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Apologies; it just didn't seem like an appropriate question for Earth Science or Biology. Let me know if I'm incorrect. Maybe Physics?

If all the ice, permafrost, ice caps, etc. melted due to global warming, and became water, would there be a significant, or even deadly (to us) change in the pressure of the air? I kind of recall something like this was what allowed larger animals to thrive millions of years ago.

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Mikey
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    I'd be inclined to post this on Physics, though it could also fit in here. – ArtOfCode Dec 22 '14 at 12:46
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    Given that ice is less dense than water - any increase in pressure - if at all - would be negligible. The change in sea level atmospheric pressure could be as much as 2 meters! (From the rise in sea level) – Scott Downey Dec 22 '14 at 13:35
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    Related question: http://worldbuilding.stackexchange.com/questions/4982/if-the-north-and-south-poles-ice-melted-instantly-what-would-be-the-effect-on/4983#4983 – James Dec 22 '14 at 14:47

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Nothing would happen due to the ice caps melting. However, the increase in temperature that would be required to cause the ice caps to melt would result in an increase in pressure.

How much of an increase in pressure? Well, for a simplistic model, let's consider the ideal gas law: $PV = nRT$.

What this says is that, for a closed volume, the increase in gas pressure is proportional to the increase in temperature. This is temperature measured in Kelvin, so even a change of 10 C would lead to an increase in pressure of only around 100*(310/300 - 1) percent, or around 3%. (Based on an initial temperature estimate of around 30 C). Our atmosphere can also expand due to heat, so this would be an upper bound, not an absolute.

Melting both ice caps would lead to a sea level rise of around 70m, which would inundate many low lying areas and increase the surface area of the oceans, so we might have a bit more water in the atmosphere as well, leading to a slightly higher pressure, but the major change would be from the increased temperature.

Historically, large creatures are predicted to have flourished in an atmosphere which wasn't significantly higher pressure than ours, but rather had a higher proportion of oxygen in it. (Around 35% at max). This is hypothesized as the reason why arthropods may have grown so big, though the theory is somewhat contentions. The largest dinosaurs lived in a climate with an atmosphere similar to our modern one.

ckersch
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    OTOH, the rise in temperature is also a rise in average speed of the air particles. Faster air means more molecules reaching escape velocity. Which means a loss of pressure. I don't know enough to run the numbers to see which effect would overwhelm the other. – Shane Dec 22 '14 at 19:24
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    A temperature rise at the bottom of the atmosphere of around 10 degrees is unlikely to significantly affect thermal atmospheric escape. Upper atmosphere temperatures are mostly driven by direct solar heating, and are on the order of thousands of degrees. Most atmospheric species that undergo significant loss are also not found in the lower atmosphere, where greenhouse heating mostly takes place. – ckersch Dec 22 '14 at 19:38
  • Out atmosphere isn't exactly a closed volume, so your calculations have little meaning. –  Dec 23 '14 at 09:31
  • I'd expect it to be an upper bound, not an absolute increase, but I'd expect some portion of that to go into increasing pressure. – ckersch Dec 23 '14 at 15:04
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I have never heard of any pressure changes that would happen because of the melting of the icecaps.

However, if the atmospheric pressure did increase significantly it could help melt the icecaps, both the pressure and the thickening atmosphere would transfer energy faster.

bowlturner
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