Up until recently, I was under a (wrong) impression that the amount of planetary cumulative water resources was finite as I believed its escape from the atmosphere was impossible. I believed that, unlike other planetary resources, it was impossible to "waste" water as any waste would simply recycle itself in nature, i.e. municipal waste would be treated and get into the oceans from where it would eventually evaporate and precipitate back into continental waters renewing our water supply.

Recently, I learned I was wrong and that there is such a phenomenon called sequestration and that it is indeed possible for the total water resources on the planet to vary.

I am curious if there are metrics on how much water the planet has been losing (or gaining in case of a reverse phenomenon). Additionally, is there anything we can do to reduce this effect?

This article says that we've lost a quarter of our water total but I was looking for more granular statistics.

  • $\begingroup$ And what about the cosmic dust that the Earth receives from space (4 x 10^7 kg/yr). These dust grains carry water from interactions between the solar wind and silicates. See Luke Daly's research on Itokawa asteroid grains. The Earth is not a closed system. $\endgroup$
    – Naz
    Commented Oct 31, 2023 at 16:58

3 Answers 3


It is not actual water what is lost to space, because in the high atmosphere water usually dissociate into other molecules or ions. The oxygen ion outflow is frequently assumed to be a proxy for the loss of water from the planetary atmosphere. In terms of global outflow rates for the Earth the rate varies from $10^{25}$ to $10^{26} s^{-1}$, depending on geomagnetic activity (reference).

On the poster of the reference (sent to me by the author) we can read:

If we assume oxygen corresponds to water loss (self-regulation, Hunten and McElroy [1970]), then an oxygen loss rate of ~$10^{25} s^{-1}$ corresponds to ~300 $\text{g s}^{-1}$ of water loss. Over the age of the solar system (4.5 billion years ~ $1.4 \times 10^{17}$ s) this loss rate gives $4.2 \times 10^{19}$ g of water.

The current loss figure is equivalent ~25,920 liters per day, or 9,467 $\text{m}^3$ per year. And the reference of that figure seem to be the paper Escape of O+ through the distant tail plasma sheet, that used measurements from the STEREO‐B (Solar Terrestrial Relations Observatory) spacecraft.

That would correspond to a total loss over Earth's history of 42,000 $\text{km}^3$ of water, equivalent to about 12 cm of sea level change. However, that's a straightforward extrapolation of the current rate, because they acknowledge that they don't know how to model how Earth's magnetosphere would have behaved in the past when the Sun was weaker. Although, for a weaker Sun it would be reasonable expect smaller losses.

This value is radically different to that of the article you pointed to. In that case they use the loss of hydrogen instead of oxygen as proxy for water loss:

Knowing how much hydrogen had disappeared from the oceans over the last four billion years enabled the researchers to calculate that the oceans have lost about a quarter of their water since the Earth’s early days.

They the say:

Today the atmosphere is rich in oxygen, which reacts with both hydrogen and deuterium to recreate water, which falls back to the Earth's surface. So the vast bulk of the water on Earth is held in a closed system that prevents the planet from gradually drying out.

That suggests that the limiting factor for the existence of water is now the abundance atmospheric oxygen (although oxygen in rocks if very abundant). So it would make sense to use now Oxygen loss as proxy for water loss, but in the past, before the atmosphere was flooded with oxygen, maybe hydrogen loss was a better proxy.

They suggests that methanogenesis can free hydrogen atoms and make them liable to be loss to space, but when it is part of a water molecule it is safe. Methanogenesis would have been much more common in the past when the atmosphere was rich in methane (50 to 500 richer than today).

To wrap up, it seem that there is no consensus about the actual amount of water loss trough Earth's history. Different proxies give different values. Some proxies might be representative to some periods in Earth's past, but other proxies are more representative of other periods. And unfortunately we don't have yet a good reconstruction of the composition and density of the atmosphere throughout Earth's history.

But we do know with some confidence, that while you where reading this answer, one or two liters of water were loss to space.

