Discussion on Evolution of Venus Temperature & Climate in the Context of Global Warming
Wolfgang G. Gasser
Original discussion – Comments or criticism can be posted there or sent to: info@pandualism.com
It is often claimed that Venus surface temperature as high as 450 - 500 degrees Celsius suggests a carbon dioxide greenhouse effect (around 96.5% of its atmosphere consists of CO2). However, if one deals with the question in an unprejudiced, honest way, then the hypothesis of such a greenhouse effect being the culprit of the high surface temperature becomes completely untenable.
What is called greenhouse effect on Venus would be quite different from both the original greenhouse effect and the global-warning greenhouse effect.
The original greenhouse effect:
The sun heats the ground of a greenhouse, the ground heats the air, and the glasses prevent the hot air from flowing outside the greenhouse
The global warming greenhouse-effect:
Radiation from the sun heats the earth, and a corresponding amount of energy is lost as thermal radiation to outer space. An increase in CO2 significantly reduces such thermal losses (outgoing radiation), whereas it does not reduce significantly the absorption of incoming radiation.
A Venus greenhouse-effect would work in this way:
Although only a small proportion of radiation from the sun reaches the ground of Venus, carbon dioxide is assumed to somehow heat up the ground.
The true reason of the high temperature on Venus is much simpler:
The very dense atmosphere of Venus has prevented the planet's surface from cooling down after its formation to a temperature similar to thermodynamic radiation-equilibrium.
So what is called greenhouse effect in the case of Venus is nothing more than atmospheric heat insulation over hundreds of millions of years. Gases are excellent insulators.
And if it were possible to cool down the whole planet Venus to zero degree Celsius, its temperature would remain near water freezing point over millions of years.
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Atmospheric carbon dioxide per square centimeter: on Venus 100
kg - on Earth 0.6 gram
wogoga in #1:
Although only a small proportion of radiation from the sun reaches the ground of Venus, carbon dioxide is assumed to somehow heat up the ground.
dasmiller in #8:
If the sunlight isn't reaching the ground because it's being absorbed by the atmosphere, then the atmosphere is being directly warmed by the sun. I don't understand the "somehow" remark - are conduction and convection not well-understood?
The question is, whether Venus surface got heated up from the sun (as suggested by the greenhouse effect ideology), or simply remained hot because of a heat flow from inside (as suggested e.g. by common sense).
It is obvious that core, mantle, and crust are hotter than the surface. The atmosphere however is colder, continuously decreasing from the surface temperature of more than 450 °C next to the surface, to around -30 °C at the upper cloud deck in 60-70 km altitude (Source).
Do you actually suggest that the atmosphere (warmed by the sun) can by conduction and convection increase surface temperature beyond its own temperature?
In Greenhouse Effect and Radiative Balance on Earth and Venus, page 4, we read:
o Only ~2.6% of the solar flux incident at the top of the atmosphere reaches the surface
o Solar flux at the surface is ~17 W/m2 (global avg.)
o Surface temperature of ~730 K maintained by an efficient atmospheric greenhouse mechanism
o Net downward thermal flux at surface ~15,000 W/m2
It seems rather astonishing to me that as much 2.6% of solar radiation should reach the surface, despite the mass of Venus' atmosphere corresponding to a mass of more than 1 km depth of water (more than 1000 metric tons per square meter). For comparison, at 100 m underwater the light present from the sun is about 0.5% of that at the surface. (Source)
In any case, how "an efficient atmospheric greenhouse mechanism" should somehow transform a "solar flux at the surface" of around 17 W/m2 into a "net downward thermal flux at surface" of around 15000 W/m2, will probably always remain a complete mystery.
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Ideology driven science is rather rule than exception
wogoga in #1:
So what is called greenhouse effect in the case of Venus is nothing more than atmospheric heat insulation over hundreds of millions of years. Gases are excellent insulators.
ben m in #7:
Nope. First of all, gases are excellent insulators against conduction and only conduction. Given convection and advection, over large distances they're actually worse insulators than solids.
Do you know evidence of significant vertical convection on Venus?
In heights where atmospheric temperatures are (significantly) influenced by the sun, there is strong horizontal convection (zonal circulation, transporting heat between the day and night sides of the planet). At lower than 50° latitudes, such east-west winds decrease from around 100 m/s at 60-70 km height to less the 1 m/s near surface. (Source)
Heat is also transported from the equator to the poles. However, near surface, such heat exchanges are not needed, as the temperature there does not depend on sun radiation, but on the iso-thermic heat from within the crust.
And they're no insulation at all against radiation, except in the normal atmospheric-greenhouse-effect effect sense.
Also liquids and solid are "no insulation at all against radiation, except in the normal" not-being-transparent "sense".
To characterize a well-known normal physical principle by a trendy ideological concept such as greenhouse-effect seems rather problematic to me.
That Venus may seem hotter than expected results primarily from the fact that planets with atmospheres have no clearly defined surfaces.
