Is Earth's total water finite?

28 February 2013

Question

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.

Ian.
Caerphilly.

Answer

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 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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