The oceans have a moderating effect on the Earth’s climate, helping prevent extremes of air temperature. This is because water takes much longer than air to heat up and cool down. Since about two thirds of the planet’s surface is ocean, this has a profound effect on climate. Outside the equatorial regions, this vast body of water tends to store heat during the summer months, and slowly release it during the winter. This is why coastal regions tend to have milder climates than areas that are far inland.
Specific heat is a measure of how much heat it takes to raise the temperature of a given quantity of a substance by a given amount. This value is more than four times higher for water than for dry air or land. Land heats up quickly, and this heat is easily transferred to the air.
Air over land, however, also cools down relatively quickly. In contrast, much more heat is required to bring about a similar rise in ocean temperature, and so the seas take much longer to heat up. Similarly, they take much longer to cool down. For this reason, in areas that experience seasons, the ocean tends to lag behind the land in terms of temperature.
Effects on Climate
This difference in specific heat affects temperature ranges on both seasonal and daily timescales. Days are cooler, and nights warmer, over the oceans than on land. This has an effect on coastal areas, keeping temperatures down during the day, and preventing them from dropping very low at night.
On a micro scale, the effects of ocean temperature can easily be seen at the beach. Usually, air at a beach is a few degrees cooler than the air just a few miles inland during the day. Likewise, during the night, the air at the coast may not cool down quite as much as at inland locations. This is why coastal areas like San Diego have one forecast for the beach, and another for inland areas. The effect can also be seen in wind direction: typically, during the day wind will blow from the sea toward the land, where heat causes the air to rise, and vice-versa at night.
On a longer timescale, in summer, ocean temperature does not reach its maximum until some time after the maximum day length. Similarly, the minimum ocean temperature occurs some time after the shortest day. This influences the climate over land, creating a similar time lag.
As oceans heat up, they release more water vapor into the air, increasing its humidity. This also affects climate, as humid air takes longer to heat up, and retains heat for longer than dry air. Again, this has a moderating influence. Without the oceans, temperatures would fluctuate far more dramatically, probably making conditions impossible for most life forms.
The extent to which the ocean influences climates inland depends on topography. Moist air from a warm ocean can help moderate the climate for a considerable distance, but if it is forced to rise by a mountain range, much of the moisture will condense, forming cloud and producing rain. On the other side of the range, the air will have lost most of its moisture, and the climate will tend to be more extreme.
The equatorial regions receive more heat from the Sun than the higher latitudes, and this difference in the degree of warming of the oceans leads to currents that circulate heat around the globe. These currents have a huge impact on the climates of some parts of the world. Perhaps the best-known ocean current is the Gulf Stream, which is sometimes known as the North Atlantic Drift. This brings warm water from the Gulf of Mexico northward to northwest Europe, where its effect can be seen by contrasting the winters experienced in this region with those in Eastern Europe. For example, Glasgow in Scotland typically experiences mild, wet winters, while Moscow — at roughly the same latitude — has freezing conditions.
While this may be the most commonly cited example, ocean currents exist all over the world. Some are cold water currents, carrying cooler water from the arctic areas down toward the tropics. These reduce evaporation and humidity, leading to drier conditions with greater temperature variations than is usual for coastal regions. ENSO (El Niño/Southern Oscillation) is a periodic warming of part of the Pacific Ocean off the west coast of South America that has a huge effect on climate all over the planet.