A black body with Venus' surface temperature (~740° Kelvin) radiates around 17000 W/m2, whereas a black body with a temperature of Venus' upper cloud deck (~240° Kelvin) less than 200 W/m2.
If we defined the boundary between Jupiter's atmosphere and Jupiter's fluid interior as Jupiter's surface, then we also could explain its high temperature by a super greenhouse effect.
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Ideology driven science is rather rule than exception
26th June 2010 #48
Wangler in #41:
If you take a look at Venus' atmosphere at a location (height) where the atmospheric pressure is similar to earth's atmospheric pressure, the Venusian temperature is higher, at least for those regions of each atmosphere where the temperature change is approximately linear with altitude change.
Isn't this higher temperature at points of similar pressure the true indication of differences in solar input and greenhouse gas effects for the two planets?
According to Wikipedia, on Venus at 50 km height, atmospheric pressure is 1.066 bar and temperature is 75°C.
Thus, at a pressure of around 1 bar, we have a temperature of 288°K on Earth and 348°K on Venus.
Venus receives around 1.9 times more radiation per square meter than Earth. As the power of thermal radiation is proportional to the fourth power of temperature, a temperature 1.18 times higher (fourth root of 1.9) results in 1.9 higher infrared emissions, thus remaining in equilibrium with the 1.9 times higher incoming radiation. If we multiply the 288°K of Earth by 1.18, we get 340°K for Venus, not far away from the above referenced 348°K.
Greenhouse-effect supporters will argue: As Venus reflects much more of the incoming radiation, the radiation energy absorbed by Venus is similar to the energy absorbed by the Earth. Thus, the fact that the incoming radiation is 1.9 times more powerful on Venus is not relevant, and the higher temperature of Venus at 1 bar is evidence of a greenhouse effect.
However such reasoning in favor of a greenhouse effect is exactly what I criticize as ideologic:
The fact that the clouds (at 60-70 km height) interact with incoming radiation is explained by normal physics. Yet an analogous interaction of outgoing radiation is explained by special (i.e. greenhouse-effect) physics. At least on Earth, clouds significantly slow down cooling at night.
(I do not call into question the physical principles of the greenhouse effect. Like solids and liquids, also gasses have "colors", determining the interaction with radiation. A change in the composition of an atmosphere can make it "darker" in the infrared, whereas its transparency in the visible and ultraviolet is not (significantly) affected.)
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An ideology
can be thought of as a way of looking at things, a set of
ideas proposed by the dominant class of a society to all members of this
society. The main purpose behind an ideology is to offer change in society, and
adherence to a set of ideals, through a normative thought process.
One billion malnourished humans and all the focus on climate change! Perverted!
Wangler in #50:
At similar atmospheric pressure, the difference in temperature is about 60 Kelvin.
What I consider relevant is atmospheric mass, not weight or pressure.
Venus gravity is only 0.904 g. So we must choose for Venus a height where pressure is 0.904 times lower than on Earth, in order to get the same atmospheric mass per square meter as on Earth. If we take further into account that average ground level on Earth is around 250 m above sea level (Source) with a pressure reduced by around 0.97 with respect to sea level, then the concerning height on Venus (Table) is 53.6 km (instead of 55 km) and temperature is 62°C (instead of 75°C).
This would mean that at similar atmospheric mass per surface, the difference in temperature is only 47°K (288°K on Earth, 335°K on Venus, where 335°K < 1.18 ∙ 288°K).
If you account for albedo, cloud effects and other effects, you possibly will be left with effect due to greenhouse gases alone. That remaining effect is most likely not zero.
The high albedo on Venus is due to its clouds. From the fact that Earth satellites cannot look through clouds in the infrared (e.g. temperature measurements of the oceans), we can conclude that clouds on Earth are not only a barrier for incoming but also for outgoing (thermal) radiation. Is there any evidence that the opaque sulfuric acid clouds on Venus affect outgoing thermal radiation significantly less than incoming radiation from the sun?
This get's back to Ziggurat's point: a primary reason Venus' atmospheric temperature at the planet surface is so high is because of the higher atmospheric pressure.
Ultimately, me too [like e.g. Iomiller in #25], I consider lapse rate rather an effect of surface temperature than a cause of it. Otherwise, (as far as I can see) I would have to retract this statement of #1:
And if it were possible to cool down the whole planet Venus to zero degree Celsius, its temperature would remain near water freezing point over millions of years.
---
The next glacial seemed rapidly approaching, when paleoclimatologists
met in 1972 to discuss this issue (a period of so-called global cooling). The
previous interglacial periods seemed to have lasted about 10,000 years each.
Assuming that the present interglacial period would be just as long, they concluded,
"it is likely that the present-day warm epoch will terminate relatively
soon if man does not intervene." (Quaternary
glaciation)
3rd July 2010 #55
wogoga in #52:
What I consider relevant is atmospheric mass, not weight or pressure.
