Climate of Extreme Depths and Deserts

Factual Questions
1

I’m writing a story about people who live on mountaintops surrounded by an inhospitable desert.

At times I’ve seen articles that stated that many years ago, the Mediterranean Sea was isolated from the Atlantic Ocean, and dried up to become a vast desert, the Mediterranean Depression. During this time, places like Malta might have been such mountaintop oases.

What would the climate have been like in the Mediterranean Depression? Would it have become too hot to survive? (60 degrees C? More?) Would there have been significantly greater air pressure at the bottom?

Are there air-pressure problems when descending into deep mines?

I got to thinking about Larry Niven’s story A Gift From Earth. AGfE takes place on Mount Lookitthat, a small continent that juts kilometres above the searing surface of a venuslike planet. It is the only place where people can live and breathe. The surface of the planet is too hot and the air is too dense.

Would it be possible for this to occur? Could a venuslike planet have a layer of breathable air with oxygen, especially if there are no oceans to replenish the oxygen?

2

Well the bottom of the Mediterranean is around 5km below current sea level, compared to around 3km for the deepest mine. That mine has air pressure of about double that of sea level.

I don’t know whether there was ever a time however when the Mediterranean basin had dried up completely like that. It became a collection of saline lakes like the dead sea, but the catchment is too large to allow it to dry up completely AFAIK.

Not problems as such AFAIK, beyond needing to inflate vehicle tyres to take account of it.

The oceans don’t play any great role in replenishing oxygen They played a major role in creating oxygen one way or the other, but these days the oxygen content of the atmosphere is set and the oceans play no significant role. If they dried up altogether the decline in oxygen levels would be almost immeasurable. Oxygen exists today because there is nothing left for it to react with, it remains because there is nowhere left for it to go, it’s that simple. It doesn’t need replenishing simply because it’s not being used at any significant rate.

3

Check out Rise of Endymion by Dan Simmons for another take on the “mountaintop” world. It’s the fourth in a series, so parts may be hard to follow if you haven’t read the other three, but I found the world where much of the action takes place to be intriguing.

The world has a thick, corrosive, poisonous atmosphere for the most part - but the toxins don’t reach the high altitudes where humans live. Mountain tops are islands in a toxic sea. There are issues of how to travel from one to another. Also, they are at high altitude - some land is unusable simply because there isn’t enough oxygen to survive.

Anyhow - for your sub-sea-level basins take a look at both Death Valley in the United States and the Dead Sea in Isreal - both are below sea level. They’re hot. The air is thick, but not so much so that it causes problems for humans. There is less harmful ultraviolet sunlight, though, since the thicker atmosphere filters more out. In fact, the Dead Sea area has long been frequented by folks with skin problems who apparently find both the salt sea and the light therapeutic.

In deep mines rising temperature is more of a problem than rising air pressure.

And that about exhausts the limits of my knowledge.

One last fictional treatment, though - there was a four book series a couple decades ago called Continuum that had a number of authors each write four related short stories on a world/theme/whatever. One of the stories was “My Own, My Native Land” and it and three others all dealt with settling a high-air-pressure world. (The same tetraology was also the first appearance of Anne MacCaffrey’s Crystal Singers and Killashandra Ree). Anyhow, I still have the books around here somewhere - but no time to search for them at the moment. If I can find 'em later I’ll give you more information. If you’re interested.

4

Not really very good comparisons though. The Mediterranean basin is 5000m below sea level. The dead sea is only 500m, Death Valley even less. There’s just no comparison.

5

Don’t forget that there are many rivers that flow into it - Nile etc. I wonder if it was like a tropical rain forest?

6

The deepest point may be 5000m, but the average depth is 1500m - far more comparable to the others.

7

The point - which apparently whooshed at least one person here - is that Death Valley and the Dead Sea are currently what we have in the way of sub-sea level environments. It’s no different than using Mt. Everest to extropolate conditions on a mountain 50,000 feet tall.

If I recall what I’ve read on the “Mediterranean Basin” theories, the basin would probably be a salt flat due to evaporation of sea water after the area was closed off from the ocean. If that’s true, there would have been a period of increasing salinity in the water (much like the Dead Sea area). After awhile, you’d have thick salt flats, like in the western United States. Some areas around the edges would lack salt to the degree that you would get plant and animal life, and of course rivers would still continue to run into the area. The lowest areas would have the greatest salt accumulations and it’s doubtful people could make a living in such a place (unless they were salt miners).

8

It’s not Like the Mojave or Sinai deserts are naturally cold and wet even in those adjacent regions at sea level. The fact that the low lying areas are also hot and dry tells us approximately nothing. That’s a point that seems to have been missed by some people. These areas are below sea level because they are in hot dry areas that don’t receive sufficient rainfall to turn them into lakes. It’s a statistical bias that some people are ignoring. We night just as well look at the Scottish Lochs that are just as far below sea level and extrapolate from there, except that we can’t because in those cold wet climates the low areas fill up with water and stay full.

9

You can’t possibly mean this… perhaps you mean the oceans don’t play any great role in creating oxygen and that they play a major role in replenishing oxygen. IIRC, about 60-80% of the atmosphere’s molecular oxygen is produced by photosynthesis in the world’s oceans.

That is, if you’re talking about molecular oxygen, but if you mean atomic oxygen, then I would say the ocean neither creates nor replenishes individual oxygen atoms.

What about Aerobic cellular respiration? Every oxygen-breathing creature on the planet uses O[sub]2[/sub] and tags those oxygen atoms onto H[sub]2[/sub]O.

