# How much water is the atmosphere losing to space?

Up until recently, I was under a (wrong) impression that the amount of planetary cumulative water resources was finite as I believed its escape from the atmosphere was impossible. I believed that, unlike other planetary resources, it was impossible to "waste" water as any waste would simply recycle itself in nature, i.e. municipal waste would be treated and get into the oceans from where it would eventually evaporate and precipitate back into continental waters renewing our water supply.

Recently, I learned I was wrong and that there is such a phenomenon called sequestration and that it is indeed possible for the total water resources on the planet to vary.

I am curious if there are metrics on how much water the planet has been losing (or gaining in case of a reverse phenomenon). Additionally, is there anything we can do to reduce this effect?

This article says that we've lost a quarter of our water total but I was looking for more granular statistics.

## 1 Answer

It is not actual water what is lost to space, because in the high atmosphere water usually dissociate into other molecules or ions. The oxygen ion outflow is frequently assumed to be a proxy for the loss of water from the planetary atmosphere. In terms of global outflow rates for the Earth the rate varies from $$10^{25}$$ to $$10^{26} s^{-1}$$, depending on geomagnetic activity (reference).

On the poster of the reference (sent to me by the author) we can read:

If we assume oxygen corresponds to water loss (self-regulation, Hunten and McElroy [1970]), then an oxygen loss rate of ~$$10^{25} s^{-1}$$ corresponds to ~300 $$\text{g s}^{-1}$$ of water loss. Over the age of the solar system (4.5 billion years ~ $$1.4 \times 10^{17}$$ s) this loss rate gives $$4.2 \times 10^{19}$$ g of water.

The current loss figure is equivalent ~25,920 liters per day, or 9,467 $$\text{m}^3$$ per year. And the reference of that figure seem to be the paper Escape of O+ through the distant tail plasma sheet, that used measurements from the STEREO‐B (Solar Terrestrial Relations Observatory) spacecraft.

That would correspond to a total loss over Earth's history of 42,000 $$\text{km}^3$$ of water, equivalent to about 12 cm of sea level change. However, that's a straightforward extrapolation of the current rate, because they acknowledge that they don't know how to model how Earth's magnetosphere would have behaved in the past when the Sun was weaker. Although, for a weaker Sun it would be reasonable expect smaller losses.

This value is radically different to that of the article you pointed to. In that case they use the loss of hydrogen instead of oxygen as proxy for water loss:

Knowing how much hydrogen had disappeared from the oceans over the last four billion years enabled the researchers to calculate that the oceans have lost about a quarter of their water since the Earth’s early days.

They the say:

Today the atmosphere is rich in oxygen, which reacts with both hydrogen and deuterium to recreate water, which falls back to the Earth's surface. So the vast bulk of the water on Earth is held in a closed system that prevents the planet from gradually drying out.

That suggests that the limiting factor for the existence of water is now the abundance atmospheric oxygen (although oxygen in rocks if very abundant). So it would make sense to use now Oxygen loss as proxy for water loss, but in the past, before the atmosphere was flooded with oxygen, maybe hydrogen loss was a better proxy.

They suggests that methanogenesis can free hydrogen atoms and make them liable to be loss to space, but when it is part of a water molecule it is safe. Methanogenesis would have been much more common in the past when the atmosphere was rich in methane (50 to 500 richer than today).

To wrap up, it seem that there is no consensus about the actual amount of water loss trough Earth's history. Different proxies give different values. Some proxies might be representative to some periods in Earth's past, but other proxies are more representative of other periods. And unfortunately we don't have yet a good reconstruction of the composition and density of the atmosphere throughout Earth's history.

But we do know with some confidence, that while you where reading this answer, one or two liters of water were loss to space.

• Do you possible have a typo there in the first sentence? By how much does the global outflow rate vary? – AtmosphericPrisonEscape Feb 1 '19 at 21:07
• @AtmosphericPrisonEscape Yes, thanks for spotting that!!! I just fixed it. Cheers – Camilo Rada Feb 1 '19 at 21:08