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Preface: I am new to the forum so apologies if everything is not in the exact required format.

I know that the higher above sea level you go the atmospheric pressure decreases and the inverse is true when you go below sea level. I have looked for answers for my specific imagined scenario but cannot find anything.

My scenario is a large continent completely surrounded by mountains that are as tall as Mt. Everest. Let's say a disaster occurs and the sea level rises until it just below the peak of the mountains and doesn't spill over onto the surrounded continent below.

Clarification: Assume that the planet is the same size as earth with the same atmosphere, the only difference is it covered in about 70% water with the land all forming the one continent. The water comes from the melting of massive continental glaciers that have a large area and are very thick. Assume they are as large as needed to reach such a high sea level.

Given that before the sea level rose the atmospheric pressure at ground level was equal to 1, my question is Because of the sea level rising does the atmospheric pressure at ground level increase or not since the ground itself did not rise or fall?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild
    Jun 9, 2022 at 5:08
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    $\begingroup$ In the scenario described by your clarification, the answer is 'no' because the volume of water over the rising sea level is just a displacement of the ice in the glaciers, and thus there is no overall volume change in the system. The top of the atmosphere does not change. It is like the myth of the melting icebergs causing the sea level to rise. The melting icebergs, already in the ocean, add no new water to the ocean. Your clarification actually changes the question sufficiently that some answers are no longer valid - they depend on NEW water being added. $\endgroup$ Jun 9, 2022 at 15:15
  • $\begingroup$ (1) It considered rude to change the question invalidating existing answers. (2) The scenario set forth in the new version of the question is impossible. The glaciers would have to be at least 18 km thick, and you cannot have mountains of ice that tall on Earth. (Ice is nowhere near strong enough. The glaciers will flow over the surrounding mountains.) $\endgroup$
    – AlexP
    Jun 9, 2022 at 18:18
  • $\begingroup$ @AlexP This clarification was made by request. We wanted it clarified where the water came from. This information was necessary in order to give a valid answer. The clarification does NOT change the question, it constrains it appropriately. Those answering the question without waiting for this clarification did so at the risk of the clarification invalidating their answer. These answers were potentially invalid from the beginning, as some of us pointed out. Nothing rude about this clarification at all. Impossible or not, it is the scenario the OP established. It s obviously NOT Earth. $\endgroup$ Jun 10, 2022 at 18:30

5 Answers 5

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The line between a pure science question and a worldbuilding question and answer is sometimes very fine indeed. In worldbuilding, context and background are important. Something is done 'in a world'.

Let's start with pure science.

Scenario 1

Take a large sealed drum, on its end. Within that drum, siting upright on the bottom, is a much smaller diameter cylinder, Cylinder A, open at the top. We will call inside this small cylinder Region A, and outside this cylinder Region B. Region A has some earth on the bottom, to Level A with respect to the bottom. Region B has water also to Level A. The pressure at the surface of the water and the earth is the same.

Now, inject more water into Region B without breaking the seal so that the water level rises. This decreases the volume of air, so the air pressure goes up. The pressure at the surface of the water AND the surface of the earth is higher, even though the level of the earth did not rise. This pressure rise is entirely due to the compression of the air in the entire sealed drum.

Scenario 2

Same setup as scenario 1, but now add a second cylinder, Cylinder B, open at the top, in Region B. It is filled to the top with water, at a level much higher that the water in Region B. The pressure at the surface of the water in Region A and of the surface of the earth is the same.

Now, open a stopper in cylinder B so that the water in it flows into Region B. The water level in region B rises considerably, much higher than the level of the earth in Region A, but no new water is added. The existing volume of water is just moved around and redistributed. The total area for the air is unchanged. It is not compressed, and so the pressure does not change at the surface of the water or the earth. It is important to know if the additional water came from within the system, or from without.

Scenario 3

Same as Scenario 1, but the drum is not sealed at the top.

Now when more water is added to Region B, the water level in this region rises, but the air is just pushed out the top, the air is not pressurized. There is no change in air pressure at the surface of the water, nor at the surface of the earth. There is no compression. There is no additional pressure. It is important to know if the drum is sealed, or open so that the air can escape (into space?).

Scenario 4

Same as scenario 1, except that the smaller Cylinder A goes right to the top of the drum, and Region A is sealed separately from Region B, as well as the drum being sealed.

Now, when water is added to Region B to raise the water level, only the pressure in Region B goes up. The pressure in Region A is unchanged. The pressure at the surface of the earth does not change.

