What are some plausible means for increasing the atmospheric pressure on a post-apocalyptic earth?

Question

I want my post-apocalyptic science-fantasy setting to be at least plausible enough for people to suspend their disbelief comfortably, rather than having to ignore obvious physical fallacies. I want there to be giant life-forms, like the dinosaurs once were. I know there are many problems with this in our current world, which we see evidenced by the fact that no creatures fill that giant-animal niche anymore (except elephants and they're at the upper limit). One of the issues with this is that giant creatures require higher O2 pressures to keep their tissues properly oxygenated, and for giant birds and pterosaurs and stuff to fly. What are some plausible ways for a post-apocalyptic earth to have increased its atmospheric pressure sufficient for this scenario? If the ice caps completely melted and we had a runaway greenhouse gas effect, could higher water vapor, methane, and CO2 concentrations do it? I am looking for suggestions as much as simple yes/no answers, thanks so much everyone!

Answer 1

Yes

The Earth's atmospheric pressure has likely fluctuated several times over history, though this field is particularly difficult to study and results have often been inconclusive and sometimes contradictory.

If you need a world with higher air pressure, you can have it. You don't even need to explain why the pressure is higher unless you want your earth scientist readers to nerd out, but more than likely you'll just piss them off. However, if you need your world to go from current air pressure (1 bar) to higher air pressure (let's say 2 bar) in a relatively short time you need to add that mass from somewhere quickly.

Earth's atmosphere has a mass of about 5.15×10¹⁸ kg, so we'll need about that much mass added to earth in gaseous elements to double the atmospheric pressure. Halley's comet has a mass of around 2.2×10¹⁴ kg, so we'd need ~25000 Halley's comets to hit earth to add enough mass, assuming almost all of it eventually settled as atmospheric gasses. Given that Halley's comet is about the same size as the asteroid that created the Chicxulub crater, this would certainly be apocalyptic.

Alternatively, some of that mass could come from within earth, released by volcanic eruptions. The largest volcanic eruptions in history expelled something on the order of 10,000 cubic km of lava, and if we assume an equal amount of gas that would be roughly 2×10¹³ kg, so we just need another 500000 of these volcanos to go off and we're golden. And you have another apocalypse.

All this to say, the Earth has a lot of atmosphere, and quickly adding enough mass to substantially alter atmospheric pressure without obliterating absolutely every living thing is going to be challenging. That's why on Earth it has happened over millions or billions of years.

One thing to consider, it would be far easier to noticeably alter the composition of the atmosphere (after all, we're doing it right now) than to change atmospheric pressure. Earth's oxygen levels are believed to have climbed to a maximum of 35 percent and then dropped to a low of 15 percent during a 120-million-year period, and high oxygen percent led to enormous bugs. It is possible to make things bigger within reasonable physical constraints, but the question of how you get there is up to you.

Answer 2

You don't need a higher overall atmospheric pressure, (you can have that if you want it but it's not completely necessary) what you really need is a higher partial pressure of oxygen. This has occurred at many stages in the history of life on Earth most notably during the Carboniferous when oxygen levels were as high as 35%. This was the era of giant insects like Arthropleura, the two and a half metre long centipede, this would also be more than enough oxygen to support larger than modern land animals of mammalian, amphibian, or reptilian types. In a post-apocalyptic setting the mere removal of human farming and forestry practices would be enough to increase forest cover massively and thus raise the oxygen level markedly.

Answer 3

Something like the Azolla Event would consume carbon dioxide and release oxygen. This happened when the polar ocean was isolated from the others. A modern equivalent could happen in Hudson's Bay if the ice does not all melt.

Don't try this at home, kids! Chucking Azolla into Hudson's Bay is unlikely to make a dent on global warming, and most good ideas for tampering with nature end up biting us in the backside. But you could base your rise in oxygen level on something like that.

Answer 4

Stable wormhole to the surface of Planet X, which has higher pressure than Earth, high partial pressure of oxygen, and giant monsters.

There was another portal hundreds of millions of years ago and some Earth life ended up on the then-lifeless Planet X - hence why Xlings have the right protein structure to eat Earthlings.

Presumably the portal was caused by the same event/s that caused the apocalypse.

This also gets you a plausible source of giant megafauna without waiting so long that your planet no longer qualifies as post-apocalyptic.

Wormholes aren't very scientifically plausible, but they're a staple of sci-fi, and - unless relativistic space travel is involved - they're an easy concept for the author to nail down all the abilities and constraints and then follow where logic and physics leads.