The US Geological Survey (USGS) recently published a remarkable image helping us appreciate the preciousness of water. We have all probably heard the oft-quoted statistic that about how 70 percent of the Earth’s surface is water-covered – most of it in the form of oceans. Though it sounds like a lot, if we gathered all that water up, how much space would it take up? The USGS, in the image below, shows us that the answer is… not much.
click for larger image (Illustration by Jack Cook, Woods Hole Oceanographic Institution (©) Howard Perlman, USGS)
Though about 97% of Earth’s water is in the form of oceans, these oceans are shallow compared to the Earth’s radius – just a couple miles deep. The image above shows us just how shallow the oceans are, using radar image data, mapping the bottom of the ocean. The blue sphere shows what would be required to contain all Earth’s water – from oceans, lakes, rivers, glaciers, snowpack, etc. The sphere has a radius of just 700 kilometers.
When this picture appeared in the news, I found myself wondering what portion of that sphere was fresh water. I noted a number of reader comments asking the same thing. It’s an important question. The Food and Agriculture Organization of the United Nations (FAO) projects that two-thirds of the planet will be water stressed by 2025. Safe drinking water and abundant water for irrigation will become increasingly scarce.
So what size sphere would be required to hold all the accesible fresh water in the world? The picture below shows the answer. But first, using USGS water data, let’s look at how the size of that fresh water sphere was calculated:
The Math:
- As the USGS demonstrated, all Earth’s water (1,386,000,000 cubic kilometers) can be contained in a sphere with radius of about 693 kilometers.
- Of that water, only about .77% is fresh water, usable by humans (ground water, lakes, rivers, etc.) N.B.: An additional 1.74% of global water is stored as glaciers, ice caps and permanent snow, but is not accessible and usable.
- .77% x 1,386,000,000 cubic kilometers = 10.7 million cubic kilometers of fresh water
- 10.7 million cubic kilometers of fresh water = a sphere with radius of 137 kilometers
Here’s the updated image, with spheres representing “All Water” and “Fresh Water” – side by side.
(source: USGS data, 8020 Vision calculations)
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