CapelDodger in #54:
Why, exactly?
I explain the high crust surface temperature of Venus by atmospheric insulation from a colder environment, and therefore the decisive parameter is a form of quantity (mass, thickness), and not pressure of the atmosphere.
wogoga in #52:
The high albedo on Venus is due to its clouds.
CapelDodger in #54:
Yes, and irrelevant. Solar radiation that is reflected away from Venus does not influence its temperature.
Albedo due to clouds may be irrelevant in your prejudiced, ideological thinking (see #48).
An informative quote:
"The effect of clouds depends upon their type and the time of day. The more interesting and important type is the low thick clouds. At night the reflection effect is zero so the greenhouse effect and reflection of thermal radiation dominate and the low thick clouds have a warming effect. One can easily see that the reflection of thermal radiation is far more important than the greenhouse effect. The greenhouse effect could at most return 50 percent of the outgoing radiation back to the Earth. Reflection from the underside of clouds probably returns 90 percent of the radiation. The two effects are not in competition. Clouds could return 90 percent from reflection and half of the unreflected 10 percent. Thus it is easy to see why there is such a difference in temperature between a clear night and a cloudy night in the winter. Since the greenhouse effect from the atmospheric gases would be the same on a clear and a cloudy night one could say that the effect from greenhouse gases is negligible compared to the effect of low thick clouds."
wogoga in #52:
Otherwise, (as far as I can see) I would have to retract this statement of #1:
And if it were possible to cool down the whole planet Venus to zero degree Celsius, its temperature would remain near water freezing point over millions of years.
Ziggurat in #53:
Then you better get ready to retract that statement, because it's NOT simply a function of surface temperature, as I detailed here.
The surface of Venus does receive some heating from the sun. But if the surface is frozen, then it won't lose much heat from radiation, and it will lose NO heat from convection (which it currently does, which is why the adiabatic lapse rate matters). So it won't need a lot of heating to unfreeze it, and it won't last close to a million years at that temperature.
The blackbody temperature of Venus is around -40°C (Source), resulting a thermal emission of 163 W/m2. Venus obviously also absorbs (nearly) the same amount of sun radiation.
(Solar irradiance: 2614 W/m2, mean irradiance over the whole sphere: 1/4 ∙ 2614 W/m2 = 653.5 W/m2, not reflected: 25% ∙ 653.5 W/m2 = 163 W/m2)
The -40°C can be seen as the temperature of an averaged thermal-emission-surface of Venus around 70 km above crust surface. The thick atmosphere below is able to insulate the more than 450°C hot crust surface from this -40°C cold radiation-surface.
And now you tell me, that such a -40°C radiation-surface could thermally not be as well insulated from a crust surface of 0°C as from a crust surface of more than 450°C!
CapelDodger in #58:
Infra-red (long-wave) radiation is not reflected by anything in Earth's atmosphere (nor Venus's, for that matter).
Do you have evidence for the non-reflectivity of atmospheres based on more than wishful thinking?
A simple radiative transfer model demonstrates that multiple reflection of thermal radiation between the atmosphere (including clouds) and the solid surface has a significant influence on the observed radiance under the condition of Venus, where reflectivity of overlying atmosphere and clouds is high.
From Warming Early Mars with Carbon Dioxide Clouds That Scatter Infrared Radiation:
Model calculations show that the surface of early Mars could have been warmed through a scattering variant of the greenhouse effect, resulting from the ability of the carbon dioxide ice clouds to reflect the outgoing thermal radiation back to the surface.
From Thermal radiation fluxes in the lower atmosphere of Venus:
It is found that with an H2O content of about 0.00001, the fluxes may agree if the clouds reflect more than 60% of the thermal radiation incident on them.
CapelDodger in #58:
You quote from someone who lacks some very basic understanding.
It doesn't matter whether somebody has a degree in a field. What matters is knowledge and scientific (logical, consistent, critical, skeptical) reasoning. I like Thayer Watkin's articles, because (unlike the results of non-transparent computer simulations, in which one only can believe or not) he uses interesting, concrete, transparent lines of thought, which I can judge for myself.
A statement of Thayer Watkins which could turn out correct in the long term:
A small change in cloudiness over the rest of the Earth's surface can be far more important than major changes in the area of the ice caps. It is important to keep such things in perspective. Climate modelers have a distinct tendency to focus on a sensational minor topic while neglecting the major topics of climate. Clouds and cloudiness are the major factors in the Earth's climate. Clouds rule the Earth's climate. Everything else, including the atmospheric greenhouse gases, is marginal.
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1) Define the problem as apocalyptic because apocalypse sells.
2) Present the apocalyptic vision as mainstream view, and dissenters as crackpots or in the pay of evil giant corporations.
3) Build massive financial support.
