There’s no denying that Venus is hot. With a surface temperature of 737K it’s even hotter than Mercury which is closer to the Sun. And with an atmospheric CO2 concentration of about 96.5% it’s no wonder CAGW-advocates have used Venus as ‘proof’ of CO2’s purportedly huge heat-catching ability. Apparently many ‘qualified’ scientists think that the number-one culprit is CO2. Pieter Tans of the National Oceanic and Atmospheric Administration states that “Venus is hot because its atmosphere is mostly CO2. Therefore if we add more CO2 to the Earth’s atmosphere the Earth will get as hot as Venus” and the always-rational Skeptical Science says

*“The greenhouse effect on Venus is primarily caused by CO2”. So what is their reasoning? The reasoning is as follows: ‘Venus is hot and it has a very high atmospheric CO2 level and we know CO2 absorbs long-wave radiation therefore CO2 must be responsible’. It really is that simple.*

The idea that Venus is hot because of the
back-radiation from atmospheric CO2 seems to be somewhat unlikely. For starters
Venus receives a maximum of 2613W/sq.m of solar radiation which corresponds to a
temperature of 464K although its actual surface temperature is around 737K corresponding
to 16,700W/sq.m. That is a difference of 14,000W/sq.m. One question that comes to mind is how can CO2 (or any gas) re-emit more radiation than it absorbs? Venus is getting a maximum of 2316W/sq.m from solar radiation, or 65/Wsq.m averaged-out over the surface and the greenhouse is generating 14,000W/sq.m? Really? This is not just a greenhouse. It is a super-greenhouse that manages to back-radiate 14,000W/sq.m from the measly 65W/sq.m it receives.

Another issue is the fact that Venus makes a full rotation once every 240 days meaning the dark-side of the planet does not see any solar radiation for 120 days, yet there is essentially no difference in temperature between the two sides. How then can the greenhouse effect be maintaining such high temperatures at night when it is dependent on solar radiation to operate and when the night-side is not seeing any solar radiation for such prolonged periods? The wind at the surface on Venus is also very weak at only 0.3 to 1.0 metre/second so any convective heat-transfer from day to the night would seem unlikely.

Another issue is the fact that Venus makes a full rotation once every 240 days meaning the dark-side of the planet does not see any solar radiation for 120 days, yet there is essentially no difference in temperature between the two sides. How then can the greenhouse effect be maintaining such high temperatures at night when it is dependent on solar radiation to operate and when the night-side is not seeing any solar radiation for such prolonged periods? The wind at the surface on Venus is also very weak at only 0.3 to 1.0 metre/second so any convective heat-transfer from day to the night would seem unlikely.

The IPCC’s logarithmic equation that relates CO2 to radiative forcing implies that the CO2 on Venus is only enough to contribute 6% to its overall temperature. To calculate this we must first find the temperature of Venus if it were heated only by solar irradiance without a greenhouse (i.e. the effective temperature). This is usually calculated with the following equation: T = [S (1-A) / (4σ)]^1/4. Where T is the effective temperature, S is the solar constant, A is albedo, and σ is the Stefan-Boltzmann constant. Venus has a solar constant of 2613.9W/sq.m and an albedo of 0.90 (Source: NASA Venus Fact sheet). Plugging in the values gives us an effective temperature of: T = [2613.9 (1-0.90) / (4σ)]^0.25 = 184K.

Our next step is to calculate the radiative forcing from the entire atmospheric CO2 greenhouse on Venus. The atmosphere of Venus has a mass of 4.8x10^20kg whereas Earth’s atmosphere has a mass of 5.1x10^18kg. The CO2 concentration on Venus is 96.5% (around 4.7x10^20kg) and Earth has a CO2 concentration of 0.039% (around 3x10^15kg). An astonishing 157,000 times the mass on Earth! On Venus the CO2 concentration is the equivalent of 61,000,000ppmv relative to Earth’s atmosphere. The IPCC apply the following logarithmic equation to calculate CO2’s radiative forcing: ΔRF = 5.35*Ln(C1/C0). Coverting the equation into a predictor of temperature increase with the Stefan-Boltzmann law we get: ΔT = [(σ*T^4 + 5.35Ln(C1/C0))/σ]^0.25

*–*T. Where ΔT is the resultant temperature increase, T^4 is the absolute temperature of the body raised to the 4th-power, σ is the Stefan-Boltzmann constant, Ln is the natural logarithm of, C1 is the final CO2 concentration and C0 is the initial CO2 concentration. So the radiative forcing from the CO2 on Venus gives us a temperature increase of: [(σ*184^4 + 5.35Ln(61000000/1))/σ]^0.25 – 184 = 47K.

