Many years ago in Slate, William Saletan wrote in the Deluded world of air conditioning:
Air conditioning takes indoor heat and pushes it outdoors. To do this, it uses energy, which increases production of greenhouse gases, which warm the atmosphere. From a cooling standpoint, the first transaction is a wash, and the second is a loss. We're cooking our planet to refrigerate the diminishing part that's still habitable.
In fact, a new study shows that the first transaction is not a wash at all. The air conditioning actually is heating up the outdoors. In Phoenix, a team from Arizona State University found that air conditioning didn't make much of a difference in outdoor daytime temperatures, but at night, when temperatures drop to about 80 degrees, heat pushed outside by air conditioning heated up the ambient air by as much as two degrees. Candace Pearson at BuildingGreen notes:
With extreme heat projected to increase this century, this positive feedback loop could prove a public-health concern or put further strain on electrical grids; in Phoenix, energy used for cooling already sometimes rises to half of the region’s total electrical consumption.
And over half of that electrical consumption comes from burning fossil fuels. Especially in Phoenix, they are indeed cooking the planet to refrigerate their buildings. But hey, it's Arizona, where nobody cares about water or climate change.
The study, Anthropogenic Heating of the Urban Environment due to Air Conditioning, is behind a paywall. More in BuildingGreen