Tuesday, 23 April 2019

Atmospheric pressure and temperature, Earth v Venus

Radical Rodent linked to this article comparing Earth's and Venus' atmospheres in the comments to this post.

The upshot is, all you need to know to work out the temperature at any altitude above any planet* is two things:
a) its distance from the nearest star ('the Sun' in the context of our Solar System), and
b) the atmospheric pressure at that altitude.
c) you do not need to adjust for, or even know, the 'albedo' of the planet, the composition of the atmosphere or anything else.

This fairly recent article by completely different authors compares all planets in the Solar System which have a proper atmosphere and comes to the same conclusion. Our physics teacher mentioned this theory in passing when I did O-Level physics way back when, and I have seen it mentioned every now and then since, so I assumed it was just an accepted scientific theory (or 'consensus' in NewSpeak). I'm not aware it's ever been seriously challenged in itself, but it would appear that it has been simply written out of history.

But Earth (atmosphere = 0.04% carbon dioxide) is of particular relevance because we live on it, and Venus is a useful comparison because its atmosphere = almost 100% carbon dioxide, which we are told causes 'a runaway greenhouse effect', plus the data is going to be most reliable, having been actually measured from up close, so let's focus on those two.

The results in the first article seemed suspiciously accurate, so I used other sources for the temperature v pressure gradients for Earth and Venus from here and here, made the 'divide by 1.176' adjustment for Venus (to compensate for the fact it is closer to the Sun) and knocked up a chart in Excel.

The tried and tested theory seems to stack up to me:

* Forgive the clunky wording, but Earth is clearly a (solid) planet with an atmosphere; while the gas giants are atmospheres without an actual planet.

12 comments:

Matt said...

I propose the problem with the Harry Dale Huffman is that the comparison between Earth and Venus is based on current conditions.

Venus is supposed to have had significant water content in the past which contributed to the heating effect (runaway greenhouse warming) but has subsequently been lost.

So it's not that the current amount of CO2 on Venus is important.

See http://www.nicholasmehrle.com/res/greenhouse/greenhouse.html for some background.

Mark Wadsworth said...

M, that's s fantastic bit of circular logic. And all of a sudden, CO2 doesn't matter. Well, well.

Matt said...

Don't have a dog in the race on this. I think there is climate change but I don't really care what the cause is.

If it is humans then I'm not buying the green idea (espoused by the Extinction Rebellion fanatics) where we regress to living in mud huts and eating turnips grown in our gardens and fertilised with our own excrement!

Tim Worstall makes some well reasoned points around the economics of climate change and how they should follow the business cycle to deliver maximum utility (rather than trash existing investments). He also makes the point that the forecasts are based on worst case models/data and that the changes already made mean these will NOT happen.

Back to this, the important part of the article I linked is the Kombayashi–Ingersoll limit which looks at what the maximum solar flux a planet can handle without a runaway greenhouse effect setting in. (https://en.wikipedia.org/wiki/Kombayashi%E2%80%93Ingersoll_limit). Note this is from 1967 well before any of the current AGW bandwagon got going.
The paper (https://journals.ametsoc.org/doi/pdf/10.1175/1520-0469%281969%29026%3C1191%3ATRGAHO%3E2.0.CO%3B2) looks at the possible effects of water on Venus (Section 4) and concludes that a runaway green-house regime is possible for a planet with oceans at the orbit of Venus.

The paper also references Manabe and Wetherald (1967) which looks to conclude the time evolution of the temperature and water vapour content of the Earth's atmosphere and their solutions tend to approach a stable equilibrium.. The paper (https://journals.ametsoc.org/doi/pdf/10.1175/1520-0469%281967%29024%3C0241%3ATEOTAW%3E2.0.CO%3B2) also touches on many of the arguments around pressure/temperature and albedo but I admit I've only skim read it.

Don't know if these works have been superseded by later papers either.

Mark Wadsworth said...

M, but that's the point. There is no "runaway greenhouse effect" on Venus. The surface is so hot because it's 90x atmospheric pressure and it's one-third nearer the sun.

Matt said...

I agree - there is no runaway greenhouse effect on Venus now. The temperature is dictated by the incident of solar radiation. Doesn't mean it didn't happen before in the presence of water however!

That's the difference between Earth and Venus now - we have a lot of water (which is the strongest greenhouse gas - at least in vapour form).

Maybe that's the reason Earth is in danger of reaching the Kombayashi–Ingersoll limit. If we do, we'll end up like Venus now.

Mark Wadsworth said...

M, that's still not the point. The point is, the composition of the atmosphere does not matter, only atmospheric pressure and distance from sun. There is no K-I limit.

Matt said...

Why do you say there is no Kombayashi Ingersoll limit? Because Huffman found a planet (Venus) that gave a consistent result for pressure/temperature against solar influx?

He didn't get the same results from other planets and moons in our solar system. Just said that he didn't know why they weren't the same.

Mark Wadsworth said...

M, did he say that? I'll have to read it again.

Anyway, see next post, there is a very clear relationship, if you adjust for atmospheric pressure.

Matt said...

From http://theendofthemystery.blogspot.com/2015/05/venus-no-greenhouse-effect-comments.html?showComment=1479204036579#c670536778750419272:

If I knew why that simple Jupiter comparison doesn't fit, of course, I would long ago have informed everyone of it. I have contented myself with saying the results of a simple comparison of the other planets with the Standard Atmosphere is not definitive as it is with Venus.

And from the original (http://theendofthemystery.blogspot.com/2010/11/venus-no-greenhouse-effect.html):

I don't know why the comparison falters slightly between 600 and 300 mb, or why it improves suddenly at 200 mb.

His hypothesis/model doesn't even seem to fit Venus all that well. Sure, you can argue that the input data introduces errors but that should have spurred Huffman to look at where this came from.

In fact, having thought about it some more, he has basically cherry picked his data to fit an assumption.

We know how a perfect black body should react to heating. The next closest to this would be a body with a uniform atmospheric make-up (like Venus where it is almost all CO2). The further you get from this the worse the fit becomes.

That's why IMHO the other planets/moons don't fit. His model is too simplistic but instead of refining it he has chosen to ignore the problem.

Mark Wadsworth said...

M, you are right. His model doesn't seem to extend to other solar planets. A

I also suspected he cherry picked data, which is why I got them from other sources, which happen to support the theory even better.

Bayard said...

"Maybe that's the reason Earth is in danger of reaching the Kombayashi–Ingersoll limit. If we do, we'll end up like Venus now."

Where do we get 89 times more atmospheric mass from? That's a fsck of a lot of mass: 5.1480×10^18 kg x 89 = 4.58 x 10^20 kg. The whole planet has a mass of 5.972 × 10^24 kg.

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