Alternate explanations for Venus

[lomiller]

Actually, complete transparency makes the adiabatic lapse rate irrelevant. You get an adiabatic lapse rate when you get vertical convection, and to get any significant vertical convection you need to gain heat from below and lose it from above. You can gain heat at the ground from direct thermal contact, but you can only really lose it at the top from radiation. And if you're completely transparent, you don't radiate..

This would only hold true if the atmosphere was completely opaque to IR, because IR is emitted in a random direction which means large amounts are emitted towards the surface. So, once again we are back to the dependence on greenhouse gasses.

Sure, but you need to know how much atmosphere you have to calculate that second part. That's my point: more atmosphere means a hotter surface temp. Composition matters too, but if Venus had the amount of atmosphere we have, even with the composition it currently has, it would be a lot cooler than it is now.

What you need to know is how much greenhouse gas you have in the atmosphere, since non-greenhouse gasses don’t interact with IR by definition. If Venus has an equally thick atmosphere composed entirely of a non-greenhouse gas like Nitrogen (and the planets reflectivity was unchanged) it would be about the same temperature as the moon regardless of pressure.



Venus has many times more greenhouse gas in it’s atmosphere then the earth has atmosphere so obviously we couldn’t reach those same levels without more atmosphere which would result in more pressure, but pressure remains an ancillary effect, not a cause.

The surface of a planet most certainly CAN cool by convection. Below the point at which the atmosphere becomes opaque to IR, that can easily be the dominant cooling mechanism

Only if the top of the atmosphere is cooling by some other means. AFAIK convection IS the dominant cooling mechanism for the surface, nonetheless top of atmosphere radiative balance ultimately determines surface temperature.

That convection brings warm gasses up to high altitudes where it can then radiate heat into space, but the transfer of heat from the ground to the upper atmosphere very much involves convection. And since we're talking about surface temperatures, it's wrong to exclude convection from the discussion.

This gets you into territory where you need extremely complex models to resolve anything. Such models are well beyond anything we could hope to discuss here but we know what these models tell us and their results are in the same ballpark as the simple top of atmosphere calculation.

http://www.realclimate.org/index.php/archives/2007/08/the-co2-problem-in-6-easy-steps/

 

[Belz...]

I've encountered someone who doesn't buy the conventional explanation for why Venus has such hellish conditions. It apparently upsets him for some reason.

Maybe he doesn't accept global warming. Ask him if he believes the conventional explanation for why Mars has such barren conditions.

 

[Travis]

Maybe he doesn't accept global warming. Ask him if he believes the conventional explanation for why Mars has such barren conditions.

It was us arguing over the issue of global warming that led to the discussion of Venus and his declaration that he didn't think there was such a thing as a greenhouse gas.

But he is old so maybe a bit of whatthebutlersaw was saying is true too.

 

[Ziggurat]

This would only hold true if the atmosphere was completely opaque to IR, because IR is emitted in a random direction which means large amounts are emitted towards the surface. So, once again we are back to the dependence on greenhouse gasses.

It holds whenever radiation is not an effective method of heat transfer within the atmosphere. If the atmosphere is transparent across a part of the spectrum, then you get no radiative transfer within that part of the spectrum. If the optical depth is very short across part of the spectrum, you get no radiative transfer. So all we need is some segment of the spectrum to have a short optical depth (we can't have it completely transparent or no radiative loss at the top), everywhere else can be transparent.

And basically all atmospheres will be opaque to some segment of the spectrum.

What you need to know is how much greenhouse gas you have in the atmosphere, since non-greenhouse gasses don’t interact with IR by definition. If Venus has an equally thick atmosphere composed entirely of a non-greenhouse gas like Nitrogen (and the planets reflectivity was unchanged) it would be about the same temperature as the moon regardless of pressure.

Sorry, but that's simply not true. An all-nitrogen atmosphere on Venus would be significantly cooler than it is now, but it sure as hell wouldn't be the temperature of the moon.

Venus has many times more greenhouse gas in it’s atmosphere then the earth has atmosphere so obviously we couldn’t reach those same levels without more atmosphere which would result in more pressure, but pressure remains an ancillary effect, not a cause.

This is wrong. The total atmospheric mass is VERY relevant to the surface temperature. A Venus atmosphere with the same composition but less of it would have a cooler surface temperature. The argument that it's just a greenhouse effect doesn't hold, because the atmosphere reaches opacity far above the surface, but the temperature profile of the atmosphere below that is not constant. It's not close to constant. It's close to the adiabatic lapse rate, because convection matters. And if convection matters, the atmospheric mass matters.

I am NOT claiming that the greenhouse effect is irrelevant. It's not. Greenhouse gasses set the boundary conditions for the crossover between convective and radiative losses, and that directly impacts surface temperature to a very large degree. But greenhouse gasses alone do not, and cannot, account for the incredibly high surface temperature of Venus. The very high total atmospheric mass is also critically important. That effect can be seen rather easily here on earth with the temperature variation by altitude.

