Whether the forecast calls for a dry spell, a balmy day, heavy snow, or a hurricane, the ocean has played a major role in it. Although the sun is the engine that drives all weather on Earth, the ocean and atmosphere steer the sun's energy along certain paths to produce both regional climates and individual weather phenomena. For example, the climate of the West Coast of the United States is kept moderate by winds warmed by the Pacific Ocean. And Hurricane Mitch, which resulted in the deaths of more than 10,000 people in Central America in 1998, was spawned—like all hurricanes—by the ocean.

The ocean plays a crucial role in determining climate because of its ability to absorb, store, and transport heat from the sun. Ocean water also affects atmospheric temperature and circulation around the world. Furthermore, seawater is the source of most precipitation.

The top 3 meters (10 feet) of water in the ocean—most of this in the tropics and subtropics—hold as much heat as the entire atmosphere. The ocean's immense heat capacity means that it takes a huge amount of heat energy to change the temperature of a region of the sea. For instance, there must be many cold days to make an area of the ocean a little cooler and many hot days to make it warmer.

The upshot of this is that the ocean responds very slowly to changes in the seasons, causing it to have a moderating effect on climate. The sea makes winters in coastal regions a bit warmer and summers near the coast a bit cooler than they are farther inland.