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Earth Has Less Water Than You Think

All the water on Earth would fit into a sphere 860 miles (1,385 km) wide. (Jack Cook/WHOI/USGS)

If you were to take all of the water on Earth — all of the fresh water, sea water, ground water, water vapor and water inside our bodies — take all of it and somehow collect it into a single, giant sphere of liquid, how big do you think it would be?

According to the U. S. Geological Survey, it would make a ball 860 miles (1,385 km) in diameter, about as wide edge-to-edge as the distance between Salt Lake City to Topeka, Kansas. That’s it. Take all the water on Earth and you’d have a blue sphere less than a third the size of the Moon.

Feeling a little thirsty?

And this takes into consideration all the Earth’s water… even the stuff humans can’t drink or directly access, like salt water, water vapor in the atmosphere and the water locked up in the ice caps. In fact, if you were to take into consideration only the fresh water on Earth (which is 2.5% of the total) you’d get a much smaller sphere… less than 100 miles (160 km) across.

Even though we think of reservoirs, lakes and rivers when we picture Earth’s fresh water supply, in reality most of it is beneath the surface — up to 2 million cubic miles (8.4 million cubic km) of Earth’s available fresh water is underground. But the vast majority of it — over 7 million cubic miles (29.2 cubic km) is in the ice sheets that cover Antarctica and Greenland.

Of course, the illustration above (made by Jack Cook at the Woods Hole Oceanographic Institution) belies the real size and mass of such a sphere of pure liquid water. The total amount of water contained within would still be quite impressive — over 332.5 million cubic miles (1,386 cubic km)! (A single cubic mile of water equals 1.1 trillion gallons.) Still, people tend to be surprised at the size of such a hypothetical sphere compared with our planet as a whole, especially when they’ve become used to the description of Earth as a “watery world”.

Makes one a little less apt to take it for granted.

Read more on the USGS site here, and check out some facts on reducing your water usage here.

Water, water, every where,
And all the boards did shrink;
Water, water, every where,
Nor any drop to drink.
– from The Rime of the Ancient Mariner, Samuel Taylor Coleridge

Comments on this entry are closed.

  • InterPur May 8, 2012, 8:33 PM

    That’s why this planet is called Earth and not Water.

    • Torbjörn Larsson May 8, 2012, 9:56 PM

      Actually it started out as “Dirt”, but a while after it got populated the popular vote went to “Earth”. (Well, it sounds better than “Soil”.)

      • Eppur_si_muove May 9, 2012, 8:38 PM

        Are you saying that before we called our planet Earth, or Terra, we started out calling it “Dirt”? …Could you elaborate, how do you know this.

        • Torbjörn Larsson May 9, 2012, 9:22 PM

          Just joking.

          Eppur scherza.

  • Torbjörn Larsson May 8, 2012, 9:52 PM

    If we had as much water as the asteroids have by content, we would drown in it.

    This is the only real finetuned constraint for technological life here.

    For the lower limit, it just so happens that Earth seems to be marginally large for having plate tectonics. (Compare with Venus and Mars.) It is water that is needed to make the subducting plates pliable enough.

    Water is also needed for massive granite formation, which is what makes up our less dense continental plate floats. It is believed that super-Earths can manage plate tectonics without the constraint of water.

    Luckily we had a large last impactor that blew off the excess volatiles into space. The Late Heavy Bombardment gave us both our ore metals (that had gone to the core during differentiation) and our volatiles back in sufficient quantity.

    That process is probably not to uncommon in planetary system formation. But it makes you wonder if the super-Earths are mostly water worlds.

    The amount of water on Earth is estimated as “one ocean below for one ocean above”. I.e. the crustal water is ~ as much as what we see above it. The mantle is too hot for crystal water I believe, so you get oxides and hydrides at best.

    This gives us that free water is ~ 5/1000 of Earth’s mass, IIRC.

  • kkt May 8, 2012, 11:25 PM

    How much water is created or destroyed each year?

    • Daddy Bud May 9, 2012, 1:11 AM

      My guess is zero…if you mean contaminated by foreign substances, melted from ice caps or distilled from salt water then you should restate your question.

      • Kawarthajon May 9, 2012, 2:18 AM

        Actually, water is created when combusting fossil fuels and, technically, you can “destroy” water by separating the hydrogen from the oxygen.

        • zkank May 9, 2012, 4:00 AM

          Water molecules are also split during photosynthesis.

          I remember hearing as a kid a common misconception years ago, that we are drinking the same water that had once been urine in dinosaurs (Wowwww…! Ewwwww!)
          Soon after in high school biology I remember reading that all the water on the planet splits and bonds several times during a period of ten Ma., so our water is “new”.

    • Torbjörn Larsson May 9, 2012, 9:41 PM

      It is a water cycle, so what organisms or photolysis breaks down is reconstituted.

      For an estimate, let us forget photolysis for laziness. Most of the water we produce is from metabolism, the oxygen from breathing and the hydrogen from biomolecules. (There are desert animals that can live on this water only.) That water is transported out several ways (breathing et cetera).

      Using humans as model organism, reasonably roughly 1 dm^3/(100 kg cells*day) or ~ 3*10^-3 m^3/(kg biomass*year).

      Earth biomass is ~ 1000 billion tonnes C including bacteria or ~ 10^12 kg, so the biological water cycling is ~ 3*10^9 m^3 or ~ 3 km^3 each year.

      So at least ~ 10^-7 of the total water mass is recycled every year. A sanity check is the biomass link that says ~ 20 % biomass is recycled every year, which roughly accords with the water production value.

      The atmospheric loss to space, mainly photolyzed hydrogen atoms, is obviously much less since Earth is ~ 5*10^9 years old and we have had similar volumed oceans the whole time.

  • sergio rigolino May 9, 2012, 2:45 AM

    the surface still looks kinda green, is the water of living beings inside this ball?

  • Beun May 9, 2012, 9:05 AM

    Reminds me of a post from 2010, which has an image with two water spheres instead of one.
    One is salt water, one is fresh water. The fresh water sphere is just a dot. Shows even better how little fresh water we have: http://beun.me/cMx

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