4) Use that lobbying support to fight the dissenters and to expand the political, economic and scientific power of the new ideology. (adapted from)
By the way, clouds do reflect (or scatter back) thermal radiation. A quote from Longwave Multiple Scattering in Clouds: Climatic Implications:
"In most global climate models, the multiple scattering of longwave radiation by particles like clouds has been neglected. As the single scattering albedos of both ice and water clouds are between 0.4 to 0.7, the multiple scattering of longwave radiation by cloud droplets increases the effective absorption path length."
Or from Longwave multiple scattering by clouds:
"… and the albedo R increases monotonically with optical depth. Of note in the thermal part of the spectrum is the rather large reflectivity exhibited in the 10 – 20 μm region by the smaller size (5μm) particles which shifts toward longer wavelengths for larger (25μm) particles. Accordingly, a substantial amount of longwave radiation is reflected by clouds reducing the cirrus emissivity at these wavelengths by a (1 – R) factor."
macdoc in #60:
Do you understand the difference between inside the box reflections which you discuss and albedo reflection in the long wave IR range?
Let us assume a planet with fully transparent atmosphere and a universal cloud deck. Then a thermal-radiation cloud-albedo of 100% would mean that the cloud deck reflects (i.e. scatters back) the whole thermal surface radiation, resulting in no heat loss of the surface by thermal radiation.
And a cloud-albedo of 70% would mean that at every moment, 70% of the amount of emitted thermal radiation comes in from the cloud deck. This obviously entails that the heat loss of the surface by thermal radiation is reduced by 70%.
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Sometimes the wrong is more instructive than the right
wogoga in #55:
And now you tell me, that such a -40°C radiation-surface could thermally not be as well insulated from a crust surface of 0°C, as from a crust surface of more than 450°Celsius!
ben m in #56:
OK, I'll tell you: A -40 C surface can be in perfectly good convective-thermal contact with a 450 C surface if they are at different pressures. In fact, the laws of thermodynamics tell you that convection between regions of different pressure will give them different temperatures.
I have thought a lot about this comment, but it doesn't seem relevant to the question whether a much smaller lapse rate (starting with a ground temperature of 0°C) could remain stable on Venus over millions of years.
I fully agree with what you wrote in #36 (emphasis mine):
"Regarding the adiabatic lapse rate: there are thermodynamic assumptions that go into that calculation. One of them is that there are adiabatic vertical air currents; the calculation that gives you the lapse rate is basically the calculation of how much a parcel of (high-altitude, low-pressure) air will heat up when descending and being compressed, or vice-versa. If air is moving vertically, then you will have this heating effect from standard textbook thermodynamics."
"But note that this is not the only way for an atmosphere to behave. An isothermal atmosphere is also a perfectly good solution to the thermodynamics. The stratosphere is another perfectly good solution."
On Earth we obviously have strong "adiabatic vertical air currents", because our atmosphere is heated up from the ground, and ground temperatures vary widely depending on several parameters such as day-night-cycle, latitude, change of seasons, reflectivity of the ground, or weather.
The near-ground atmosphere of Venus however consists of isothermal layers, where temperature essentially only depends on the distance from the isothermal ground. And in such a horizontally isothermal atmosphere, there is nothing which could give rise to adiabatic vertical air currents of significant amount.
To sum up: You consider both the current Venus lapse-rate of around 10°C / km and an isothermal atmosphere (with a lapse rate of zero) as possible. So why do you oppose my claim, that also a lapse rate in between would be similarly stable?
Ziggurat in #57:
You seem to be having some problems understanding the concept of energy flow. The surface gets some small amount of heating from solar radiation. Even with a small amount of heating, the surface must lose energy to stay at a constant temperature. And it must do so at the same rate that it gains energy from solar radiation.
The average solar flux reaching the ground is around 17 W/m2 (ref. of #10). The black-body temperature of 17 W/m2 is only -140°C. The ground however is at +470°C (corresponding to 17000 W/m2).
If we assume an emissivity factor (over the corresponding spectra) of 60% for both ingoing solar and outgoing thermal radiation, then absorption of 10 W/m2 is confronted with emission of 10000 W/m2 [instead of 17 vs. 17000].
Thus, the assumption of a net radiation energy flow to the hot ground from the cold clouds seems completely absurd to me, or do I overlook something?
Think also about Venus' poles where the solar flux is virtually zero all the time. Ground temperature there is essentially the same as on the equator.
wogoga in #70:
On Earth we obviously have strong "adiabatic vertical air currents", because our atmosphere is heated up from the ground, and ground temperatures vary widely depending on several parameters such as day-night-cycle, latitude, change of seasons, reflectivity of the ground, or weather.
Ziggurat in #72:
Same thing happens on Venus.
If you think you are right, then you should be able to detail.
"Only about 11% of the solar radiation absorbed by the planet reaches the surface, and most of it is taken up in the clouds at altitudes of 60–70 km." (Source)
"The movement of super-rotation starts around 10 km of altitude, develops regularly up to 65 km, where it reaches a speed at the equator of about 540 km/h, to decrease and cancel themselves around 95 km." (Source)
Ziggurat in #74:
You overlooked the fact that the cloud layer also emits/reflects radiation downward.
