If all the water in the oceans disappeared, how would it affect air pressure on Earth?

Question

I wondered about the effect of water disappearing on Mars for years when I was bored in elementary school. At first glance, it seems that if the oceans disappeared, the air pressure at the deep ocean floor would be the same it is as it is at sea level today.

The thing is, the average depth of the ocean is two miles, which means that half of it is even deeper than that. It seems to me that all the air would slide down to the really deep places (8 miles deep).

Cities would be effectively 8 miles high, the altitude that jets fly. Everyone in the cities would suffocate.

You could walk around on the ocean floor next to the Titanic, but you'd still suffocate from a lack of air 6 miles above the lowest dry surface.

Anyway, that's what I concluded in elementary school. Was I wrong? This probably isn't a legitimate Earth science question, but I'm actually curious after all these years.

Answer 1

If the ocean's waters are suddenly magicked away, well, anything goes. Magic (if it exists) is non-scientific. Anything goes.

So, I'll look at a mechanism by which the Earth can lose all of its water. The Sun gets ever more luminous as it ages. In about a billion years (well before the Sun becomes a red giant), the Earth's surface temperature will exceed the boiling point of water. The Earth's oceans will boil away. This is the moist greenhouse effect.

This added water vapor in the atmosphere will raise atmospheric pressure (and surface temperatures) by a great deal as water vapor is a very powerful greenhouse gas. Plate tectonics most likely will come to a halt as plate tectonics is widely hypothesized to depend on water for lubrication at subduction zones.

One side effect of this will be to stop the deep carbon cycle. The conditions that led to large igneous provinces such as the Siberian Traps and Deccan Traps will become the primary mechanism by which the Earth's interior cools off. (Large igneous provinces and volcanic activity are the tectonic mechanisms by which Venus cools its interior.) Lots of carbon from the Earth's interior will enter the Earth's atmosphere and will stay there without subduction to pull excess CO₂ to the deep Earth.

Meanwhile, the water vapor in the upper Earth's atmosphere will be dissociated by sunlight. The hydrogen will escape while the oxygen will mix into the lower atmosphere, where it will oxygenate anything and everything. More CO₂. The Earth will eventually become Venus 2.0: Very high pressure and high temperature at the surface. What is sea level now will be very hot and at a very high pressure. What was the seabed will be even hotter and at an even higher pressure.