The “runaway greenhouse effect” on Venus is often used as a stick to warn us as to what could happen to planet Earth unless we mend our ways. Can this possibly be true? I think that to understand the climate on Earth we must at least be able first to explain the climate on Venus. There have been some excellent discussions about this whole subject: see for example [1] below.
Venus has 300,000 times more CO2 in its atmosphere than does the Earth. Mars has 30 times more CO2 in its atmosphere than the Earth. Both atmospheres are composed of approximately 95% CO2 but differ wildly in thickness (pressure). On Mars CO2 is an insignificant greenhouse gas warming the surface by only about 5 degrees, whereas on Venus there is a crushing ~ 500K “greenhouse effect”. Is it really possible that CO2 could alone be responsible for both climates ?
Energy flow and temperatures on Venus - from Dave Crisp, JPL "Greenhouse Effect and Radiative Balance on Earth and Venus"
Mars has 125 times less atmospheric pressure than on earth, but despite this IR photons leaving the Mars surface still encounter 30 times more CO2 molecules than they would do in leaving the Earth. The low Martian pressure narrows a bit the absorption bands for CO2, and there is little H2O to absorb in other bands. In contrast, on Venus the pressure is a massive 90 times higher than Earth and this leads to some broadening of CO2 bands thereby increasing slightly the bite taken out of the IR spectrum by CO2 – but this is not even half the full story. Venus is special because it is heated from above by thick clouds in its upper atmosphere. Venus receives nearly twice the Earth’s solar radiation (TSI=2636 W/m2 – average S0= 660 W/m2). However 76% of this radiation is reflected by clouds and of the remaining 170 W/m2, 90% of it is absorbed by these H2SO4 clouds at the top of the atmosphere. Just a tiny 2.5% of incident sunlight (just 17 watts/m2 !) reaches the surface [2]. That is equivalent to one light bulb heating a small room. So the narrative of a runaway CO2 greenhouse effect on Venus caused by CO2 absorbing IR emitted from the surface thereby causing warming through “back radiation” (IMHO) simply cannot be true. Instead I think Venus is “heated” from the top of the atmosphere downwards via the adiabatic lapse rate. If 90% of available sunlight is being absorbed some 50 km above the surface then this is the level that must reach the effective temperature Teff of Venus (260K ) needed to balance absorbed Solar radiation with IR radiation out to space. This sets the scale of the lapse rate to be 50 km above the surface. Convection then drives the temperature gradient towards the adiabatic lapse rate by pumping heat down to the surface as the temperature at 50 km is now normalised to 260K. The adiabatic lapse rate depends only on gravity and Cp of the atmosphere. It must be the driver of massive heat flows downwards to heat the surface. It acts like a global heat pump with the surface mainly heated from above through convection. Maintaining a lapse rate needs some net heat flux and greenhouse gases allow energy loss from the top of the atmosphere to space, but the radiating level on Venus is set at 260 deg.C 50 km up in the atmosphere because it is directly heated by the sun. So Venus surface is heated through convection from above and not directly by CO2 greenhouse effects.
To see why CO2 alone cannot be the cause of Venus’s high temperatures consider the effects on Earth. The forcing of a Mars like 30 fold increase in CO2 in the Earth’s atmosphere is 5.3Ln30 = leading to about 5 degrees of warming. A Venus like 300,000 fold increase in CO2 only leads to just 19 deg. C. However, the greenhouse effect on Mars is negligible and on Venus it is a whopping 500 degrees. So there must be more to it than CO2. CO2 only absorbs IR in selected bands. The logarithmic effect is due to the decreasing influence of extra CO2 once these bands get saturated. Although there is some pressure broadening on Venus this is insufficient to explain the crushing greenhouse effect on Venus. In reality Venus has a host of other greenhouse gases present as well – SO2,OCS,H2S,H2O. Water vapour is also present in the clouds.
How high does the convective troposphere extend ?
Temperature profiles with height Venus, Earth & Mars
The lapse rate extends for between 3-4 scale heights on each planet and seems to be independent of pressure. The scale height is the height at which pressure or density reduces by 1/e.