As to your "cloud layer emits":
The temperature of the cloud layer (at a height of 60 to 70 km) is in the order of 30°C below zero, yet ground temperature is 470°C. A heat transfer from the colder to the hotter is thermodynamically impossible, also in the case of thermal radiation.
By the way, if the atmosphere under the cloud deck were transparent for thermal radiation, as you suggest, then the ground would lose substantial heat to the clouds (i.e. there would be a strong thermal radiation flow from 470°C to -30°C). This would entail a significant increase in the thermal-emission-surface temperature (see #55) of Venus. As higher thermal-emission-surface temperature (corresponding to higher blackbody temperature) leads to higher thermal emissions of the planet, significant cooling of the crust surface would be an inevitable outcome.
As to your "cloud layer reflects":
If only 1% of the 17000 W/m2 blackbody radiation (i.e. 170 W/m2) were emitted by the ground to the clouds, and as much as 90% of this emitted thermal radiation came back to the surface, then the resulting heat loss of 17 W/m2 would already be as high as solar radiation reaching the ground on average.
We conclude: In the same way as a sea bed below tens or hundreds of meters of water does not significantly interact via thermal radiation with the atmosphere, the crust surface of Venus does not significantly interact via thermal radiation with its higher atmosphere.
The pressure found on Venus's surface is high enough that the carbon dioxide is technically no longer a gas, but a supercritical fluid. The density of the air at the surface is 67 kg/m3, which is 6.5% that of liquid water on Earth.
That 17 W/m2 is not the total amount of power that gets absorbed by the ground, and it's not the total power that gets emitted from the ground, it's the difference in the power the ground emits (through radiation and conduction/convection) and the power the ground absorbs from the atmosphere.
If the dogma of a runaway greenhouse effect on Venus had something to do with reality and science, then something similar to what you write here were actually a correct scientific conclusion.
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If greenhouse-effect science concerning Earth is as
catastrophically biased, unscientific, and illogical as the one concerning
Venus, then we should remain very skeptical
Ziggurat in #77:
I'm not arguing for greenhouse gas effects.
What I'm disputing, is the claim that 1) radiation from the sun has been heating up the crust surface of Venus from a significantly lower temperature to its current 470°C, and that 2) a hypothetical ground temperature of 0°C would make such a heating much faster.
It seems that the myth of a runaway greenhouse effect on Venus can be traced back to the authority of Carl Sagan. In the meantime the myth has become a dogma.
And if I'm not completely mistaken in my thermodynamic evaluation, then everybody supporting the idea of such a temperature increase on Venus is (intentionally or unintentionally) working for the greenhouse-effect ideology, even if the assumed temperature increase is (honestly or insidiously) attributed to another mechanism. The reason is simple: Alternative explanations of non-existing 'facts' give further credibility to such 'facts'.
From the premises
o The lowest mean crust temperature on Venus is at crust surface
o The highest mean atmosphere temperature is at crust surface
I conclude
o There is a tiny net heat flow from the crust to the atmosphere
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Whereas a scientific hypothesis is refuted by facts and logical
reasoning, a dogma refutes facts and logical reasoning
wogoga in #73:
Think also about Venus' poles where the solar flux is virtually zero all the time. Ground temperature there is essentially the same as on the equator.
Ziggurat in #74:
It's close to equatorial temperatures, because of convection. You can't get that kind of temperature homogeneity across the entire planet without it. Why on earth did you think that this piece of evidence undermined rather than supported what I've been saying?
Horizontal winds near surface are slower than 1 m/s, i.e. less than 100 km/day. Because the distance from the midday point to the midnight point on the equator is around 20000 km, more than 200 days would be needed for temperature homogeneity to be achieved by near-surface convection.
And even if we start with the highly questionable claim that as much as 2.6 percent of the incoming radiation reaches the crust, on the equator at midday we get only around 70 W/m2 in order to heat up a ground of 470°C.
Ever tried to heat up e.g. melted Zinc of 470°C with weak daylight?
By the way, the atmospheres of both Earth and Mars are heated up by sunlight from the crust surface, and an obvious logical consequence of such a heating process is an inhomogeneous temperature near surface.
ben m in #75:
Every source I can find says that near-surface vertical air motion is somewhere in the 1 mm/s or 1 cm/s ballpark.
A horizontal wind of 0.2 m/s and a slope of 5% are enough to entail a vertical air motion of 10 mm/s.
ben m in #75:
Your hypothesis does not predict an isothermal atmosphere, nor a non-circulating atmosphere. Your hypothesis, "Venus's surface is heated from below", predicts that the lower atmosphere is circulating like crazy due to convection. That's what happens when you heat something from below.
Am I actually the only one here, being capable (not without effort) of recognizing how mistaken such statements are?