Scenario 5

The first four scenarios ignore gravity to a great extent, and thus the weight of the air causing pressure on the surfaces of the water and the earth. So this scenario is like Scenario 3, only let's add 'weight' to the water. In this scenario, the drum is VERY big, and is in a vacuum, but siting upright in a gravitational field. Gravity prevents the air from spilling over the top of the drum. In this scenario, Cylinder A goes to the top of the open drum. There is just enough 'air' in Region A and Region B to fill them up half way. Thus, air can not go between Region A and Region B ('over the top of the wall', as it were).

Adding water to Region B to raise the water level 'pushes' the air up in Region B, but does not compress it. The air is unconstrained, and can move up freely and unrestricted. The water level goes up higher than the level of the earth in Region A, but there is no effect on the volume or mass of air in Region A, and thus no pressure change at the earth level. There is only a very slight pressure decrease on the water, because now the air is further away from the center of gravity, so there is less 'pull' on it. Whether the additional gravity from the additional water compensates, is moot. There is no change in Region A.

Scenario 6

Same as Scenario 5, except that Cylinder A does not go all the way to the top, and there is enough air in the drum such that the air at the top of the drum can flow over the top of the cylinder between regions A and B. The top of the drum is still not enclosed, so the volume of air is unconstrained. The top of the air does not reach the top of the drum, so it cannot flow out of the drum.

Now, when water is added to Region B from outside the system to raise the level, the water displaces air further up in the drum. The pressure of the air does not change, as it is not constrained and there is no compression. The volume the air is contained in does not change, it just moves higher up in the drum. The top level of the air goes further up the drum, and can 'spill' over the top into Region A. There is no overall pressure change of the air, as it is not compressed. But now there is more air in the air column over Cylinder A (the height of the top level of the air has gone up) so there is more mass of air on the surface of the earth in Region A. The additional 'weight' due to gravity of the air in Cylinder A and Region A causes an increase in pressure at the level of the earth. Thus, it is important to know what the height of the atmosphere is, with respect to the height of the wall.

Also, considering Scenario 5 with this scenario, is there free movement of the air at the top of the wall between Region A and Region B? Are there weather patterns that prevent this? Like inversions, for instance. Is the air pressure already higher over the land than the water, due to other variables (local heating, atmospheric composition, air turbulence), that causes a local high pressure system preventing air movement from Region B into Region A?

Scenario 7

Same as Scenario 6, but now Cylinder B is added inside Region B, that holds the water that will flow into Region B (a frozen glacier that melts?).

As the water flows out of Cylinder B and into Region B, the water level in region B goes up, but the overall volume available for the air does not change. Air is not 'pushed up' in Region B, it just fills in the space where the water in Cylinder B came from. The overall height of the air in the drum does not change, and thus there is no change in height of the air in Region A, and thus no pressure change at the surface of the earth. It is important to know how the water that is used to raise the level of the water in Region B affects the overall volume available in the drum. Is the water just displaced from somewhere else, and thus the overall volume remains unaffected?

This is similar to the common misconception over melting icebergs causing the sea level to go up. If the iceberg is already floating in the water, its melting will not significantly 'add water to and raise the level of' the sea level.

Scenario 8

Same as Scenario 6, only there is a huge pool of hydrogen at the bottom of the drum, below the water in Region B and the earth in level A. The air is composed of oxygen.

This pool of hydrogen is released into the air in Region B, combines with the oxygen, and forms water. This water fills up Region B, and raises the water level. But this water is just replacing the volume taken up by the oxygen. In this scenario, the volume of air goes down, perhaps more than the volume of water goes up. So the air pressure actually decreases in the system. The air pressure over the water AND the earth goes down. It is important to know where the water comes from.

A bit of trivia, this was once the theory of where water originally came from on earth - hydrogen from volcanoes mixing with oxygen in the atmosphere - until the 'water from comets' theory gained dominance.

Scenario 9

Same as scenario 6, only the atmosphere is made of a gas that is very highly soluble in water. Ammonia, for instance.

Thus, when water is added to region B, the air is dissolved into the added water, the volume of air goes down, perhaps lower than it started out to be, and thus perhaps the overall height of the top of the atmosphere goes down. It is important to know the effects of the added water on the existing air.

Scenario 10

Same as scenario 6, only the air goes to the top of the drum.

