Here is a very interesting article on Low Tech Magazine by Kris De Decker on how we can actually heat and cool cities using the solar energy. The technology to do so has always been available all these while.
The reason why it has not been put into has to do with the lifestyle that we choose. For example, such a city would need to radical rethink in the zoning laws to be adopted. Check out the possible architectural shapes that potentially arises from such a situation.
Compared to conventional zoning practices, the solar envelope produces a different geometry — the limits of the envelope derive their vertical dimensions from the sun’s daily and seasonal movements. Thus, while conventional zoning envelopes are shaped like a box, the solar envelope has both vertical and sloping spaces.
As a result, the buildings and city blocks that fill these imaginary solar envelopes are more likely to have unique shapes. One side of a building would not look like the other, nor would each side of the street. In the northern hemisphere, development would tend to be lower on the south side of a street than on the north where a major southern exposure would be preserved. Streets take on a directional character where solar orientation is clearly recognised.
Buildings within the solar envelopes shown above. Ralph Knowles.
The way things are going right now with regard to fossil fuels, I expect it would only be a matter of time before such a city will be built. Architecturally, it will be an awesome place.