How the Earth Changed History Water World 神奇水世界(3-4)

Out there is a river valley that’s been carved into the rock. It’s been carved by running water, just flew down here smoothly off this rock bed and then cascaded down to the valley north there. Six thousand years ago, that was a big river.

Satellite images reveal that the river bed I’m standing in is just one of a network of past river valleys that crisscrossed the Sahara desert. Ten thousand years ago, this dry, empty place was entirely different. Little is known about the early Saharans who lived here then, but we do know that they depended entirely on water. Water formed the lakes in which they swam. Water nourished the plants which fed the animals they hunted. Water filled the clay pots from which they drank, but then the climate changed.

About five and a half thousand years ago, the Sahara began to dry. The rains failed, the river shrank and the lakes dried out. For the early Saharan people, there was only one option – to follow the rains and abandon the desert.

The fortunes of the early Saharan people revealed a universal timeless truth: our fate is inextricably linked to water. The problem is the water never stands still. It’s always on the move across the planet. We think of this as a blue planet, but while water is abundant, most of it is no use. More than ninety-seven percent of the earth’s water is salty ocean which we can’t drink or use to grow crops. Less than three percent is fresh water on which all human life hangs. What’s more, that tiny fraction is often hard to depend on, because fresh water has a life cycle all of its own.

The water seems so familiar, doesn’t it? But to see its remarkable qualities, you have to go to some extreme length.

The fresh water that we depend on begins its life in the oceans. As the sun’s rays beat down on the surface of the sea, they heat the water molecules until some evaporate. It’s the start of an extraordinary journey.

Here as water evaporates, it feels like it vanishes into thin air there. Although we barely notice it, water molecules are suspended around us all the time, just that we are only aware of it when they clump together as clouds.

a journey one time, less than a thousandth of the world’s water is up here in the atmosphere. It may not seem much, but this is what spreads water from the seas to the land.

The water molecule doesn’t hang around up here for very long. It fact, it spends less time up there in the atmosphere than the any other time of its journey, a mere nine days until a typical water molecule crashes the earth as rain.

For most of us, rain is perhaps the most familiar stage of the water cycle, but notoriously the least reliable.

As the water falls as rain, it joins a big system, cascading and carving its way across the land surface as streams and rivers. Look at that, water, absolutely everywhere.

Rivers and rain are of the parts of the water cycle that we depend on and yet they are only a tiny proportion of the world’s fresh water, amazingly two percent of all fresh water on the planet, the rest of the earth fresh water is locked away, down there on the ground.

What a landing.

The vast majority of it is stored as ice. Most of the rest seeps deep into the earth where it’s known as groundwater. Hidden away down here is the planet’s second large store of fresh water, but in the end, all water arrives back in the oceans and the cycle begins again.

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