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Perhaps a simple question, we know 71% of the earth's surface contains water as oceans. If Earth's age is 4.543 billion years, then I guess it should be decreased with drying or should have been dried so far. Why doesn't it dry or decrease?

If we put some water in sunlight, it evaporates. The oceans are the chief source of rain, but lakes and rivers also contribute to it. The sun's heat evaporates the water.

So I wonder why doesn't the 71% water coverage not evaporate, decreasing until gone? Why is it still 71% after billions of years? Does water keep coming from somewhere? Or, does moving water not evaporate? Why is it still here?

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    $\begingroup$ Water evaporates, but comes down back as rain or other forms of precipitation. $\endgroup$ – Gimelist Apr 30 '17 at 6:19
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    $\begingroup$ Where do you think the water goes when it evaporates? $\endgroup$ – immibis May 1 '17 at 4:20
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    $\begingroup$ tl,dr: Because earth's atmosphere can only hold so much evaporated water, far, far less than earth's oceans hold. $\endgroup$ – RBarryYoung May 1 '17 at 14:48
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    $\begingroup$ How would anyone know that it was 71% billions of years ago? $\endgroup$ – Tim May 2 '17 at 7:34
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    $\begingroup$ Who said it doesn't evaporate :O :D? It just also precipitates :D. $\endgroup$ – Teacher KSHuang May 2 '17 at 11:49
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There are two ways this problem needs to be looked at. The first is more astronomy than Earth science. The Earth as an entire system is largely contained. Its gravity and magnetic field retains nearly all of its elements. Earth does lose hydrogen and helium and cosmic rays will split water molecules leading to a loss of an impressive amount of hydrogen and as an indirect result a loss of water, but this is loss irrelevant compared to the size of the oceans. More detail here. Space dust, comets and asteroids contain water so some water is returned from space too.

By the upper estimate in one article, 50,000 tons of hydrogen per year works out to about 450,000 tons of water lost every year. (and 400,000 tons of oxygen added as a result). Compared to the mass of Earth's oceans those numbers are small. 450,000 tons per year, or 450 trillion tons over a billion years is nothing compared to the 1.3 million trillion tons of water Earth has in its oceans. By the highest estimate, it will take 30 billion years at the current rate and at the Sun's current luminosity for Earth to lose just 1% of its oceans. (Will look to update with other estimates).

As for the rest of the question, once we recognize that loss into space is insignificant, then virtually all water is continuously cycled though the water cycle or hydrological cycle. Very little water gets destroyed or chemically transformed. Nearly all of it, even of millions or billions of years, evaporates, or, turns into ice, or gets absorbed by plants, or seeps underground, but it always returns. Evaporated water returns to Earth as rain. Water that gets frozen on the ice caps eventually melts back into the oceans. Water absorbed by plants or that seeps underground does eventually get returned to the surface by plate tectonics or volcanism. Plants that store water return it when the plant is eaten. Water is very hard to destroy, so it stays remarkably constant on Earth over time.

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  • $\begingroup$ I think you have a typo in "gets absorbed by planets". $\endgroup$ – Dragomok Apr 30 '17 at 15:44
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    $\begingroup$ One problem with this answer is that 50000 tons per year is nowhere near the highest estimate. A fairly common estimate is 3 kg/sec, which is equivalent to twice your rate of 50000 tons per year. Some estimate even higher current mass loss rates. Another problem: Oxygen comprises 8/9 of the mass of a water molecule. Lose the 1/9 that is hydrogen and the water it's gone. One last problem: That ~3 kg/second is the current rate. It was arguably orders of magnitude higher in the distant past and will be orders of magnitude higher in the distant future. $\endgroup$ – David Hammen Apr 30 '17 at 15:47
  • $\begingroup$ @Dragomok Thank you. That should be gets absorbed by plants. Fixed. $\endgroup$ – userLTK Apr 30 '17 at 21:24
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    $\begingroup$ @DavidHammen I did the math for the 8/9ths. That's why 50 tons of hydrogen works out to 450 tons of water. Also, I know it was orders of magnitudes higher in the distant past, certainly higher when Earth was forming and very hot and higher during the late heavy bombardment and higher prior to the magnetic field. I didn't want to get into that, but I should probably add a notation. As to the estimate, that's a good point. I should adjust the estimate. $\endgroup$ – userLTK Apr 30 '17 at 21:28
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Why doesn't 71% water of the earth dry or evaporate?

The simple answer: Because it rains.

The not so simple answer: By some estimates, the Earth has already lost about a quarter of its water, and it is predicted to lose almost all of its water in a billion or so years from now.


It rains because temperature decreases with altitude. This lapse rate means that moist air becomes saturated at some point in the atmosphere. You can see this point on somewhat cloudy days. While cumulus clouds have puffy tops, they have flat bottoms. Those flat bottoms reflect the point where the humidity level reaches 100%.

