Gill 5: Smoke and Mirrors? Some comparisons between Earth and Venus

Collection of five Surrey Mirror Group Website Articles written by Peter F Gill and posted from about March 2015 to January 2016. They now no longer exist on the Internet unless archived by someone.

Scientists and non-scientists often give Venus as an example of a planet with a runaway greenhouse effect. However, some years ago I came across an analysis with a different conclusion. I shall say more about this later. Firstly, let us look at some vital statistics:

Earth

Venus

Average Distance from Sun

149.6 million km

108.2 million km

Diameter

12,756 km

12,104 km

Mass

5.972×1024 kg

4.867x 1024 kg

Density

5520 kg/m3

5250 kg/m3

Atmosphere

78% N2, 21% O2, 1% Ar

96% CO2, 3% N2

Albedo (reflectivity)

0.37

0.76

Acceleration due to gravity at surface

9.8 m/s2

8.9 m/s2

Pressure at surface

1 bar

90 bar

Average Surface Temp.

2880 K (150C)

7380K (4650C)

Orbital period

365 days

225 days

Rotation period

Approximately 1 day

Approx. 225 Earth days

Moons

1

None

Although the size and density of the two planets are similar, the factor that most people find striking, is the extreme difference between the planets’ surface temperatures. On Venus, the surface temperature is well over the melting pint of lead and some 450C above the melting point of zinc whilst on Earth the average temperature is only 150C above the freezing point of water. Is the difference due to the fact that most of Venus’ atmosphere is composed of carbon dioxide? Well many scientists think not (some examples: H.D Huffman, A. Miatello and D J Cotton). To explain why, I will have to get into a little bit of physics and a little bit of mathematics. Please accept my apologies in advance to those who struggle with these disciplines.

Firstly for reasons that may become obvious a little later let’s look at the temperature at the height in the atmosphere of Venus where its atmospheric pressure is the same as that on Earth at ground level (approximately 1000 millibars). On Venus, the average temperature at that height is about 338.6 degrees Kelvin (about 65.50C) compared with an average of 287.40K just above the earth’s surface (14.30C). Of course, as we have seen from the data above, Venus is closer to the sun than Earth. The amount of solar radiation received depends on the inverse square of the distance of the planet from the sun. So roughly the radiation received by Venus in comparison with Earth is (149.6/108.2)2 = 1.911. To convert this difference in radiation received to equilibrium temperatures then according to the Stefan-Boltzman Law we have to take the fourth root of 1.911, which is 1.176. So, if Venus was at the same distance from the sun as Earth it would have a temperature at the pressure altitude of 1000mb of 338.6/1.176 or about 287.90K (about 14.70C). You will notice that, although I have ignored the difference in reflectivity (albedo) for the two planets, the small difference in the planet disc facing the sun, the fact that the Earth’s rotation is fast compared to Venus and some other factors, this simple calculation appears explain the temperature difference without the need to call upon a greenhouse effect!

In case you think that this is all smoke and mirrors let’s take a slightly different approach and only talk about Earth. You may know that generally as one goes higher the atmospheric temperature reduces. The rate at which this proceeds depends on whether the air is dry or contains water vapour. We call this reduction in temperature with height the lapse rate. The simplest lapse rate is the dry adiabatic lapse rate or the DALR. It can be expressed as a rate of change of temperature T with height h as: dT/dh = -g/cp where g is the acceleration due to gravity and cp is the specific heat of air in the atmosphere at constant pressure. Although g changes slightly with h and cp changes slightly with T to a first approximation one can regard both g and cp as constants. So using integral calculus, we can generate the formula:

Ts –Th = -g/Cp * (hs-h)

Where: Ts is the average temperature at the Earth’s surface and Th is the temperature at height h and hs is height at sea level which by definition is zero. Now the moist lapse rate has many more terms in it but we are interested only in a rough calculation so I will insert the average moist lapse rate for -g/cp as -6.50K/km (or C) the equation simplifies to: Ts = Th+ 6.5*(h). Now let us substitute two numbers for height and temperature at that height. Typically, at say 5km above sea level the air temperature is about 2550K. So using this information let us calculate the surface temperature. It is 255+6.5*5, which is 222.50K or 287.50K (about 14.40C). So, again, without using any radiation formulae we have deduced the average surface temperature correct to within less than one degree Kelvin.

This leaves us with the thought that, if there is a greenhouse effect, it is likely to be small. It also follows that, a change in the amount of carbon dioxide in the atmosphere, is not likely to have very much effect on temperature. On the other hand, because the solubility of carbon dioxide in water (and sea water in particular) varies inversely with temperature one expects to see increasing levels of carbon dioxide in the atmosphere when, for whatever reason, the atmosphere heats up. Interestingly is what we have been seeing since we emerged from the little ice age. Nevertheless, if I was an English Literature graduate I may still be thinking that it is all smoke and mirrors.

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5 Responses to Gill 5: Smoke and Mirrors? Some comparisons between Earth and Venus

  1. oldbrew says:

    ‘this simple calculation appears explain the temperature difference without the need to call upon a greenhouse effect!’

    Yes, Huffmann pointed this out eight years ago.
    http://theendofthemystery.blogspot.co.uk/2010/11/venus-no-greenhouse-effect.html

    Atmospheric pressure and temperature are always going to be related. Trying to pick apart the supposed behaviour of constituent gases is not proving productive, the failure to determine so-called ‘climate sensitivity’ being one example.

    • Peter F Gill says:

      Actually as Cp (dry) is largely made up of the specific heats of Nitrogen, Oxygen, Argon and carbon dioxide one can try varying the CO2 content and according to your assumptions as to the source of the CO2 the O2 content too to see how much effect this makes (Hint its rather small).

  2. Rockyredneck says:

    I found this series very interesting. A few months ago, I wrote an essay on the broader aspects of climate change, That is rapidly gaining popularity on the internet. It is based on my years of evaluating related research, climate data, and personal observations.
    Perhaps not so surprising, it seems to align quite closely with the conclusions of Mr. Gill and yourself. I am providing a link, https://goo.gl/9Ns3ux

    • Scottish-Sceptic says:

      Nice article. I haven’t read it in detail (because I’ve already read the same type of article multiple times), but scanning the graphs, you’ve picked the ones I think give a reasonably balanced picture. The only thing I would add in is a graph of Arctic and Antarctic ice. You need to pick a long enough graph to show previous reductions, recovery and then the modern reduction for Arctic. These may be hard to come by as good records are not available till the satellite era – so it may be best to ignore this until we get a sustained recovery.

  3. Robert Holmes says:

    You are correct, there is no net warming from the so-called ‘greenhouse effect’ on Venus or any other planet for that matter.
    This was shown in my recent paper published here;
    Robert Ian Holmes, Molar Mass Version of the Ideal Gas Law Points to a Very Low Climate Sensitivity, Earth Sciences. Vol. 6, No. 6, 2017, pp. 157-163. doi: 10.11648/j.earth.20170606.18
    And will be detailed more comprehensively in my next paper which is due out soon.
    Planetary surface temperatures are not determined by the greenhouse effect, but by a combination of auto-compression and insolation.

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