10

Yes wevets I do mean that, because it’s an objective fact. 60-80% of the atmosphere’s molecular oxygen is produced by photosynthesis in the world’s oceans and 60-80% of the atmosphere’s carbon dioxide is produced by respiration in the world’s oceans. Every molecule of oxygen created is counterbalanced by a molecule of CO[sub]2[/sub] so there is no net gain.

I know your HS science taught you that plants create oxygen via PHS but what it didn’t teach you is that plants only use PHS to create sugars that they can burn via respiration to produce energy and CO[sub]2[/sub]. There’s no net gain. The only oxygen ‘produced’ is the relatively tiny amount that is deposited as oceanic sediments.

Respiration burns complex organic molecules created via PHS in the first place. There can never be any net change because the oxygen being consumed has already been paid for in advance by the oxygen produced during PHS.

Every oxygen-breathing creature on the planet uses O2 and tags those oxygen atoms onto H2O but they do that by combining the oxygen with complex organics that were produced by splitting O[sub]2[/sub] from H[sub]2[/sub]O in the first place. It’s simply a reversal of a reaction that has already occurred. It’s not using oxygen that then needs to be replaced any more than a person spending money is using notes that need to be replaced. The whole cycle is one big shuffling exercise in which no O[sub]2[/sub] is used.

11

Strongly recommended reading: Harry Turtledove’s Down in the Bottomlands is set in an alternate world in which the Mediterranean never filled; the protagonist is a scientist studying the unique fauna and flora of the dead sea bottom who is serving a turn as a tour guide in the park established to protect them.

A quite similar scenario is part of the tapestry woven by Julian May in her Pliocene Exile tetralogy, notably the first two books. Lester Del Rey also set a story in a similar scene. But the Turtledove book is the one that really pays attention to climate and lifeforms.

12

This is not true. Photosynthesis is not perfectly balanced by respiration.

For evidence: Dig down and you can find carbon sequestered by the producers including bacteria, algae, and plants (fossils, mud, peat, oil, limestone, etc…).

If a rose dies in the garden, it doesn’t vaporize into water and CO[sub]2[/sub]. You can still hold the flower and stem in your hands. The flower and stem are primarily made of cellulose, which is itself made of sugars. It would also not be wise to assume action by decomposers and consumers is 100% efficient. After all, it’s difficult to find any systems that are 100% efficient.

The biosphere produces some net oxygen through photosynthesis that is not counterbalanced by respiration.

A review of the [http://paos.colorado.edu/~fasullo/pjw_class/oxygencycle.html”]Global](”[url) Oxygen Cycle could be helpful.

So why does our atmosphere have a stable composition?

Because there are abiotic processes involved. Oxidation reactions can remove oxygen from the atmosphere without the mediation of living organisms. Processes like volcanism can add CO[sub]2[/sub] to the atmosphere.

The stability is a property of the entire system, not of individual components of the system such as the oceans.

How does this relate to the OP?

If you dry up the oceans, you have eliminated the lion’s share of the processes that contribute oxygen to the atmosphere, and the oxygen levels in the atmosphere could be depleted. See [http://paos.colorado.edu/~fasullo/pjw_class/oxygencycle6.html”]page](”[url) 6 of the previous cite.

To have oxygen content in the atmosphere remain the same, you would have to fiddle with other processes involved in the carbon and oxygen cycles, such as volcanism.

13

My apologies for poorly done url coding there. :frowning:

The links in my post above should be:

Global Oxygen Cycle

page 6

14

wevets I can only assume that you missed in my first post where I said “it’s not being used at any significant rate”. Sequestration in marine sediments isn’t occurring at any significant rate. More importantly it would be non-existent if the oceans dried up, which is what we are discussing. That would leave peat and coal formation, a trivial contribution. I won’t even get started on the lack of terrestrial plant growth if the oceans dried up.

No it couldn’t because you have also eliminated the lion’s share of the processes that remove oxygen from the atmosphere as I explained earlier.

That page you referenced is also very misleading in that it assumes that there are no abiotic processes creating oxygen which is not the case. In actual fact the rate of creation due to water destruction in the upper atmosphere would more than balance the rate of loss from the processes described there.

15

These stories were by Poul Anderson and have been republished as a short novel named New America which was also the name of the planet with the high pressure atmosphere.

16

I just disagree that the rate is insignificant. Every source I’ve seen takes a contrary position to yours.

For example:

(bolding mine) From The University of Hamburg Dept. of Biology.

1/2000th every year may appear insignificant (it’s only 0.05%), but on geological time scales this is huge. The key words are every year.

Perhaps my sources are wrong, but no offense, I would like to see cites of your own on which you base your position that atmospheric composition would not change.

Again, removing the huge amounts of photosynthesis (yes: and respiration, but as I have mentioned, respiration is a lesser contributor to atmospheric composition than photosynthesis) that take place in the ocean would have a drastic effect on the atmosphere. You’ve also mentioned that terrestrial plant life would suffer reduced growth if the oceans dried up, also further reducing the input of oxygen into the atmosphere. Presumably the decomposers and consumers would continue to survive off dead producer life for some undetermined small amount of time, continuing to respire. I still don’t see how you wind up with the same atmosphere.

17

Thanks, everyone. I can see I’ve hit a fertile territory, so to speak. :slight_smile:

My original idea was to have only the mountaintop oases, and nothing else but desert, but I may now have only a vast deset basin, and maybe an ocean on the other side of the planet…