China is considering using deep ocean water to air-condition Hong Kong high-rise office buildings -- which are virtually man-made "cliffs" right by the ocean -- by utilizing the differential between cold ocean depths and warm surface temperatures circulating deep ocean water through a district wide system of pipes.

The technique can be applied using lakes and rivers, instead of ocean water, as long as the water source is cold enough, and even may be suitable for communities, universities, hospitals or hotel resorts.

Projects underway in Singapore, Gibraltar, Stockholm and Honolulu are using deep, cold ocean water to cool large building complexes and downtown areas.

Cornell University has a pilot project underway utilizing lake water for its cold-water source. Pumps draw cold water from a nearby Lake Cayuga. The water is pumped to a heat exchanger at the shore where the campus and a school share a cooling loop, and the warm water from the buildings flows down to push cool water up to the campus. The system is both elegant and cost effective. Enwave also uses lake water, rather than sea water to cool downtown Toronto. Its interactive diagram shows you very clearly how the deep-water cooling works using the icy-cold water of Lake Ontario as its source of naturally chilled deep lake water.

As it gets colder, the density of water increases, causing it to sink to the bottom. Even in warmer weather, while the surface water warms up, but it remains at the surface because it's not dense enough to sink. No matter how hot the summer, the water at the bottom remains very cold.

We could meet our need for a livable temperature range renewably, because historically we built so many cities next to water, for the convenience of trade and transportation.