When water is added to raise the water level, the air is pushed out of the drum completely and into space. Atmosphere is lost, as was the case on Mars. Thus, it is indeterminable as to what happens to the column of air over Region A, and thus the gravity-created pressure on the surface of the earth. It is important to know what happens to the air that is 'pushed up' or displaced by the rising water. Is it still constrained by gravity to the planet? If the water suddenly filled Region B with a 'whoosh', it is more than likely that much of the air was just 'blown off' the planet.

TL:DR

In worldbuilding, as opposed to a science lab demonstration, it is important to take into consideration the entire context of the world, and not to assume the perfect science lab conditions and control of all of the variables. The real world is very different from a contrived and proscribed science demonstration, and is full of 'uncontrolled unintended consequences'. Thus, taking a purely science approach to a worldbuilding question could lead to incorrect answers, that work perfectly in the laboratory but fail completely in the context of the world.

There is just not sufficient content, context, and criteria in the original question to assume only scenario 6 under the perfect atmospheric and climatic conditions applies. This question begs that the answer approaches the question from many perspectives, not just the purely laboratory response.

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This answers assumes that the new sea level is obtained by teleporting water from somewhere else. An edit to the question clarifies that the water was always there, in the form of ice. As such, the answer is obviously that the atmospheric pressure won't be changed significantly.)


Atmospheric pressure equals the weight of the column of air with a cross section equal to the unit area.

Since total amount of the atmosphere stays constant, and gravitational acceleration won't change significantly, the pressure at the new sea level will almost equal the pressure at the old sea level. (Not exactly, because now you have a reasonably large depression below the new sea level, which will contain a non-negligible amount of air, but close enough for government work, as they say.)

The pressure at the bottom of the depression will increase just like you expect. For example, on Earth, the atmospheric pressure at the bottom of an 8 kilometer deep depression would be about 2.4 atm (2400 hPa).

The main problem will be that the bottom of the depression will be verrry hhhot. The adiabatic lapse rate on Earth is about 10 °C (18 °F) per kilometer, meaning that temperatures on the bottom of the hole will easily reach 90 °C (200 °F).

The other main problem is that at such pressure (1) oxygen becomes toxic, and (2) photosynthesis doesn't work. (Even assuming that plants would somehow survive the heat.)

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  • $\begingroup$ The new sea level is at a higher altitude, which means greater diameter, which means greater circumference. The existing air would be spread over a greater distance. The atmosphere would not be as thick, and therefore the pressure would be less. $\endgroup$ Jun 7, 2022 at 23:47
  • $\begingroup$ And that much water would hold a LOT of oxygen, hydrogen, and other currently gaseous elements, thus decreasing the amount available to create the atmosphere. $\endgroup$ Jun 7, 2022 at 23:48
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    $\begingroup$ @JustinThymetheSecond: As I said, it's not exactly the same pressure but close enough. It always pays to estimate the importance of the changes before bringing them up as significant factors... The area at the new sea level would be a whopping 0.25% larger, that is, a factor of 1.0025. Surface gravity will be 0.12% lower. And water holds very little oxygen and nitrogen in solution, about 20 mg per liter total at normal atmospheric pressure; for comparison, a liter of air at normal pressure is about 1200 mg. $\endgroup$
    – AlexP
    Jun 8, 2022 at 0:40
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    $\begingroup$ @JustinThymetheSecond: Yes, it will change the density, by a factor of about 0.9975 * 0.9988 = 0.9963, or about 0.4% lower, or 757 mm Hg instead of 760 mm Hg, or 1009.5 hPa instead of 1013.25 hPa. The difference is about the same size as the ordinary fluctuation in pressure between night and day. (And forty times smaller than the pressure drop at the center of a hurricane.) $\endgroup$
    – AlexP
    Jun 8, 2022 at 0:52
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    $\begingroup$ Yeah, what matters is mostly "height above mean surface level" (which is what they use for other planets that don't have oceans, but in our case, ocean counts as surface whenever it's above the land). In this scenario the mean surface increases by almost as much as the sea level does, so the people end up several km below the mean surface, and the rest is all second-order stuff — yes, pressure goes up, by a lot. $\endgroup$
    – hobbs
    Jun 8, 2022 at 16:51
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Frame Challenge

There are a lot of unknowns in this scenario, that make the question extremely difficult to answer.

Climate

The amount of water vapor in the atmosphere would directly contribute to its density. It is hard to imagine this scenario without envisioning most of the surface becoming water, and thus most of the surface exposed to evaporation. This would increase the amount of water vapor in the air, and thus the water cycle would be drastically altered. The density of the atmosphere could not possibly be the same.

