Eli5 How is water renewable?


Every bit of information I find just talks about water cycling through its different states of matter, I’m aware that water evaporates then rains and returns to the earth but none of them say at which point more water is actually produced or how. Is it actually renewable or is it the concept that there are other sources such as ice and humidity that can keep refilling our supply?

In: 6

It’s pretty much a closed system. The Earth doesn’t “produce” new water.

Water is “renewable” in that, again, it’s a closed system, so water can’t really get _lost_.

With that said, _drinkable_ water is a very scarce resource (compared to how much water covers this planet), and we absolutely can run out of _that_.

>but none of them say at which point more water is actually produced or how

New water is basically never produced. Maybe fifty million tons a year from burning fossil fuels, but that’s peanuts on a global scale, when it comes to water.

Okay, so, there’s bascially zero new water entering the system. But that’s only a problem if the existing water is leaking out somehow. Where are you worried the system is losing water?

How do you think it is being used up? If you are talking about a steam engine, the water only changes states. If you are talking about something like electrolysis, then the H2 and O2 produced will eventually recombine to form water.

In our ionosphere, there is enough energy for the reaction to happen spontaneously.

> at which point more water is actually produced or how

Essentially no new water is produced, but also none is really lost or consumed – so it’s renewable in the sense that it just moves around between different locations and states of matter.

Like, when animals “use” water, water molecules aren’t being destroyed. All the water you drink ends up either in your tissues, or leaves in your pee, your breath, sweat, or feces and going back out into the world water cycle.

On the planetary scale, no water is leaving Earth so there’s no need for a mechanism continually producing it either. It just moves around between being liquid water on and under the ground, humidity in the air, falling rain, inside biomass (plants and animals), etc.

The water on Earth is a fixed amount with very little loss or production of new water. No matter what you do with water, it just reenters the water cycle. It does not escape.

This is because the only way for it to escape is to leave Earth’s atmosphere and escape into space. Short of us building rockets to throw it out of Earth’s gravity well, the only mechanism for this is to float to the very highest reaches of the atmosphere and then get hit by the solar wind hard enough to be driven away from the Earth over time.

This happens with hydrogen and helium. However, a water molecule is too heavy to float that high and is too heavy for the solar wind to matter that much.

So, all water stays on Earth. On Earth, almost any way you can imagine to “use it up” simply puts it back into the water cycle.

Essentially, water on Earth is a nearly perfect recycling system. No new water need apply.

Edit: Corrected error on the rate of escape of hydrogen and helium.