( This is what we depend on for our daily needs. Most fresh water is locked up in the polar ice caps in the Arctic and Antarctic. If all this ice were to melt, the oceans would rise more than 40m (130 feet), as high as a 13-storey building!
We Earthlings depend on a tiny fraction of the total amount of water on Earth - less than 1%. Much of this drinkable water is hidden underground in different layers of rock. A smaller amount lies in the lakes and rivers. The rest is floating in the atmosphere as a gas. )

Winds push waves along the top surface of the ocean, but the tides move the entire ocean. At one time, people thought the daily rise and fall of the ocean meant that it was breathing. Now we know that tides are caused by the gravitational pull between Earth, the moon and the sun.

Advanced knowledge

The water cycle

The most noteworthy characteristic of any small body of fresh water-be it a pond, a stream, an icicle, or a rain cloud-is its impermanence. Ponds evaporate, streams flow to the sea, icicles melt and dribble away, rain falls; water is forever on the move, repeatedly changing its state-liquid, ice, or vapor-in the process. In the word, it is dynamic. In much the same way that every living organism has a life cycle, water has a water cycle; it circulates. Indeed, all the water on earth is constantly circulating.
In one form or another it is familiar to almost everybody. Water falls to earth as rain or snow, some on land and some at sea; much of what reaches the land drains into the sea, by surface routes below the ground. The rest evaporates back into the atmosphere, either directly or through the vegetation: all plants on the land absorb water from the ground and exhale most of it into the air, as vapor, in a process called transpiration. The vapor from the land mixes with vapor evaporating from the sea, and, together, they condense into rain-giving clouds again. The cycle keeps on indefinitely, with the world ocean as the reservoir. The bulk of all this water is therefore salty sea water. Only the water temporarily withdrawn from the ocean is fresh.
Now for some statistics. The total amount of water on earth at the present time is approximately 1.4 billion cubic kilometers, an impossibly large quantity to visualize. If it were solidified into a cube, each edge of the cube would be about 1,120 kilometers long, approximately twice the length of Lake Superior.
The amount of fresh water in the world today is approximately 36 million cubic kilometers, a mere 2.6 percent of the total; of this fresh water, only 11 million cubic kilometers (0.77 percent of the total, or 30 percent of the fresh water) counts as part of the water cycle, in the sense that it circulates comparatively fast. Of the water not in circulation, most is immobilized in long-lived polar ice sheets, and some is trapped, stagnant, under the ground.
No single answer can be given to the question, how fast does the water circulate? This is because the time taken by a given drop of water to complete the cycle, from ocean back to ocean, varies tremendously. It ranges from minutes or hours, as when a rainstorm blows inland from the sea, to thousands of years, the time during which a water drop may be frozen into a glacier. Indeed, there isn't a sharp distinction between circulating and noncirculating water: given enough time-hundreds of thousands, or millions, of years-all water circulates.
These statistics apply to what is happening at the present day. Until the 1980s, it was believed that the total amount of water on the earth had remained the same, with only negligible changes, throughout the lifetime of the planet; in other words, that the earth's water cycle was virtually closed-no new water ever entered it and no existing water ever left it. New discoveries in the late 1980s have inspired new speculation. Many scientist now believe that the earth's water supply is growing all the time, though not fast enough to solve humanity's water-shortage problems. According to the new theory, loosely packed "snowballs" of nearly pure snow, the size of small house, are entering the earth's gravity field from the outer parts of the solar system every few seconds; they have been dubbed small comets, and most of them appear to weigh between 20and40tons.They melt and vaporize when they get near the earth. If this "rain of snowballs" has been going on at the current rate for billions of years (and there is no reason to suppose that it hasn't), the amount of new water we are continually gaining is equivalent to about 6 millimeters of rainfall over the whole earth, or 3 trillion tons of water, every 10,000years.
The amount of fresh water on earth, as a fraction of the sum total of all water, fresh and salt, varies from one geological epoch to another. So does speed of the water cycle. At times when the whole world was warm-no ice sheets anywhere-and shallow, inland seas were more extensive than they are now, the amount of liquid water in circulation would have been much greater than it is at present, and the ratio of salt water to fresh would undoubtedly have been different. Conversely, at the height of an ice age, liquid fresh water was comparatively scarce, because much was immobilized in ice sheets. At the same time, the low temperatures would have reduced evaporation rates and precipitation rates, slowing the whole cycle.
To return to the present, to the fresh water the world holds now. People is general are at last becoming aware that human demands for fresh water are beginning to outstrip the supply. Quite probably, fresh water will turn out to be the factor that limits population growth, as limited it obviously must be.
Fresh water has a "natural history" of its own: the water cycle can be thought of as a "life cycle". Water comes from the sea as a vapor, travels for a time as a liquid, sometimes lingers as ice, and then returns to the sea again. Fresh water" behaves" in a number of different ways: it moves over and through the ground, sometimes fast and sometimes slowly; it pauses in lakes and ponds; it freezes; it vaporizes; it creates habitats for a wide range of ecosystems; it shapes the land. In brief, it is active and powerful, indeed more active and more powerful than the living things whose lives depend on it.

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