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How Much Water Is There On Earth?

Magellan Would Be Shocked

Water feels good when we're thirsty, cools us when we're hot, looks great in a pond, a lake, a cloud. Because we ourselves are two-thirds water, we like the stuff, we're drawn to it. We search the universe looking for signs of water and when we look back at our planet, we are small and pale, yes, but we're dazzlingly blue.

It's good to be wet. Compared to our neighbors — Mars, Venus, the dust-dry Moon — we look the wettest. More than two thirds of our surface is ocean. Another twentieth is ice. Water, water, everywhere. We've got so much of it.

Or do we?

There's an oceanographer in Hawaii, Mike Mottl by name, who's been measuring — or trying to measure — how much water we have on this planet, and his calculations are a little startling. When you stand on the shore of Lake Michigan, or the Pacific, the water supply looks impressive. But Mike changes perspective; he steps off the planet and considers the whole sphere, the surface, the crust, the mantle, the core and he asks, how much total water is there? Is Earth carrying a lot? Or a little?

So he starts counting. First, of course, are our oceans. On a map there are five big ones, but combines them into one "global ocean." Then come all the rivers, all the lakes, big and small. Then the water in our atmosphere, the clouds, the fog, the moisture that hangs invisibly in the air. Then, hidden below us, are the underground pools of water, aquifers, that show themselves in wadis or oases in the desert, that you touch when you dig a well deep into the ground.

When Mike Mottl counts, he counts in Global Ocean units, all the oceans together. So when he adds up all the air, oceanic/lake/river and underground water he gets, roughly 1.2 "Global Oceans;" basically surface Earth is mostly ocean with a scoop of fresh water on top.

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