Currently, only an extremely small fraction of the moisture in the air makes its way to the top of the thermosphere. The tiny amount of moisture that does make its way to the stratosphere can migrate throughout the stratosphere. The top of the stratosphere is unprotected from the nastier parts of the Sun's output. Ultraviolet radiation dissociates water into hydrogen and oxygen. Some of that hydrogen escapes into space. That lost hydrogen represents lost water.

The Earth is currently losing about three kilograms of hydrogen into space every second. At that rate, it would take about a trillion (1012) years for the Earth to lose all of its waters. There are signs that the escape rate was significantly higher in the distant past. This is a byproduct of solutions to the Faint Young Sun problem. The signs of this early high escape rate are written in stone (serpentine, to be specific). Very old rocks have a different deuterium ratio that is currently observed.

The escape rate will inevitably become significantly higher in the distant future as the Sun gets warmer and warmer. The Earth will eventually lose all of its water.

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    $\begingroup$ A related question is how much of Earth's water that locked in the mantle will be locked in the mantle and not be lost to space. Might not relevant though because that water is not available for liquid water on the surface. $\endgroup$ – Gimelist Apr 30 '17 at 21:30
  • $\begingroup$ is predicted to lose almost all of its water in a billion or so years from now.. Strange, considering the most upvoted answer suggests that the Earth will lose about 1% of its water after 30 billion years! $\endgroup$ – user1993 May 2 '17 at 16:08
  • $\begingroup$ @user1993 - That answer reflects the current loss rate. The Sun grows more luminous as it ages. It will be about 10% more luminous a billion years from now. That increase in luminosity will eventually trigger a runaway greenhouse effect. $\endgroup$ – David Hammen May 2 '17 at 19:04
  • $\begingroup$ @Harry .. as Michael mentioned above there may be a very large amount of water locked in the Earth's mantle. See this article astrobio.net/news-exclusive/… (there are many others). However, contrary to what Michael said, the researchers think that this water may have been a buffer helping the Earth maintain a constant (even more than expected) volume of surface water. $\endgroup$ – Jack R. Woods May 14 '17 at 1:04
  • $\begingroup$ @DavidHammen, Hi, is it measured beyond doubt that the earth is currently losing kilograms of hydrogen into space every second? Or is it speculation that has its critics? $\endgroup$ – Pacerier Aug 23 '17 at 12:15
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The bigger question is really: where would it go?
Because of course matter isn't created or destroyed. So it'd have to go elsewhere.

The only two places it could go are:

  • Into the Earth. But water is fairly light (1 g/mL versus Earth which appears to increase from 1.02 g/mL near the surface to 13.09 g/mL at the center). Plus, even if water could/did continue to transition into the Earth in large amounts, it'd just build up there until it reached a saturation level. And there's only so much room between the highly compressed rock.
  • Out to space. However gravity generally keeps the atmosphere here, just like it does for us. A very small percentage of gas does escape. However, it's predominantly lower mass gases like hydrogen and helium. The question on our site of What “g” would be needed to keep helium on Earth? is quite useful for information on this.

Or, alternatively,

  • Out to space as its constituent gases. (Monotonic) Oxygen and especially Hydrogen do escape somewhat more readily. Electrolysis and other water splitting methods do cause a small percentage of Earth's water to be broken down at any given time. However, based upon this estimate of 95,000 tons of hydrogen lost per year, and this estimate of $1.4 \times 10^{21}$ kg of water, it'd take 1.8 trillion years to lose all of Earth's water by hydrogen loss.

Otherwise, if it isn't leaving the system, it could only build up in the atmosphere... potentially until reaching a level where the air could hold no more. Theoretically that'd be at 100% relative humidity. Unfortunately the troposphere can only hold the equivalent of a layer of a couple inches\few cm of water. Additionally water tends to leaves the air when enough areas of it reach 100% RH, through processes like dew and precipitation.

And so over the long run, it can only reach an equilibrium, where the amount of water the air held was in balance with the amount it lost. And that seems in line with what we see. Moisture levels vary, but aren't rapidly changing in any direction. Considering that there's 96000 times more water in the oceans than there is in the air currently, the atmosphere surely couldn't hold it all anyways! If it can't leave, and the atmosphere cannot hold it all... well, it has to stay on the Earth. Certainly we see the equilibrium in practice too, as sea\lake levels aren't dropping consistently. Lake levels actually vary quite a bit, showing evaporation has the ability to drain at least the lakes fairly fast. Yet they aren't dropping consistently in any noticeable way. We're very dry in Florida right now, and lakes are way down... but in places like California and the Piedmont they've been seeing great flooding. But neither lasts in the long run.

Looks like studies do estimate that the Earth has lost 1/4 of its water in its estimated lifetime. So indeed it's not that nothing leaves. It's just that it's very slow. If it wasn't... well we'd be seeing it quickly change now! Over our lifetimes, over the past few thousand years. But there's no indications that general lake/ocean levels have drastically changed.