If you heat up an area on Earth to 470°C, then the atmosphere near that area will be "circulating like crazy due to convection". The reason is obvious: The air near the hot spot is heated up and its density (mass divided by volume) becomes lower than the density of the surrounding air. The heated air moves upwards, and is replaced by not yet heated air.
The situation on Venus however is not such a dynamic one. The hottest air (i.e. the one next to the ground) also has the highest density. And because not higher temperature but lower density ultimately causes such vertical convection, the air near the surface will simply remain where it is.
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Medieval superstition in modern society
ben m in #86:
On Earth, the hottest air is also closest to the ground and also has the highest density – it still convects.
On Earth, the air closest to the ground is the hottest with the highest density only on average.
Likewise in the Sun---the interior is at higher density and higher temperature than the surface, but there's still convection.
Also correct, if we assume "on average".
You need to compare the densities at equal pressures, and the pressure-change needs to be tracked adiabatically.
That we "need to compare the densities at equal pressures" seems quite wrong to me. If the air at a higher altitude is less dense, then no downward force arises in the first place [even if the warmer air below would be less dense at equal pressure], and without movement no adiabatic changes.
On Venus, a surface air layer of 67 kg/m3 is not replaced by a superior layer of 66 kg/m3, even if the colder superior layer would turn out denser "at equal pressures".
When you work it out, under a reasonable set of assumptions, you find that convection occurs when the temperature lapse rate is lower than the adiabatic lapse rate;
This statement seems not unreasonable to my Kantian synthetic a priori reasoning.
Let us make a rough estimate:
o Venus atmosphere is around 1000 metric tons per square meter.
o The lowest 150 m atmosphere result in 67 kg/m3 ∙ 150 m = 1 ton/m2.
o So at a height of 150 m, we have 99% [99 ton / 100 ton] of ground level pressure.
o Assuming ideal gas law, we conclude that absolute temperature of air at 150 m height must be at least 1% lower than at ground level, in order to sink due to higher density.
o One percent of ground level temperature (740°K) is 7.4 K.
o If actual lapse rate is less than 7.4 K / 150 m = 50°C/km near surface, then convection cannot start.
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An inconvenient truth: Rising sea levels tend to moderate global
climate
ben m in #89:
Sorry, you're simply wrong; you are probably used to thinking about buoyancy only for incompressible objects and incompressible fluids.
I've changed my mind several times. Just after having convinced myself once again that I'm rather wrong than right, I noticed in the table posted by Ziggurat in #19 a column with the name autoconvective lapse rate. Its value is 47 Kelvin/km for Venus, which comes quite close to my above rough calculation (showing that vertical convection on Venus near ground only starts if actual lapse rate is higher than around 50°K/km).
Then I found this quote from Basic Convection (for students of meteorology):
In many situations, surface heating is so intense that even convection is not enough to transfer the heat. This condition can happen near the surface in quite shallow layers, at locations where the vertical motions generated by convection are limited by the shallowness of the layer. Then it is possible for extremely high lapse rates to develop. The lapse rate can become sufficiently high so that density increases with height. It can be shown that a hydrostatic atmosphere in which density doesn't change with height has a lapse rate of g/R. This rate turns out to be about 34°C/km [on Earth] and is known as the autoconvective lapse rate. If the environmental lapse rate exceeds this value, not even a butterfly's flapping is necessary to initiate overturning; overturning is spontaneous in the same way that a brick held and then released in the air falls spontaneously!
ben m:
Perhaps the answer is more obvious if you treat it as a Hamiltonian system---does the system's total gravitational potential energy increase, or decrease, as the net effect (including adiabatic heating, hydrostatic pressure increase, etc.) of swapping Air Parcel #1 with Air Parcel #2?
Sounds reasonable.
If the answer is "decrease", then there's a buoyancy force working to carry out that swap.
It is rather the contrary: If actual lapse rate is between adiabatic (10.5°C / km on Venus) and autoconvective (47°C / km) lapse rate, then only a forced vertical convection opposing buoyancy can start a movement. Buoyancy itself has a stabilizing effect.
Only after a movement in the right direction has started, it is more and more reinforced by buoyancy, because adiabatic temperature changes (in function of changing height) are weaker than corresponding volume changes. In the case of sinking air parcels, this means that, as long as they sink, they become heavier and heavier in comparison with their environment [of the new lowered altitude].
Thus, an actual lapse rate between adiabatic and autoconvective leads rather to what can be called a meta-stable atmosphere. I do not exclude that actual lapse rate on Venus may have been in the lower meta-stable range over hundreds of millions of years.
A further quote from Basic Convection:
Unless the lapse rate exceeds the autoconvective, the parcels will not rise spontaneously.