Only if the top of the atmosphere is cooling by some other means. AFAIK convection IS the dominant cooling mechanism for the surface, nonetheless top of atmosphere radiative balance ultimately determines surface temperature.

That's what I've been saying. And if convection matters, then the atmospheric mass matters. You cannot get Venus surface temperatures without a lot of atmosphere, no matter what you make it out of.

 

[Ziggurat]

Its quite obviously the Venicians who have mastered weather control and are conspiring to stop silly humans like ourselves ruining their planet by making it as inhospitable as possible for us.

I think you mean the Venusians. The Venicians (or more commonly, Venetians) can't even keep their streets dry.

 

[lomiller]

It holds whenever radiation is not an effective method of heat transfer within the atmosphere.

Which means it does not hold in cases where there are no greenhouse gasses in the atmosphere, making it completely irrelevant to any point you are trying to make.

The total atmospheric mass is VERY relevant to the surface temperature. A Venus atmosphere with the same composition but less of it would have a cooler surface temperature.

The same composition but less mass means less greenhouse gas. It should be obvious that if you want to show (as you are attempting) that its pressure not greenhouse gasses causing the change you need to hold the amount of greenhouse gas constant.

This is wrong. The total atmospheric mass is VERY relevant to the surface temperature. A Venus atmosphere with the same composition but less of it would have a cooler surface temperature.

With an all nitrogen atmosphere Venus would be sitting at approximately its blackbody temperature. After accounting for differences in albedo Venus receives similar amounts of energy per m^2 as the moon and would therefore be similar in temperature.

That's what I've been saying. And if convection matters, then the atmospheric mass matters. You cannot get Venus surface temperatures without a lot of atmosphere, no matter what you make it out of.

No, it really isn’t what you are saying. Surface temperature is set by what’s going on at the top of the atmosphere. Here’s another nice realclimate post of the subject.
http://www.realclimate.org/index.php/archives/2010/01/plass-and-the-surface-budget-fallacy/

The very high total atmospheric mass is also critically important. That effect can be seen rather easily here on earth with the temperature variation by altitude.

The only sense in which this is true is that CO2 can’t give you a greenhouse effect as strong as Venus without much more atmosphere then the Earth presently has. (There is enough Carbon and Oxygen for that to happen though)

 

[Zeuzzz]

Which means it does not hold in cases where there are no greenhouse gasses in the atmosphere, making it completely irrelevant to any point you are trying to make.

Do you frequently partake in global warming themed discussions here by any chance?

I swear that many pro anthropogenic global warming environmentalists tend to really overplay the significance of greenhouse gasses. No matter what planet, seemingly.

 

[Belz...]

It was us arguing over the issue of global warming that led to the discussion of Venus and his declaration that he didn't think there was such a thing as a greenhouse gas.

Of course. If he accepts what happened to Venus as true, it throws a wrench into his beliefs about Earth.

 

[Andrew Wiggin]

I think the conditions at the poles are not much hotter than the hottest locations on Earth where we find life. So life on Venus is not entirely ruled out, just very unlikely.

There's that whole sulfuric acid rain thing to contend with though. As you say, very unlikely. Might be a good place to land and put a (acid proof) habitat though, if we ever get around to manned interplanetary exploration.

 

[Ziggurat]

The same composition but less mass means less greenhouse gas.

Once you get below the optical depth, the composition is irrelevant.

If you added a completely transparent gas to Venus's atmosphere, you would increase its surface temperature. If you removed all the nitrogen from the earth's atmosphere, you would decrease its surface temperature. Composition matters, but so does atmospheric mass, even of the completely transparent kind.

It should be obvious that if you want to show (as you are attempting) that its pressure not greenhouse gasses causing the change you need to hold the amount of greenhouse gas constant.

But that's NOT what I'm trying to show. I've stated explicitly that composition matters. But so does total gas content. That is a point you have been denying, and you're simply wrong about it. BOTH composition and pressure (which is really just a measure of total atmospheric content) matter. Composition determines the boundary conditions at the point where the upper atmosphere becomes opaque in whatever region it absorbs in, and total content determines how far down you follow the adiabatic lapse rate below that. I've been saying from the start that both matter.

No, it really isn’t what you are saying. Surface temperature is set by what’s going on at the top of the atmosphere.

That isn't sufficient. There's a gradient between the top of the atmosphere and the ground. And that gradient is determined by the combination of the adiabatic lapse rate and the total amount of atmosphere below that top part. So pressure (a measure of the total amount of atmosphere you have) matters. Obviously. Pressure alone isn't enough to determine surface temperature, but you're simply wrong to claim it isn't a big factor. It's a HUGE factor.