the rooftop greenhouse for some passive cooling as a result of the passive-stack effect. In short, rooftop greenhouses can have a huge positive impact on building efficiency.
While the gains in energy efficiency can be significant, the issue of food production in urban centers is just as important. For as long as cities have existed we have had problems with getting enough food to city dwellers (think of the Roman Empire shipping in tons of grain from Egypt). In the latter half of this century we thought we had solved those problems for good by cultivating produce that transports well and shipping food all over the world with cheap fuel. Now that we know about global warming and pollution, we have realized that our solution to the problem of 'enough' has generated a number of new problems related to 'how.' Urban homesteads help mitigate this issue, whether in a greenhouse, a backyard, or in planter boxes on a 20th story balcony. If even a percentage of produce can be home grown instead of trucked in, the reduction in fuel consumption is directly related to the reduction in demand for transported goods.
Some would argue that home grown vegetables, especially heirloom varieties preserved by seed bankers (many seed bankers will give you a few seeds for the cost of postage, just to keep their favorite varieties in wide circulation), can greatly improve your quality of life as well. The taste of freshly picked vegetables is distinctly different, and many people will tell you (though I don't know enough about it to unequivocally agree) that they are much more nutritious. Whether or not the health benefits are true, keeping a vegetable patch is worth it just for the flavor. And while maintaining a quarter acre lot requires a large amount of work, keeping a little patch inside or on your porch requires very little maintenance.
A while back I made a sauce from different heirloom tomatoes for a braised oxtail, and reserved two slices from each one for a caprese appetizer. Both the sauce and the caprese were delicious. The sauce was much more complex for the variety of tomatoes, and the Caprese really highlighted the unique and different flavor of each variety. It was one of the more amazing culinary evenings at Chez Cross.
Oxtail braising in Heirloom Tomato Sauce
Heirloom Tomato Caprese