eli5: Earth’s water doesn’t increase or decrease?

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So I have always read/heard that Earth’s water never increases or decreases but didn’t it come from icy comets in the first place? Wouldn’t even the small ones that hit us all the time and burn up in the atmosphere still release evaporated water?

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It’s not *100%* constant, but the amounts added by asteroids (not comets; comets are pretty big and we would notice real quick if one hit us) or removed by the solar wind are so small as to be basically irrelevant compared to the amount of water on Earth.

In the present day, the amount of water being added to the Earth’s ecosystem from space debris is negligibly tiny compared to the amount that’s already here.

As a rough estimate, about 40,000 tonnes of matter falls to Earth from space every year—and that’s a *total* estimate, most of which is rocky dust and not icy bodies (this close to the Sun, ice tends to sublimate into water vapor and not form solid chunks). The Earth’s hydrosphere contains about 1,400,000,000,000,000,000 tonnes of water, so even if all the space debris hitting us was pure ice chunks, it would increase the amount of water on earth by 0.00000000000286% per year, and it would take 3,500,000,000,000 years (8,000 times the age of the Earth) to double the amount of water on the planet.

Water is also produced from combustion and respiration

Combustion from fossil fuels is estimated to create 1.2 × 1013 kg per year (https://iopscience.iop.org/article/10.1088/1748-9326/aa8390)

There is about 1.37 × 1021 kg of water in the oceans (https://hypertextbook.com/facts/1998/AvijeetDut.shtml)

So 0.000001% of the mass of the ocean is created yearly.

Nice comments about water increase

What about decrease? Do we lose anything to space?