Anyway, as actual lapse rate on Venus is even lower than the adiabatic one, your claim of #75 that heating the atmosphere from the crust would predict "that the lower atmosphere is circulating like crazy due to convection" is simply untenable.
wogoga in #81:
Even if we start with the highly questionable claim that as much as 2.6 percent of the incoming radiation reaches the crust, on the equator at midday we get only around 70 W/m2 in order to heat up a ground of 470°C.
Ziggurat in #82:
Why is that highly questionable?
Venus atmosphere is around 100 times denser (mass/surface) than our atmosphere, and contains around 67 times more molecules per surface. (On average, atmosphere molecules on Venus have 50% more mass than on Earth.)
Average solar radiation arriving at the top of the Earth's atmosphere is roughly 1366 watts per square meter. However, as the Sun's rays are attenuated by the atmosphere, surface insolation is reduced to approximately 1000 watts per square meter for a surface perpendicular to the Sun's rays at sea level on a clear day (Source).
Thus, the effect of our atmosphere is a reduction of solar radiation by a factor of 1000 W / 1366 W = 0.73. According to the Beer-Lambert law, we would expect for an atmosphere which is only 15 times denser, a reduction of radiation energy to 0.01 (and for one 30 times denser, a reduction to 0.0001).
Venus atmosphere however is 67 or 100 times denser, and in addition to that has a consistent thick cloud deck. On Earth thick clouds easily reduce the energy of insolation reaching the surface to 1% (i.e. by a factor of 0.01) (Source).
So if Venus crust surface were actually heated up by the sun, then we would have to attribute this fact rather to an incredible transparency of Venus atmosphere for solar radiation than to a non-transparency for thermal radiation.
I'm glad to see that I'm not alone with my assessment. Relevant quotes from Global Warming and Venus, by Robert Clemenzi:
"Notice that both Earth and Venus have surface temperatures that are hotter than their respective black body temperatures - this difference is the Greenhouse Effect. In both cases, this is caused by the atmosphere holding heat at the surface.
Also notice that the Venusian surface temperature is near constant day and night - diurnal (daily) temperature change is about zero."
"The only important item is that the atmosphere of Venus has about 100 times more gas than Earth between the surface and space."
"The Venus lapse rate and surface temperature appear to be constant day or night (by itself, this implies that the Sun has little effect on the planet's temperature)."
"The point of this is that to cool the Earth, hot air only needs to rise about 2 km (an easy task) to get above most of the green house gases - on Venus, it has to rise 55 km. In my opinion, this is the reason Venus is so hot - in fact, if the Venusian atmosphere had the same composition as Earth, it would be even hotter (because water vapor is a much better green house gas than CO2).
The fact that the Venusian surface temperature is the same after 58 days of sun light and 58 days of darkness is really the main reason I claim that the Sun does not heat the surface. (Venus rotates with respect to the Sun once every 116.75 Earth days.)
On Earth, the minimum expected difference would be 50°C for a 2,800 hour day, on Venus, no difference is reported.
The official explanation for Venus having a constant surface temperature is strong winds - I am not convinced and I have not seen any evidence to support that position.
From NASA – Venusian wind speeds: 0.3 to 1.0 m/s (surface)."
"Based on the number of visible craters, it is believed that the surface of the planet was liquid rock 500 million years ago. If that is true, then it is safe to assume that the crust is much thinner than on Earth. As a result, the internal core heat is perhaps the most important reason that Venus is so hot.
It is frequently stated that because of the high albedo and thick atmosphere, almost no solar energy reaches the surface of Venus. If this is true, then how come the surface temperature is given as over 900F? Obviously, all this heat is coming from the planet itself, and not the Sun.
Coupled with the thicker atmosphere, and the fact that the surface temperature is near constant day and night, this is why Venus is hot."
"To be fair, Earth's oceanic crust is fairly young (still being created) ... and very thin. Yet the oceans are not boiling.
So, maybe the crust thickness is irrelevant - just the atmospheric thickness is important."
---
Maybe a good example of a greenhouse-gas establishment's
insidious fight against its opponents – Fake opponent, little grey rat: "I am far more interested in holocaust history than
global warming. I fully expect to die in a world that still believes in
fantastical homicidal gas chambers even though I am certain it is absolute
bollocks."
Ziggurat in #95:
You're treating a spectrum-averaged value as if it were the value for every wavelength in the entire spectrum. It isn't, which means you can't use those equations in the manner you are trying to use them. Let me give you a simple example: suppose that the atmosphere is absorptive across half the spectrum, and not absorptive across the other half. At some depth, the radiation from the absorbed part is cut in half, for a total transmission of 0.75. At twice the depth, what's the absorption? Well, the absorbed part gets cut down to 0.25, but that's half the spectrum, so the total transmission is 0.625. But according to your naive interpretation, it should be 0.5625. Go really far, and the transmission should approach 0.5, while your naive interpretation indicates it should approach 0.
I fully agree. However, I never had the intention (#92) to show that atmosphere and clouds of Venus attenuate incoming radiation to
(1000 W / 1366 W)67 ∙ 0.01 = 10-11 = 0.000,000,001%
I only wanted to demonstrate in very rough, simple, and transparent way that the assumption of attenuation to a value as high as 2.6% is a priori rather unlikely.