There is another possible explanation for Venus’ high surface temperature that is consistently overlooked by greenhouse effect enthusiasts: pressure. Venus has a high atmospheric pressure of 92000mb, accompanied by a huge atmospheric mass of 4.8x10^20kg and density of 67kg/m^3. According to Gay-Lussac’s law pressure is directly proportional to temperature, so if pressure increases (and volume stays constant) temperature also increases. Suns for example generate their super-high temperatures through pressure; gravity compresses hydrogen into an increasingly smaller space until temperatures reach 10 million degrees Kelvin and hydrogen fuses. If pressure (density and mass) can account for the ‘anomalously high’ surface temperatures of Venus then one would expect that the Ideal Gas law would be able to explain such temperatures, which describes the relationship between pressure, volume, density/mass and temperature. The Ideal Gas law looks like this: PV = nRT. Where P is pressure, V is volume, n is the density/molecular mass, R is the Universal Gas constant (0.082) and T is temperature. So according to the Ideal Gaw law the surface temperature of Venus from the pressure, density and molecular mass should be: T = PV/nR = 92000/(67000/43.45*0.082) = 727K. According to NASA’s Venus Fact sheet the surface temperature of Venus is around 737K.

The Ideal Gas law can also be deployed to predict the temperature of other planets and its predictive power is really quite impressive. Assuming Earth has an atmospheric pressure of around 1000mb, a density of 1.217kg/m^3 and a mean molecular mass of 28.97 we get a temperature of: 1000/(1217/28.97*0.082) = 290K. This works for all planets with atmospheres. Using this method with data from NASA’s Fact sheets we get a predicted diurnal temperature range for Mars of ~238K/182K (based on the pressure range of 6.9-9mb measured by Viking 1) when the actual diurnal range is 242K/184K; a predicted temperature for Jupiter at 1 bar of ~169K when the actual temperature at 1 bar is 165K; a predicted temperature for Saturn at 1 bar of ~133K when the actual temperature at 1 bar is 134K; a predicted temperature for Uranus at 1 bar of ~77K when the actual temperature at 1 bar is 76K and a predicted temperature for Neptune at 1 bar of ~73K when the actual temperature at 1 bar is 72K.

If the Ideal Gaw law accurately predicts the temperature of planets based on pressure, density and molecular mass, then the conventional method of applying the Stefan-Boltzmann law to the surface of a planet with a sufficiently large atmosphere may need to be revised. It also means we would not need to invoke back-radiation to explain away the temperature disparity. In Venus’ case, because of the very dense atmosphere (CO2 at the surface is technically a supercritical fluid) most of the solar radiation absorbed by the planet is in the mid-atmosphere rather than at the surface and as these atmospheric gases warm they expand, rise and then fall in the planet’s gravitational field, and as they fall they compress the atmospheric column below it doing ’work’ and heating it adiabatically. The adiabatic model and the gravitational effects on the mass of an atmosphere already explains the temperature of planets without back-radiation from a colder troposphere (see paper by Chilingar, Khilyuk and Sorokhtin).

The video below with Steven Hawking and Carl Sagan talking about the assumed runaway greenhouse effect on Venus paints a rather terrifying picture for all of us if we keep emitting CO2 at the rate we are doing:

It all sounds so dramatic. But I think it’s important that we keep a sense of proportion over it. The argument that CAGW-proponents put forward for this runway greenhouse occurring on Earth is apparently brought about by the so-called positive feedback loop. The argument is as follows:

*‘Increasing CO2 increases temperature, which increases evaporation-rates, which increases water vapour, which continues in a viscous cycle until the oceans have been run dry and our planet is rendered uninhabitable’. But they seemed to have ignored one chain in the link: the negative feedback of cloud cover. Increasing water vapour means there is more of the stuff to condense into clouds, which increases albedo and acts as a negative feedback by reflecting incoming solar radiation back out into space. It’s essentially homoeostatic. The Earth has had ample opportunity to go into a runaway greenhouse and has not done so. See this graph from paleo-climate reconstructions going back 550 million years showing CO2 as high as 7000ppmv. The Earth has endured asteroid collisions, continental drifts, super volcanoes and 7000ppmv of CO2, and people think that increasing it from 280ppmv to 560ppmv might potentially destroy the planet?*

* The IPCC’s equation above may overestimate the radiative forcing from CO2.