Large bodies of water affect the climate by serving as heat sinks. Oceans and other large bodies of water absorb heat during summer months and radiate it back out during the winter, moderating temperatures for nearby land masses. They also carry heat from tropical regions to other areas via currents.
When the sun's energy strikes land, a small amount of heat is absorbed by the uppermost layer of the crust and the rest is reflected back into the atmosphere. A large body of water, on the other hand, has a surface made of molecules that are constantly in motion, allowing warmer water to circulate away from the surface and bringing cooler water up to absorb more heat. This allows oceans and other bodies of water to absorb much more heat than land masses do. During the summer, the ocean absorbs heat from the atmosphere, cooling the air that flows from the sea onto the land. In the winter, stored energy from warmer months works its way to the surface, warming the breezes that pass onto the shore.
Currents in the ocean are also very important to world climate. Each ocean has its own set of circular currents that flow throughout the body of water, many extending from the tropics well into the colder latitudes. This system transports warm water into oceans that would otherwise remain frigid, allowing them to moderate climate even in countries near the poles.