Already when I started this thread, I doubted the statement "The cloud cover is such that very little sunlight can penetrate down to the surface, and the light level is only around 5,000–10,000 lux with a visibility of three kilometers".
According to this source, on Earth, illumination (more or less proportional to radiation energy) of an overcast day is around 1000 Lux and of a very dark overcast one around 100 Lux.
wogoga in #92:
So if Venus crust surface were actually heated up by the sun, then we would have to attribute this fact rather to an incredible transparency of Venus atmosphere for solar radiation than to an non-transparency for thermal radiation.
Ziggurat in #95:
Here's a simple test. If the surface of Venus is heated by the sun, then light from the sun is hitting the surface (and vice versa). If we were to put a camera on the surface of Venus, it should be able to take pictures with visible light.
Even on a very dark overcast day, we can take pictures with visible light. So this cannot be considered evidence that atmospheric attenuation of incoming radiation on Venus crust surface is only to around 2.6% (i.e. only by 97.4%).
In contrast, if no light is reaching the surface from the sun, the only light should be blackbody radiation, which (like looking inside a hot kiln) won't reveal any detail to our images. Now, what do you think will happen? Or, should I say, already happened.
Because Venus ground temperature (730°K) is not far away from the Draper point (798°K, above which almost all solid materials glow as a result of blackbody radiation), radiation in the near infrared could actually have contributed to the picture of Venus' surface (if the picture was taken without infrared filter).
And because emission spectra correspond to their absorption spectra, thermal radiation should reveal details in a similar way as external illumination (see also infrared photography).
---
"Exposed to Facts, the
Misinformed Believe Lies More Strongly" – And what if the authoritative majority has been
misinformed in the first place?
25th September 2016 #99
The discussion of 2010 has shown that the extremely high temperature on Venus' surface is at most marginally influenced by a greenhouse effect.
Unexpected for me, lapse rate (temperature decrease with increasing altitude) has become the central point of the discussion. There are three types of lapse rate:
o Actual lapse rate of ~8°Kelvin per km (Source)
o Adiabatic lapse rate of ~10.5°K/km
o Autoconductive lapse rate of ~47°K/km (Source)
A lapse rate lower than adiabatic (10.5°K/km on Venus) leads to a stable atmosphere, i.e. without vertical convection. If we transfer an atmospheric volume-sample to a lower altitude then the original sample gets compressed to the pressure of the lower altitude. Compression leads to temperature increase. Yet this adiabatic temperature increase of our volume-sample is bigger than the actual temperature increase of the environment (from higher to lower altitude). Therefore the sample's lower density leads by buoyancy to an opposite force (upwards, back to the original altitude).
If we transfer an atmospheric volume-sample to a higher altitude then the original sample gets decompressed leading to temperature decrease. Yet this adiabatic temperature decrease is bigger than the actual temperature decrease of the environment. Therefore the sample's higher density leads to a downwards force.
A lapse rate between adiabatic and autoconvective leads to a metastable atmosphere. Air at lower altitude still has a higher density, but if a given sample-volume is somehow forced downwards then adiabatic heating is less than heating of the new environment. As the lowered volume-sample turns out be colder and thus denser than its new environment, the downwards movement is reinforced by buoyancy (and weakened by heat transfer via conduction and mixing). In case of upwards movement, reinforcement works analogously.
A lapse rate higher than autoconvective (47°K/km on Venus) would imply that atmospheric layers at higher altitudes, due to substantially lower temperatures, have higher densities than layers below. Buoyancy alone is in principle enough to start vertical convection. (A stabilizing effect results from the fact that the layer of a given altitude cannot sink as a whole at the same time; see #90.)
Summary: Since on Venus actual lapse rate of 8°K/km is lower than adiabatic of 10.5°K/km, no vertical convection can arise in the first place on the isothermal crust surface. Thus the lower atmospheric layers act as excellent heat insulators keeping crust surface at a temperature which can be found on Earth only at a depth of 15 - 20 km below crust surface (Source). Nevertheless, Venus as a whole has (maybe apart from exceptional events) always been radiating away more energy than receiving from the Sun.
Quote from NASA climate modeling suggests Venus may have been habitable, 2016:
Venus may have had a shallow liquid-water ocean and habitable surface temperatures for up to 2 billion years of its early history, according to computer modeling of the planet's ancient climate by scientists at NASA's Goddard Institute for Space Studies (GISS) in New York.
The findings, published this week in the journal Geophysical Research Letters, were obtained with a model similar to the type used to predict future climate change on Earth.
It is rather unlikely that a model entailing such unreasonable findings for Venus' past could lead to reasonable findings for the Earth's future. In any case, a runaway greenhouse effect (implying a much colder Venus in the past) is rather the result of ideology than of science.