  • $\begingroup$ Do you possible have a typo there in the first sentence? By how much does the global outflow rate vary? $\endgroup$ Commented Feb 1, 2019 at 21:07
  • $\begingroup$ @AtmosphericPrisonEscape Yes, thanks for spotting that!!! I just fixed it. Cheers $\endgroup$ Commented Feb 1, 2019 at 21:08
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    $\begingroup$ March 21, 2021 - Was Earth once a water world? By Phil Plait suggests that as the Earth cools, the mantle absorbs water at 100,000 times the rate of loss to space! $\endgroup$ Commented Jan 4, 2022 at 21:33

Hate to break bad news, but we don't know. As oxygen is one of most abundant elements on earth by mass (not merely crustal), free protons or hydrogen atoms (as electrons are abundant) are more likely to be a limiting factor for water than oxygen by factor of 100:1. Of course, tracking hydrogen is downright difficult, as it just turns back into water under normal conditions...therefore its a proxy that resists observation. But one way we gain net hydrogen -- aside from measureable debris infall -- is not from any chemical means, is from solar and cosmic showers -- neutrons, gamma & muons, protons. Of those, thermal neutrons are somewhat well absorbed by nitrogen-14, which becomes carbon-14 and emits a proton. This proton is gonna meet an electron guaranteed, and likely to become water, as vapor or precipitate out. Of course, jeans escape on the other hand, does the opposite, allows for natural hydrogen escape. Since we do not have any way of tagging this activity, tracking such protons & hydrogen...we can't really say if we are even losing or gaining net water, or just essentially re-venting incoming protons. All we know, is both opposing processes occur, but we can't say for certain if there is overlap or not. 2021 published studies cite solar wind as a significant chunk earth's watery origin.

  • $\begingroup$ No explanation as to the downvotes?? Sounds reasonably well reasoned to me. You could certainly expand it a bit into something more sectioned out, and with the link to the study, but sounds reasonable to me. $\endgroup$ Commented Dec 17, 2022 at 8:58

Water may not be lost from the earth’s atmosphere; however, an apparent loss does exist from the “free” water in the atmosphere. It has been absorbed by the increase in living beings (people an animals) and therefore not available in abundant quantities as in previous generations. This is a simple, yet profound, explanation as to why there are more areas with droughts and water shortages than in any known time in history.

  • $\begingroup$ How do living things absorb water from the atmosphere? We emit water to the atmosphere via perspiration & exhaled breath. Even plants exhale water vapor. How much do we absorb atmospheric water? Could you please provide references verifying this. $\endgroup$
    – Fred
    Commented Aug 28, 2022 at 23:35
  • $\begingroup$ It's not like there's been vast increase in creatures in recent history, so no reason why there should be suddenly be "more areas with droughts". Lets say humans are 75% water by weight. And the average mass of a person is 100 kg (both are very generous). That's 150 billion gallons (600 million m^3) for all of humanity... or about 2 weeks worth of the discharge of the paltry Colorado River. Once. One small river. Or alternatively, 1/9th of Lake Okeechobee in Florida. Out of all the water in the world? $\endgroup$ Commented Aug 29, 2022 at 2:07
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    $\begingroup$ @JeopardyTempest He said people and animals, so you'd have to add a few billion cow, sheep, pigs, etc. I'm sure you'd get to at least 3 weeks worth of discharge, or 1/7 of lake. ;) $\endgroup$ Commented Aug 29, 2022 at 7:36
  • $\begingroup$ Drat, I had included a comment that discussed that, but apparently repasted the wrong one when I edited. Totally agree. Even said if you throw in agriculture (disregarding that it replaces existing vegetation), it still wouldn't likely equate to just the state of Florida's water... out of the whole world's fresh water... plus the atmosphere... plus, oh yeah, oceans? So suffice to say it's trivial... quite a few orders of magnitude more than the space loss... outpacing space loss by a few orders of magnitude in recent years perhaps, but nowhere near truly touching the vast scales of water here. $\endgroup$ Commented Aug 29, 2022 at 8:12

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