Science Questions

Is Earth's total water finite?

Thu, 28th Feb 2013

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Ian James asked:

Hi Dr Chris,


I am curious about the amount of water on our planet. If we (and all other animals) consist mostly of water, does this mean that as animal populations increase, the amount of water on the planet has to decrease to compensate?


Would love to hear your comments.







Naked Scientist Hannah Critchlow posed this question to Phil Robinson from the Royal Society of Chemistry. He had this to say on a subject.

Phil - The simple answer is yes. The Earth is effectively a closed system and the total amount water it contains is essentially constant. 

Now, some of that water is stored in humans temporarily while they're Impact of a drop of water.alive. So, the more humans there are then the greater the volume of water that will be stored in that reservoir. 

Now, on average, a human will hold about 40 litres of water and if we take the world’s population at around 7 billion, that gives a total volume of about 280 billion litres held in humans which is a lot at almost 1/3 of a cubic kilometre.

Hannah - Oh dear! Well, since the world population is estimated to have increased by 3 billion in the last 50 years and is anticipated to continue to rise, should we all be sensationally stock piling personal supplies of water in preparation for disaster? Fear not! Phil has more on the topic:

Phil - However, the total volume of water that exists on the whole of the Earth, in whatever form – liquid, solid, gas or biological is actually about 1.4 billion cubic kilometres. So the volume represented by people is just a tiny fraction.  It’s not even a billionth of the total amount of water. In fact, to make it a billionth, we’d have to increase the world’s population about 5 times. So, in short, yes, humans are a reservoir for the world water, but the amount of water that that represents is really just a drop in the ocean.

Hannah -  So yes, Ian. You are perfectly right. Increasing human populations will decrease the amount of water left on Earth, but not by any significant amount.  


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The majority of the water on the planet is in the ocean, or is "Brackish Water", with another large chunk of water locked up in ice.

Global warming, of course, goes with the hypothesis that the water in the ice will slowly melt and be released, causing a rise in the oceans, but no "net" change in water.

Burning fossil fuels does create water out of air and hydrocarbons, although there may also be a tendency to inject water into the wells, so the balance of water near the surface may be largely unchanged.

One could consider the water locked up in the human bodies, but it would take an awful lot of humans to make a dent in the amount of water in the oceans.  And, when a person dies, the water is released.

The biggest concern is not really a decrease in the overall quantity of water, but rather competition for water resources.  Humans consume a lot of water when considering drinking, bathing, washing, and cleaning.  Furthermore, most of our agriculture is water intensive.  The water, of course, is part of the water cycle so it is not lost forever.  However, it is not uncommon to essentially 100% drain a river, or significantly lower water tables.

Global warming will undoubtedly change precipitation patterns, increasing evaporation in many places, but with it, also increasing rainfall.  No doubt some places will benefit from increased rainfall, but others will suffer from changes in precipitation patterns.

It is possible to extract fresh water from salt water, but it is generally quite energy intensive.  It is much better to let Mother Nature do it for us.  Furthermore, Nature's water cycle cools the planet by absorbing energy in the oceans during evaporation, and releasing the energy in the upper atmosphere during condensation.

CliffordK, Thu, 21st Feb 2013

"Water supplies" are surface and near-surface quantities which can be accessed reasonably cost-effectively.  There's more which could be accessed with new technology and plenty which we'll never get to without a geological upheaval we don't really want to see.  Without new comet strikes, though, the planetary total is pretty much fixed. eddwilson, Thu, 21st Feb 2013

The water to create new life comes from the environment. The amount of water on the planet remains constant. The water trapped in ice in the artic doesn’t really change to adapt though. That only changes due to global warming which only changes due to inordinate changes in greenhouse gases. The water in rivers and oceans and clouds is just a tad less, that’s all. If we changed all the animals in the world into water and dumped that water into the ocean then you’d be unable to detect that change by the naked eye.
Pmb, Fri, 22nd Feb 2013

Water degradation is also a problem - pollution, or use for mining or fracking can make it unsuitable for drinking or agriculture.

We have dammed many rivers, which interferes with fish migrations. Modern reinforced-concrete dams have about a 100-year lifetime before they collapse from corrosion, or fill up with the sediments which would otherwise wash downstream.

Water allocation is a contentious border issue where rivers cross national borders.
There is an amazing amount of water that goes into 1kg of wheat or beef, so one solution is to "export" water in the form of wheat or beef to areas that have insufficient water supplies. See: evan_au, Fri, 22nd Feb 2013

At least in the Northwest USA, Wheat is grown in a semi-arid region using dryland farming, with no irrigation. 

Some crops such as corn require much more water, but can successfully be grown without irrigation provided there is sufficient rainfall.

The valley where I'm at gets more rainfall than Eastern Washington.  With cattle, all of the grass and hay that we used was non-irrigated.  At least around here, grass and hay was far better at utilizing the springtime sun and rain than most other crops. CliffordK, Fri, 22nd Feb 2013

Clever people can improve water supply.Stupid people can diminish water supply.Increasing quantity of people creates competition.Competition makes people be cleverer.However people increasing  is dangerous to many kinds of animals. simplified, Sat, 23rd Feb 2013

Related to the last comment: global sea-levels are also rising due to human abstractions (pumping) of groundwater because it is not replenished (recharged) fast enough to make up for what is withdrawn. Here's a great article by USGS folks who quantify its contribution to global sea-level rise:

Also, people should all be aware that water supplies that we depend on may or may not be sustainable in the long-term, and this is generally the case in a number of areas that rely on groundwater for irrigation especially, and also with increased use of groundwater to supply growing megacities (sometimes in not-so-dry areas).
NRossman, Wed, 13th Mar 2013

Keep in mind that in many cases water can be reused several times before it makes it to the ocean.

So one communities waste water eventually makes its way back into the aquifers, and into the next communities fresh water.  Minus evaporation and other losses, of course.

It always helps to be UPSTREAM!!!

Crop runoff from one farmer may get directly back into the irrigation canals for the next, although there can be issues with crop runoff and potentially unwanted herbicides and pesticides. CliffordK, Wed, 13th Mar 2013

dab waz ere lolz, Tue, 2nd Aug 2016

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