Global Warming and Venus

I hear it all the time Well, that's just not true. These are the main reasons why Venus is hotter than the Earth Actually, the fact that Venus's atmosphere is CO2 helps to cool the planet more than a mixed atmosphere like the one on Earth.

The fact that it is closer to the Sun apparently has no effect - really.

Relevant Data | Closer to Sun | Thick Atmosphere | Burning Coal and Gasoline | Lapse Rate
Thin Crust | What Scientists Say | Conclusions | References

Relevant Data

Note: Liquid water is about 1,000 kg/m3

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. Note: The NASA page is wrong - Diurnal temperature range is NOT 0 K, the daily change is 0 K ... not the range (min and max are both 737 K).

Venus is Closer to the Sun

Logically, Venus is hotter than the Earth because it is closer to the Sun. However, the data I've seen indicates that the solar energy simply does not reach the surface.

  Venus Earth
Aphelion 67,693,488 miles 94,509,130 miles
Perihelion 66,782,651 miles 91,402,725 miles
Semi-major axis 67,239,070 miles 92,956,041 miles

For the purposes of argument, I will use just the Semi-major axis and 3 significant figures to estimate the relative energy received from the sun. I know that more accurate values can be computed, but this is good enough to make the point.

In other words, Venus gets about two times as much energy per unit area as the Earth - this is a lot.

Some people will argue that Venus has a higher albedo than Earth (0.65 vs 0.367) and, therefore, the energy absorbed per unit area is about the same. It should be noted that these values are for the reflection of visible light, not IR. While I have not found good numbers for the IR albedo (of either planet), the data I've seen indicates that most of the IR energy is either reflected back into space or absorbed in the upper atmosphere (that's right - it does NOT heat the surface).

Venus Has More Atmosphere

One way to compute the amount of atmosphere is to figure out how much it weighs ... adjusting for the local gravity. Without any adjustments, it is obvious that Venus has about 92 times as much atmosphere as Earth - this is a very big difference.

This is the same as being cold ... so you add 92 more blankets.

This is speculative - I am not sure the science is correct, but ... correcting for a weaker gravity, the Venusian atmosphere should produce

Dividing by 0.904 is correct because the same amount of atmosphere on Venus would weight more on Earth.

Most reports indicate that the atmosphere of Venus is either 90 or 92 times heavier than on Earth. Since I have not found any Raw data, I do not know if that is actual measured pressure or if it has already been corrected in some way. In this document, I have tried to consistently use the 92 ATM's value, but I suspect that the atmosphere is actually 100 times thicker on Venus.

Note: In addition to the arguments above - If the atmosphere of Venus was placed around a larger Earth it would not be as thick because it would fill a larger volume (because of the difference in size) and because the gas would be pulled closer to the surface (because of the difference in gravity). However, this is not relevant in this discussion. The only important item is that the atmosphere of Venus has about 100 times more gas than Earth between the surface and space.

Burning Coal and Gasoline

Burning fossil carbon does not increase the thickness of the Earth's atmosphere - it actually makes it thinner.

For coal

Thus one oxygen molecule (2 atoms) is consumed for each carbon burned - no net change in the number of gas molecules. Eventually, that carbon dioxide will be made into rocks and the atmosphere will actually get thinner.

For more complex petroleum products, the chemistry is more complex, partly because most carbon atoms have 2 or 3 hydrogen atoms attached, and partly because the number of carbons per molecule varies. This example assumes a simple 8-carbon molecule (octane - the main component of gasoline).

Here, 25 gas molecules (Oxygen) become 34 (16 carbon dioxide and 18 water) - but the increased water in the atmosphere will eventually become rain, and some of the carbon dioxide will become rocks (or fossil fuel), and again the atmosphere becomes thinner.

But don't worry that this is actually causing Global Cooling - there really isn't enough fossil fuel on the planet to worry about the atmosphere getting too thin or about running out of oxygen. The most damage humans can do is to remove only about 0.04% of the total atmosphere.