Also we absolutely know the effects on air pressure of tornados, hurricanes, and such. Extend this to a global scale, and it becomes difficult to predict. For instance, would there be a permanent high pressure or low pressure region over this area, caused by atmospheric turbulence, air temperature, the difference in temperature between the land and the water and such?

Vegetation

Currently, a lot of the makeup of our atmosphere is due to vegetation, and plant and animal respiration. Take away the land surface, you take away the flora and fauna. This would change the makeup of the atmosphere completely, and thus affect its density.

Atmospheric temperature

The hotter the air, the less dense it is. The cooler the air, the more dense it is. This is a factor in air pressure and storms. A planet almost completely covered in water would change its albedo, and thus the amount of sunlight reflected back into space. This would drastically change the temperature of the air.

Interesting, however, is that the air pressure at the top of Mt. Everest is 10% HIGHER in the summer time, when it is hotter, and with climate change and global warming climbers may not need supplemental oxygen at the crest.

Water from the current atmosphere

If all, or a lot, of the water came from the condensation of water vapor in our current atmosphere, and in the generation of water from the hydrogen in the ground and oxygen in the air, there would be a lot less atmosphere, and that would greatly affect the air density.

Absorption of atmospheric elements by the water

That is a LOT of water. It would hold a LOT of dissolved elements and gases currently in our atmosphere. This would drastically reduce the amount of atmosphere currently 'up there', and thus the air pressure.

Deflection of the Earth's crust

For sure, that much water would compress the earth's crust, thus lowering the current 'ground level' (the diameter of the earth's land surface). The land surface in your enclosed pit would certainly not be at the same elevation from the Earth's center that it is before all the water covered the surface.

Circumference of the new 'surface'

Essentially, you are changing the circumference of the 'surface' of the earth. Pushing everything 'up' as it were. At this new circumference, the existing atmosphere would be spread out a lot further, covering lot more surface, and this spread a lot thinner.

Also, the further away from the earth's center, the lower the gravity. The lower the gravity, the lower the air pressure at the new 'surface' level.

However, the greater the mass, the higher the gravity. If all of this water is new 'mass' to the Earth, there would be a definite change in the Earth's gravity, and thus the 'pull' on the atmosphere, and thus the air pressure.

Surrounded by water vs. earth

A mine sunk in the earth is, of course' surrounded by earth. This pit would be surrounded entirely by water. Water, of course, has very different heat retention properties than earth. The bottom of the ocean is COLD, just above freezing, because water is not a gas, it is not compressible. The ideal gas law does not apply. The bottom of a mine is HOT. Surround this enclave with very cold ocean water, and it is bound to have a significant cooling effect. The ocean would be a very large heat sink. This would cool the mountain rim around the pit, and thus the land within the pit. This changes the air pressure significantly. Thus, one can not compare the conditions of this pit to that in a mine. The surrounding containing temperatures are exactly the opposite.

This, of course, depends on the width of the mountain rim keeping the water out.

The Law of Unintended Consequences

A host of consequences that would be unforeseeable and unintended. Take, for instance, ... well, what part of 'unforeseeable' are you having trouble with? Take Venus, for example.

Some perspective

Data and statistics

from NASA

Terrestrial Atmosphere Surface pressure: 1014 mb
Surface density: 1.217 kg/m^3
Scale height: 8.5 km
Total mass of atmosphere: 5.1 x 10^18 kg
Total mass of hydrosphere: 1.4 x 10^21 kg
Average temperature: 288 K (15 C)
Diurnal temperature range: 283 K to 293 K (10 to 20 C)
Wind speeds: 0 to 100 m/s
Mean molecular weight: 28.97
Atmospheric composition (by volume, dry air):

Major      : 78.08% Nitrogen (N2), 20.95% Oxygen (O2), 
Minor (ppm): Argon (Ar) - 9340; Carbon Dioxide (CO2) - 415
             Neon (Ne) - 18.18; Helium (He) - 5.24; CH4 - 1.7
             Krypton (Kr) - 1.14; Hydrogen (H2) - 0.55 
Numbers do not add up to exactly 100% due to roundoff and uncertainty
Water is highly variable, typically makes up about 1%

If all the hydrogen in the atmosphere and earth combined with oxygen to form water, that is a substantial contribution to surface water compared to existing available water in the atmosphere. That is a lot of water. Note also that there is more mass in the water in the hydrosphere than there is mass of atmosphere. The current hydrosphere does not even come close to what it would be if water covered the Earth to a depth of Mt. Everest. It is estimated that up to two oceans' worth of water is locked away in sub-surface water. Only a portion of Earth's hydrosphere is on or above the ground.