Density vs. Altitude
rho = rho0 exp( - z / H) ; mass density
n = n0exp( - z / H) ; number density
Where Scale Height: H
H = kT/mg (or H = RT / Mrg)
Mr g(m/s2) Ts (K) H(km) Cp LR (deg/km)=g/cp
Venus CO2 44 8.88 733 16 850 11
Earth N2,O2 29 9.81 288 8.4 1003 9.8 (av=6.5)
Mars CO2 44 3.73 220 11 850 4.5
Note:cp=9R/2M for triatomic(CO2), cp=7R/2M for diatomic(N2)
M=0.044kg for CO2, M=0.029 for air
The tropopause on Earth ranges in height from between 10 – 16km. Mars is at about 45km and Venus is at 55km. I suspect the tropopause occurs when radiative heat transfer exceeds convection. Mars has such a low greenhouse effect that radiation occurs mostly from the surface. The height of its tropopause is likely due to low gravity falloff with pressure. On Venus the tropopause lies at 55 km at the top of the H2SO4 clouds where IR radiates freely to space. Clouds only exist so high up in the atmosphere because the surface pressure is so high.
So the bottom line question is that if you were to replace 90% of the CO2 molecules on Venus with an equal mass of N2 molecules would the surface temperature change ? I don’t think so – the surface temperature would remain exactly the same. Correction: Cp would increase and the lapse rate decrease slightly so the surface temperature decreases somewhat. Some small greenhouse effect is needed to maintain the lapse rate, but then convection does the rest pumping heat down to the surface from the solar absorbing cloud cover above Venus. I think this is the real cause of such high temperatures on Venus.
References
[1] http://scienceofdoom.com/2010/06/12/venusian-mysteries/
http://wattsupwiththat.com/2010/05/06/hyperventilating-on-venus/
[2] Dave Crisp, JPL “Greenhouse Effect and Radiative Balance on Earth and Venus” presentation , 2007.
Convection does not drive the heat engine. The heat engine drives the convection, and the greenhouse effect is a very imortant factor in that both on the Earth and on the Venus.
I agree that the greenhouse effect is necessary to drive convection. However, once started convection then drives temperatures towards the DALR. Since 90% of available solar radiation is absorbed 50 km up on Venus thereby heating the upper atmosphere, the surface temperature must rise to maintain the lapse rate. The very high pressure on Venus must also play a crucial role. I suspect this is the reason why the troposphere and H2SO4 cloud height is so high.
Why is the troposphere so high on Mars ?
Maintaining the temperature gradient requires continuously work. Thus a atmospheric heat engine is needed to maintain the gradient, not only to start the convection.
Because convection is so much more powerful than conduction it need not be strong to maintain the gradient but the basic principle of a heat engine is that external heat must enter the system at high temperature. The convection will always cool the surface in a stationary system. The little solar radiation that reaches the surface is enough just because the greenhouse effect is so extremely strong.
The amount of CO2 is important in determining the height of the troposphere. Through that it influences the surface temperature of Venus. At the high pressure of lower Venusian atmosphere the concentration of CO2 is not important, but the total amount of CO2 is important for the height of troposphere.
The rather large amount of CO2 is important also for the height of the Mars troposphere as it determines the ratio of atmospheric emission to the radiation from surface at the most important wavelengths. Without water and pressure broadening the overall effect on the surface temperature is small but the that’s largely decoupled from the atmospheric radiative processes, because most wavelengths of the surface emission pass through the atmosphere freely while radiation emitted at one point in the atmosphere is usually absorbed near to the point of emission as that radiation is dominated by the narrow emission/absorption peaks.
@Pekka Piri:
Does the troposphere end once the mean free path for (greenhouse relevant) IR photons exceeds the remaining height of the atmosphere ?
Your argument about CO2 governing the total height of the troposphere on Venus seems convincing. This also sets the height for the cloud layers. However, you are still claiming the direct cause of the greenhouse effect is the very small amount of solar radiation absorbed by the Venusian surface. 90% more energy than this is absorbed in the upper cloud layers, which is a far larger power source to drive the atmospheric heat engine.
Suppose no solar radiation reached the surface. Are you proposing that the atmosphere below 50km and the surface would become isothermal at Teff ?
Without heating from below the atmosphere is likely to become stratified with a lapse rate too small to allow for convection. There would certainly be some disturbances that would cause mixing and maintain some temperature below the strongly convective layers – as we do have occasionally some mixing of the generally stratified stratosphere.
