Climate Solutions: Chapter 4
As seawater warms up, it expands, increasing the volume of the global ocean. 
—Gerald Meehl and his IPCC colleagues, 2007
Global warming raises the potential of unlocking large amounts of fresh water now frozen in the vast Greenland ice sheet and in Arctic Ocean sea ice. Warming air temperatures could also increase evaporation in low latitudes and transport freshwater vapor toward high latitudes, where it falls as rain or snow into the oceans. Could these factors tip the freshwater balance in the North Atlantic in the future? 
—Jerry McManus and Delia Oppo, Woods Hole Oceanographic Institution, 2006
As we have learned, the ocean is a vast reservoir, not only of water, but also of heat. The thermal layers are not uniform. Surface water warmed by the sun tends to contain more heat than layers 100 meters below the surface or deeper. The bigger the temperature difference between the warmer top water and colder bottom water, the more potential exists to convert that difference into other kinds of energy, such as electricity. Ocean Thermal Energy Conversion (OTEC) is an energy technology that converts solar radiation to electric power. OTEC systems use the ocean's natural thermal gradient—the difference in temperatures of the ocean's layers of water—to drive a power-producing cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 20°C (36°F), an OTEC system can produce a significant amount of power, with little impact on the surrounding environment. As the OTEC Web site notes, “The oceans are thus a vast renewable resource, with the potential to help us produce billions of watts of electric power.”  According to some experts, this potential may be as large as 10,000 billion watts of continuous baseload power generation.
Essentially, the technology involves pumping cold deep ocean water to the surface, exchanging the thermal energy between the two reservoirs in a heat engine, and returning the water to the mixed layer between the warm top and cold deep layers. Experimental OTEC stations have been in operation since the late 1990s. The by-products of the heat exchange include clean freshwater (which rivals in quality that of modern desalination plants) and cold "waste" water, which could be used for marine aquaculture or even for growing plants on land, as the Seawater Greenhouse project shows.*