I do really get that there can be skepticism at billions of years. It's certainly something we can't verify too easily, visually, personally. And with Biblical\other religious sources casting potential dispute upon it, that increases the question.

Honestly, a reasonable scientist should welcome skepticism and try to address it with fairness and levelheadedness. Unfortunately there certainly are plenty who are dismissive and belittling, either openly, or in their attitudes.

But while questioning is a very worthwhile endeavor, this particular line of reasoning doesn't appear to hold much mustard. Be wary not to jump too quickly on any potential disagreements you see to accepted theories and thus run hard with them, or you're no different from those overly rigid scientists. Keep seeking for the truth as something of the greatest value. Regardless of what it is, and in the end it alone shall stand, and shall set you free!

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    $\begingroup$ informative, your opinion is much appreciated. $\endgroup$ – Harry May 1 '17 at 5:17
  • $\begingroup$ There's one big problem with this answer, which is that almost all of the hydrogen that escapes from the Earth's atmosphere was originally in the form of water. Sunlight dissociates water in the middle atmosphere into hydrogen and oxygen. Hydrogen escaping into space means water is lost into space. $\endgroup$ – David Hammen May 14 '17 at 1:11
  • $\begingroup$ @David Hammen: I'm confused, that's what I'm suggesting by option 3. I didn't know the water dissociation region was primarily the middle atmosphere, as you suggest, and can add that... but my calculation equated any hydrogen loss to be water loss, so wherever the water splitting occurs, the hydrogen loss is being counted as water loss. You sound like it's a bad answer! $\endgroup$ – JeopardyTempest May 15 '17 at 8:15
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There are good answers for this question but Earth Science is one of the few sciences that lends itself really well to experiments around the house.

To see the effect you're talking about (water evaporating, but not going away) take a large rag, and soak it in water. Get it very, very wet. Place the large rag in a zip-lock bag and make sure to seal it tight. Now place the bag with the rag out in the sun.

You will notice how the water never leaves the bag. During the sunny part of the day the water evaporates into the air in the bag, and when there is enough in the air, the water condenses and drips (rains) back down onto the rag.

The rag is like the "ground". The bag is like the atmosphere.

Just like with the rag, after water evaporates it "condenses" in the atmosphere (on the bag) and rains (drips) back down to earth (the rag).

This is a over simplified example of the process, but it demonstrates it very well.

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  • $\begingroup$ sounds logical, but don't you think reprocessing of water that evaporates and comes down in rain form should decrease in amount? we see when it rains there's puddles and some water amount in streets for 1 or 2 days then it evaporates but eventually it gets dried. $\endgroup$ – Harry Apr 30 '17 at 19:11
  • $\begingroup$ It doesnt go anywhere just soaks into the ground or moves by evaporating and raining somewhere elae $\endgroup$ – coteyr Apr 30 '17 at 19:15
  • $\begingroup$ Barring some really really small amounts that "leak" into space via various amounts (very very small). All the water on earth stays on earth. It just moves around from place to place. Some times it's ice, some times it's water, some times clouds, but always here. $\endgroup$ – coteyr Apr 30 '17 at 19:23
  • $\begingroup$ as clouds travel, so evaporated water of oceans should fall as rain form somewhere on dry part of lands. still it's 71% after so many years as you mentioned some water soaks into the ground as well? $\endgroup$ – Harry Apr 30 '17 at 19:30
  • $\begingroup$ Essentially yes. With out getting silly with numbers there is always the same amount of water on earth, it just moves around. $\endgroup$ – coteyr Apr 30 '17 at 20:24
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"If we put some water in sunlight it evaporates." Really? If I take some water in a closed bottle and put it in the sunlight, then it will evaporate? Obviously not. Why not? Because the air (gas) in the bottle is saturated with water. The relative humidity of that air is 100%, or close to it. You probably know this already. So you should realize that your question is flawed. Not only does water not evaporate in the bottle, but water evaporates at night and inside houses without any sunlight. A better question, imho, is: "Why isn't the Earth's atmosphere saturated with water after 4 billion years?" The atmosphere has a mass of about 5E+18 kg (that is 5 followed by 18 0s, eg 5,000,000,000,000,000,000). The water in the oceans has a mass of about 1.3E+21 kg, meaning about 250 times more mass than the atmosphere. Simply put: there's not enough atmosphere to hold all that water. The reason why the atmosphere is not saturated, does not have 100% relative humidity, is that the temperature of the air varies a lot. Both day to night, and season to season. The air is frequently being cooled and the water lost via precipitation. When this air is reheated, both by direct sunlight and by contact with the surface which warms up even more in direct sunlight, it is low in relative humidity. This is the air that will allow water to evaporate. Because of these temperature cycles, air never has the time to reach saturation.

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I don't want to post a super long answer so I'll keep it simple.

Pressure and gravity due to the mass of the earth keeps the water within the atmosphere and cycling through the 3 states of solid, liquid, and gas.

Our water is constantly being recycled, and some things create water, like humans (we create water in our lungs when we breathe.)

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