Basically the point is, however, that without any heating from below the convection must stop and stirring the upper levels will have less and less influence on the lower levels as stratification gets stronger and stronger.
One word was left out:
.. some temperature gradient below ..
What about IR radiation downwards from the solar heated clouds absorbed by higher density CO2 near the surface ? Once an energy flow is established convection begins. The lapse rate is mainly gravity driven.
The lapse rate maintained by convection that’s driven by the atmospheric heat engine. The atmospheric heat engine is based on a situation where external energy enters at a point that’s warmer than the place where the energy leaves the system. On Earth the energy enters mostly at surface and leaves over a wide range of altitudes, on Venus the role of radiation absorbed within the atmosphere is probably larger but the surface has also an important effect.
The gravity enters when the atmospheric heat engines maintains the convection, it does not drive anything. Another essential factor is the expansion of gas when the pressure is reduced (and compression when pressure is increased).
There’s always dissipation that would stop the convection if the atmospheric heat engine would not drive it continuously.
You are right to correct my imprecise expression. Gravity doesn’t drive the lapse rate. It sets the scale for the lapse rate. I would like to propose one possible way the atmospheric heat engine might work on Venus without solar radiation reaching the ground. Differential Solar heating in the upper atmosphere from equator to poles will set up winds transferring heat polewards . At the equator this must pump air upwards from the ground which then has to descend at higher latitudes like Hadley cells. Rising and descending air sets up the lapse rate and with Teff set at 260K some 60 km up the ground level rises to 700K. Greenhouse gases absorb and emit IR as before and radiate to space from the upper cloud level.
Thanks for your patience !
Yes Clive. The compression heating of the descending air in the pressure gradient of the atmosphere set up by gravity acting on atmospheric mass is what underpins the lapse rate. Venus is indeed heated from the top down. Not by IR absorbed in high altitude clouds.
A simple look at Venus planetary winds shows that the poles are possibly the hottest point of the planet as the high altitude gas spirals down into them. It’s a proven fact too. Planetary wind is the major gas stirring mechanism. The higher winds travel at 190 mph. A ten (Earth) day circulation rate is no slouch!
The poles are suffering high pressure and the equator is effectively a low pressure zone. High altitude gas is constantly lost through circulating to the poles where the gas gets pressurised, therefore heated.
Venus (albedo) is very much like my stainless steel kitchen sink. It reflects a lot of the Sunlight into space but re-radiates very little so when its touched.. Ouch!
Peka, what convective engine are you talking about girlfriend? There isn’t one to speak of on Venus. The lower gas movements are proven to be extraordinarily light.
As Clive said, if only 17 watts of +2300watts is reaching the surface. Surface heating gas for convection is something I have issues with. Because most light is taken up in the high atmosphere.
At 60Km up, the gas pressure is 0.56 atmosphere and 20C.
Or a little lower, gas pressure; 1 atmosphere and 27C.
This is not hot considering the Sun has almost double the Earth’s radiation.
This is why I have issues with the C02 absorption bands of light being the major driver to planetary heating when is obviously the pressure.
Haven’t they found a fault with the “Unified theory of Climate” yet?
Thanks – this is a good answer as to how venus is most likely heated top – down !
The “Unified theory of Climate” , Loschmidt effect etc. are really all about the hypothesis that gravity alone can produce a temperature gradient in a planetary atmosphere as well as a pressure gradient.
> 90% of physicists will say no – a thermally isolated atmosphere in a gravitational field will be isothermal – a single temperature. This is because the 2nd law of thermodynamics which rules out heat flow from a cold body to a warm body. However, the entropy argument is slightly more complex – gravity clumps molecules near the surface.
To resolve this it would be relatively easy to perform an experiment to measure such a hypothetical effect using a centrifuge to simulate high gravity.
I could of sent a more simple answer for Pekka Pirilä by asking him if he has ever seen or heard of a gas or electrically heat pumped fridge? No moving parts but requires a small heat source to pump the heat.
The “free ride” is from planetary circulation.
Have to laugh. Venus has more volcanic activity than any planet in the solar system – where do you think those sulphur clouds came from? So you have volcanic heat unable to escape due to thick clouds and you still obsess over greenhouse gas as a cause of the high temperature? Bonkers.