Data and science behind Earth's barometric pressure and climate

What Is the Range of Barometric Pressure?

Record Barometer Readings The highest barometric pressure ever recorded was 32.01 inches. This reading was taken in Agata, Siberia, on December 31, 1968, during clear and extremely cold weather. The lowest known barometric pressure was recorded over the Pacific Ocean during a typhoon on October 12, 1979. The pressure was 25.9 inches.

That is a naturally occurring pressure difference of 25%, due entirely to climate effects.

Keeping all of these factors exactly the same stretches credulity, and would take a lot of handwaving to make them go away. Any answer would be assumptions surrounded by assumptions mixed in with more assumptions.

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Just regarding the air pressure I assume there will be not much difference for the region outside this continent:
taking earth as example. the surface of earth is ~30% land and ~70% water. if you rise all the landmass about 5000 meters (which is way more then the average single mountains) you have the mass of the air in that volume, which needs to distribute around the earth increasing the air over the whole planet about 1500 meters (30% * 5000m distributed to 100%).
That would increase the pressure like you descend from a 1500m mountain to see level. which is not much.

I think other effects (like described by @Justin-Thyme-the-Second) will be more dramatic.
Where does the water came from?
How would that mass redistribution change gravity?
Would that continent sink from that additional weight?

And it all would depend on the size of the planet and the distribution of earth and water.

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  • $\begingroup$ It was not specifically clarified in the question if the mountains were NEW mass, or pre-existing, surrounding the continent. If they were an additional mass they would indeed displace a considerable volume of air. They would have a considerable effect on wind patterns and differential air pressure between the land and the water before rising sea levels. These air and wind patterns and their effects on air pressure would be greatly disrupted if the 'other side' was now filled with water instead of air, and wind blew across at the same level instead of up the mountains from the ocean side. $\endgroup$ Jun 9, 2022 at 15:44
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All other things being equal, the atmospheric pressure at sea level will always be the same

As has been pointed out in other answers, for all practical purposes, the atmospheric pressure at sea level remains fundamentally the same regardless the rise or fall of said sea level when the average surface of the ocean is measured from the center of the planet. We can discuss things like the distribution of mass over the continent, the change in diameter (which also changes the surface area). Yada, yada, yada... but in the end, the change in air pressure is basically negligible.

Your real problem is how much you want to ignore "reality" with your protected continent

  • If the protected continent is deep, the air pressure on the floor will be enormous. This can be easily solved by making the protected continent not terribly deep (like 100 meters below sea level).

  • You have a massive problem with drainage. Because the continent is below sea level, it will become quickly submerged with everything from wind-blown waves over the peaks to rain. There's nowhere for the water to go. Yes, some of it will absorb into the strata below the continent — but that strata will be pressured to become saturated by the sea water (salty sea water...) pressing on the sides of the mountains. If you had a perfectly calm world, eventually water will seep through the mountain sides and fill the continental bowl with water (salty sea water...).

Which means that what you really want is a continent 100 meters above sea level. That way the water can drain and you have the ability to retain freshwater lakes, rivers, and a (very) small aquifer.

  • If that continent is too far below the peaks, you'll rarely see the sun. Your continent is surrounded by water, which means it's surrounded by humidity. If the peaks aren't very high above sea level (let's say the peaks are only 200 meters above sea level) then there's not really anything stopping clouds from cresting the peaks. But if the bowl of the continent is really low (let's say 2 km below sea level) then there's nothing to let the moisture out! The combination of higher air pressure, heat, and moisture would conspire to have, IMO, a fairly permanent cloud at or below peak level. No sun.

That's primarily solved by pushing the continental floor to above sea level, but that might result in a continental desert because land always heats up more than water....

There will come a moment in your worldbuilding when you need to decide whether or not to change the rules of your world from "scientifically plausible" to "what I want." Scientifically, you're not going to have a paradise in the situation you've described. The conditions will be harsh one way or another — and that's ignoring the climate problems caused by being a water world. If you want a paradise, choose to have it and ignore all of us.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild
    Jun 9, 2022 at 5:07
  • $\begingroup$ The clarification that this continent is 30% of the surface area of the planet effects your answer somewhat, and thus not a 'water world'. Same land mass as we now have. $\endgroup$ Jun 9, 2022 at 15:49

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