Oceans and Seas and the Water Cycle

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The oceans are, by far, the largest reservoir of water on earth — over 96% of all of Earth's water exists in the oceans. Not only do the oceans provide evaporated water to the water cycle, they also allow water to move all around the globe as ocean currents. Oceans are the storehouses of water nature uses to run the water cycle.

Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.

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Water cycle components  »  Atmosphere  ·  Condensation  ·  Evaporation  ·  Evapotranspiration  ·  Freshwater lakes and rivers  ·  Groundwater flow  ·  Groundwater storage  ·  Ice and snow  ·  Infiltration  ·  Oceans  ·  Precipitation  ·  Snowmelt  ·  Springs  ·  Streamflow  ·  Sublimation  ·  Surface runoff

The ocean as a (huge) storehouse of water

The ocean and shore, showing past shorelines when ocean level was higher.

The oceans contain the vast majority of all water on Earth.

The water cycle sounds like it is describing how water moves above, on, and through the Earth... and it does. But, in fact, much more water is "in storage" for long periods of time than is actually moving through the cycle. The storehouses for the vast majority of all water on Earth are the oceans. It is estimated that of the 332,500,000 cubic miles (mi3) (1,386,000,000 cubic kilometers (km3)) of the world's water supply, about 321,000,000 mi3 (1,338,000,000 km3) is stored in oceans. That is about 96.5 percent of all Earth's water. It is also estimated that the oceans supply about 90 percent of the evaporated water that goes into the water cycle.

The water in the oceans is saline (saltwater), but, what do we mean by "saline water?" Saline water contains significant amounts (referred to as "concentrations") of dissolved salts. In this case, the concentration is the amount (by weight) of salt in water, as expressed in "parts per million" (ppm). Water is saline if it has a concentration of more than 1,000 ppm of dissolved salts; ocean water contains about 35,000 ppm of salt.


The volume of the oceans does change ... slowly

Map of eastern U.S. showing coastlines in the past.

Of course, nothing involving the water cycle is really permanent, even the amount of water in the oceans. Over the "short term" of hundreds of years the oceans' volumes don't change much. But the amount of water in the oceans does change over the long term. During the last Ice Age, sea levels were lower, which allowed humans to cross over to North America from Asia at the (now underwater) Bering Strait.

During colder climatic periods more ice caps and glaciers form, and enough of the global water supply accumulates as ice to lessen the amounts in other parts of the water cycle. The reverse is true during warm periods. During the last ice age glaciers covered almost one-third of Earth's land mass, with the result being that the oceans were about 400 feet (122 meters) lower than today. During the last global "warm spell," about 125,000 years ago, the seas were about 18 feet (5.5. meters) higher than they are now. About three million years ago the oceans could have been up to 165 feet (50 meters) higher.


Oceans in movement: Tides

Of course the oceans are always in movement. The moon influences daily tides, which make the beach a more interesting place to go. Tides vary greatly around the world, and in some places can be quite dramatic. The highest tides occur in confined estuaries, such as the Bay of Fundy, Nova Scotia, Canada, Ungava Bay, Quebec, and Bristol Channel in Britain. The Bay of Fundy has maximum tides of up to 53 feet (16 meters) during certain times of the year (Bay of Fundy Com).


Oceans in movement: "Rivers" in the oceans

If you have ever been seasick (we hope not), then you know how the ocean is never still. You might think that the water in the oceans moves around because of waves, which are driven by winds. But, actually, there are currents and "rivers" in the oceans that move massive amounts of water around the world. These movements have a great deal of influence on the water cycle. The Kuroshio Current, off the shores of Japan, is the largest current. It can travel between 25 and 75 miles (40 and 121 kilometers) a day, 1-3 miles (1.4-4.8 kilometers) per hour, and extends some 3,300 feet (1,000 meters) deep.

Map of Atlantic Ocean showing Gulf Stream current moving warmer tropical water towards northern Europe.

The Gulf Stream carries warm, salty water northward along the Northeast Shelf (Gulf of Maine to Cape Hatteras, NC), bringing heat from the tropics to higher latitudes. The northward and landward extent of warm Gulf Stream waters affects ecological processes  in the ocean, including the distribution of commercially important fish species. It also affects weather in the region. The Gulf Stream returns a considerable amount of heat to the atmosphere. 

Credit: NASA. Map by Robert Simmon

The Gulf Stream is a well known stream of warm water in the Atlantic Ocean, moving water from the Gulf of Mexico across the Atlantic Ocean towards Great Britain. At a speed of 60 miles (97 kilometers) per day, the Gulf stream moves 100 times as much water as all the rivers on Earth. Coming from warm climates, the Gulf Stream moves warmer water to the North Atlantic. Cornwall, at the southwest corner of Great Britain, is sometimes referred to as the "Cornish Riviera" because of the milder climate attributable to the Gulf Stream—palm trees (true, a hardy variety) can even grow there....all because of the Gulf Stream.

This map shows sea-surface temperatures of the North Atlantic Ocean. Data are from NASA satellite observations. Cold waters are shown in darker colors, whereas orange and yellow indicate the warmest temperatures. The Gulf Stream is visible as a warm water current traveling northward along the coast of North America and eastward into the central Atlantic Ocean. As it continues its journey heat from the ocean is lost to the atmosphere, warming the air above it. Cornwall and its palm trees are located southwest of London, and if you draw a line westward, you'll end up near Newfoundland, Canada. Cornwall and Newfoundland might be at similar latitudes, but you would be hard-pressed to find any palm trees growing in eastern Canada!


How much water exists in the oceans?

Bar chart showing the distribution of water on, in, and above the Earth.

Where is Earth's water?

For an estimated explanation of where Earth's water exists, look at this bar chart. You may know that the water cycle describes the movement of Earth's water, so realize that the chart and table below represent the presence of Earth's water at a single point in time. If you check back in a thousand or million years, no doubt these numbers will be different!

Notice how of the world's total water supply of about 332.5 million cubic miles of water, over 96 percent is saline. And, of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground. Fresh surface-water sources, such as rivers and lakes, only constitute about 22,300 cubic miles (93,100 cubic kilometers), which is about 1/150th of one percent of total water. Yet, rivers and lakes are the sources of most of the water people use everyday.

One estimate of global water distribution 

Water source Water volume, in cubic miles Water volume, in cubic kilometers Percent of total water
Oceans, seas and bays 321,000,000 1,338,000,000 96.5%
Total global water 332,500,000 1,386,000,000 --

Source: Gleick, P. H., 1996: Water resources. In Encyclopedia of Climate and Weather, ed. by S. H. Schneider, Oxford University Press, New York, vol. 2, pp. 817-823.