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How much water does the Earth have?

lpetrich

lpetrich

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I'd previously posted on exoplanets with deep oceans in Can oceans be too deep for life? and Why aren't the Earth's oceans deeper?

I will now take a closer look at the Earth's water itself. I start with our planet's surface and nearby.  Water distribution on Earth

Total volume: 1.386*10^18 cubic meters
  • Oceans: 1.338*10^18 m^3
  • Ice and snow: 2.44*10^16 m^3
  • Groundwater: 2.34*10^16 m^3
    • Saline: 1.29*10^16 m^3
    • Fresh: 1.05*10^16 m^3
  • Lakes: 1.76*10^14 m^3
    • Saline: 8.5*10^13 m^3
    • Fresh: 9.1*10^13 m^3
  • Atmosphere: 1.29*10^13 m^3
  • Swamps: 1.15*10^13 m^3
  • Rivers: 2.12*10^12 m^3
  • Biological Water: 1.12*10^12 m^3
The oceans contain about 96.5% of the Earth's surface and near-surface water, with ice and snow being 1.76% and groundwater being 1.69%. Lakes are 0.013%, and the others even less.

The average depth of the Earth's oceans is 3.7 kilometers. They cover 71% of our planet's surface area, and relative to that area, their depth is 2.6 km. That thickness is 47 meters for the Earth's snow and ice, 45 m for groundwater, 34 centimeters for lakes, 2.5 cm for the atmosphere, and 2.2 millimeters for biological water.

Human body water is currently about 3*10^8 m^3, about 0.6 microns when spread over the Earth's surface.
 
I now turn to the Earth's crust.  Abundances of the elements (data page) has estimates for the Earth's continental crust, and Abundance in Earth's Crust for all the elements in the Periodic Table is what it says, from Wolfram Research's curated data. Water is H2O, and of its two constituent elements, there is much more oxygen than hydrogen in the Earth's crust. So its chemically-bound water will be bounded from above by its hydrogen.

I find an estimate of 0.15% kg per kg of hydrogen. The total mass of crust is 2.77*10^22 kg, from Mass and Composition of the Continental Crust Estimated Using the CRUST2.0 Model, also Crust, and thus the total mass of hydrogen is 4.2*10^19 kg. Combined with oxygen, that gives 3.7*10^20 kg or 3.7*10^17 m^3. Spread over the Earth, that is 720 meters.

Some of this hydrogen is in hydrocarbons, and to estimate that, I turned to Petroleum - Status of the world oil supply | Britannica.com. It quotes a estimate of 3 trillion barrels. At 140 kg/barrel, that is 4*10^14 kg. Saturated hydrocarbons are roughly (CH2), and unsaturated ones have less hydrogen, so I will use saturated ones as an upper limit. That's 6*10^13 kg of hydrogen, giving 5*10^14 kg of water or 5*10^11 m^3 -- about 1 millimeter depth.

Turning to tar sands or oil sands, I find Oil Sands, and Canada has about 2 trillion barrels of oil in those sands. Going further, to black shales, I find Organic-Rich Shale of the United States and World Land Areas has an estimate of 900 trillion short tons or 8*10^17 kg. Assuming saturation, that gives 10^17 kg of hydrogen, 10^18 kg of water or 10^15 m^3 - about 2 meters depth.

So crustal hydrocarbons are a rather insignificant reservoir of crustal hydrogen.
 
How much water is in the Earth's interior, below its crust, is not very well-known. Estimates range from 1.5 to 11 oceans. The Earth's crust has about 0.28 oceans chemically bound in it, more than groundwater, at 0.017 oceans. The Earth's oceans have about 0.023% of the Earth's total mass.

I decided to estimate an upper limit for chemical binding, and I considered a clay mineral,  Kaolinite. It is Al2Si2O5(OH)4, and it is 1.6% hydrogen by mass or 16% water by mass. That means that the Earth's crust could hold something like 3 oceans of water if its minerals were completely hydrated.


I went through all this trouble because I wanted to consider what a dry planet might be like.

If an otherwise-Earthlike planet has less than 1% of the Earth's surface water, then from the Earth's amount of groundwater, nearly all of that water will be groundwater. So a planet like that will have no oceans or lakes or other surface water.

What made the planet dry will likely have given it very little of other volatiles, so the planet's atmosphere will be very thin. The planet will thus be a warmer and larger version of Mars.

In the Solar System, we have some extremely dry large celestial bodies: the Moon and Mercury. Neither of them has much water or a very noticeable atmosphere, and the Moon's rocks contain very little of volatiles. The Moon got dried out because of how it formed, from material that was baked by the heat of a giant impact, and splattered off and into orbit. This baking forced volatiles to evaporate from this material.
 
Thanks for this. It certainly puts any thoughts of 'water sequestration' to bed as utterly impractical.
 
Well, since it looks like we're going to fail to save the icecaps, I idly wondered whether there was something else to do with all the water, instead of letting the sea levels rise.

For example, there are signs that climate change is actually making the Sahara more hospitable to life, and efforts are underway to reclaim land from it. I wondered if a vegetated Sahara (with presumably, lakes and more groundwater) would use up enough water to reduce sea level rise. According to your numbers, it likely won't put a big dent in it.
 
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