Lapse Rate

Lapse Rate is a number that defines how fast the atmosphere's temperature changes with height.

When hot dry air rises, it cools at a lapse rate of 9.78 C/km - this is known as the dry adiabatic lapse rate (DALR).

If the actual lapse rate of the atmosphere is less than the DALR, then the atmosphere is stable and convective mixing does not occur unless there is external forcing (like a hot sun).

When the sun heats the ground, it is possible to heat the air near the ground enough that it becomes lighter than the air a few hundred feet above the ground. When that happens, the hotter air begins to rise. Based on the temperature difference, it is possible to compute how high it should rise before the rising air reduces its temperature (through adiabatic expansion) to that of the surrounding air. Since the surrounding air also decreases in temperature with height, the following simple equation can be used.

Rearranging the terms and solving for the same temperature at an unknown height.

Without getting into a lot of graphs, the following table shows the atmospheric thickness and the ground temperature offset necessary to move heat above the insulating atmosphere to a zone where it can radiate into space (ie, above most of the water vapor and CO2). Note that the Earth values are *typical* and will vary considerably from hour to hour. Even the sign of the lapse rate will vary between morning and afternoon. 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). (This data is open to disagreement - my sources are good but not as rigorous as they should be.)

Lapse Rate
Delta T
Venus 55 km 7.7 148 Because probe data shows that the surface temperature has less than 1C difference, it is obvious that there is no convection high enough to cool the planet

Note: the DALR on Venus is 10.4 C/km

Earth 2 km 6.5 6.6 Humid air creates clouds at about this height
Earth 5 km 6.5 16.4 Hotter air rises higher and faster
Earth 11 km 6.5 36.1 11 km is the normal top of the troposphere
About 7 km at the poles and 17 km at the equator
Earth 16 km 6.5 52.5 This would not be reasonable for dry air - Note that the lapse rate is 6.5 up to 11 km ... not 16 km. From 11 km to 16 km, the lapse rate is zero.
Earth 16 km 4.5 84.5 The lower lapse rate is because the temperature does not change from 11 km to 16 km
Earth 23 km 3.0 156 The tops of Cumulonimbus clouds can reach 23 km. This is possible because humid air carries a lot more heat than the temperature alone indicates. As a cloud forms, heat is released and the air is allowed to rise higher than the starting temperature alone would predict. Basically, for dry air to rise to 23 km, it would have to be 156C (312F) hotter than the surrounding air.

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 50C 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)

Venus Has a Thin Crust

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.

What Pro-Global Warming Scientists Say

Paraphrased, the basic mantra is
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.

For instance, in the NOVA program "What's Up With the Weather?", Dr. Pieter Tans of the National Oceanic and Atmospheric Administration (NOAA) says

Venus has an extremely high concentration of CO2 in its atmosphere, and it's blazing, boiling hot at the surface of Venus.
and then he presents a pretty bogus experiment demonstrating how CO2 absorbs IR radiation ... but he never says that the Venusian atmosphere is any thicker than the Earth's.


Based on the available facts I conclude that


The NASA Planetary Fact Sheets are the primary source for a lot of data.

This Wikipedia article on Venus is an alternate reference for physical properties - Wikipedia also provides additional data on the atmosphere of Venus.

The Soviet Exploration of Venus provides a very interesting chronicle of the history of Venus exploration - it was a long and expensive road to collect the data I've presented above. I highly recommend this page.

The planetary fourier spectrometer (PFS) onboard the European Venus Express mission is basically a proposal for future research - but it has lots of graphs showing what is known from previous probes. Even though the satellite has been on station around Venus for over one year (since April 2006), I can not find any current data.

I have spent many hours searching for raw data from Venus probes - I can not find any online. I am specifically frustrated that there appears to be none from the Venus Express probe that has been on station for almost one year (at this writing). I presume that, when the data becomes available (Last Update: 13 Nov 2006), it will unambiguously support or refute the speculative claims I have made above.

Author: Robert Clemenzi
URL: http:// / Global_Warming / Venus.html