By Andy May

We are often confronted with the idea that if the carbon dioxide level in the Earth’s atmosphere gets too high we will suffer from a “runaway greenhouse effect” and the Earth will become like Venus. This would be catastrophic. The temperature on the surface of Venus is roughly 737K (464°C or 867°F). It is hotter on the surface of Venus than on the daytime side of Mercury! In addition, Venus has very thick clouds of sulfuric acid. It is not a hospitable place. Could the Earth “go runaway” and become like Venus with the addition of more carbon dioxide to our atmosphere? James Hansen famously wrote in 2009, page 236:

“. .. if we burn all reserves of oil, gas, and coal, there’s a substantial chance that we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome [the runaway greenhouse] is a dead certainty”

Could this be true? He got the idea by studying Venus. Is Venus “runaway?” Figure 1 is a profile of the Venusian atmosphere.

Figure 1

We cannot say for sure why the surface of Venus is so hot. It is definitely hot, the temperature and pressure of the surface have been measured several times by Russian spacecraft. We also have several temperature and pressure profiles from 30 km up. We have very little, maybe no data, between the surface and 30 km. The conditions on the surface are too extreme for human equipment to last long or be reliable so what data we have is suspect. Steve Goddard has written that the extreme pressure in the Venusian atmosphere on the surface (92 atmospheres or 92 times the pressure on the surface of the Earth) causes most of the extreme heat. This is very plausible; one would expect the PVT Law to work on Venus. Others contend that the carbon dioxide greenhouse effect is sufficient to cause most of the 464°C surface temperature. Since the Venusian atmosphere is 96.5% carbon dioxide with very little water vapor this seems unlikely given the properties of carbon dioxide. No one, as far as I can tell, denies that pressure plays a role in the high temperatures. However, these two factors are not mutually exclusive. High surface pressures and high carbon dioxide concentrations, may both be involved, along with several other factors which we will discuss below. This is not an either/or debate. It’s about establishing the relative ratios between the different proposed factors.

Planetary black bodies

A planet that maintains a constant temperature emits as much energy as it absorbs from the sun. This sort of hypothetical planet is called a blackbody. The name derives from the fact that while it absorbs all radiation wavelengths, including visible light, it radiates energy as “outgoing longwave” infrared radiation (often abbreviated “OLR”) that cannot be seen by the human eye. Thus the object appears black. The specific OLR wavelengths emitted are determined by the black body temperature. The object reflects, scatters and deflects some of the incoming solar radiation due to its Bond Albedo. The geometric albedo only accounts for the direct reflection back toward the sun and this is not an appropriate value to use when computing the blackbody temperature. The Bond albedo takes into account all radiation scattered back into outer space.

The blackbody temperature of Venus is 227K, Earth’s is 254K. Neither Venus nor the Earth a true blackbodies, thus their calculated blackbody temperatures have no physical meaning.  We just use the theoretical blackbody temperature as a convenience.  Venus is closer to the sun, but has a lower blackbody temperature because of a higher albedo. The albedo of Venus is 0.77, Earth’s is 0.31. These temperatures are a hypothetical starting point; the greenhouse effect or other warming processes raise the surface temperature of Earth to 288K (15°C) on average. Temperatures on Earth range from 71°C to -92°C but the average is about 15°C. We have less data on Venus and the magnitude of its greenhouse effect and other warming processes are an open question. We have no answer, but we can discuss the problem.

Venus facts that affect climate

I’ve prepared a spreadsheet comparing many critical properties of Earth and Venus, it includes references, download it here. Below is a table containing some of the data.

Table 1

Venus has a very weak magnetic field, virtually zero and only about 0.000015 times the Earth’s magnetic field. This provides almost no protection from the solar wind or cosmic rays. Further complicating the comparison to Earth is Venus has a fairly strong electrical field, whereas Earth’s is very small, practically undetectable. This electrical field causes electrons to drift higher in the atmosphere and increases the loss of oxygen and hydrogen. It may explain why there is so little water on Venus, Venus loses twice as many hydrogen atoms as oxygen atoms to space suggesting the atoms came from water molecules being broken up in the electrical field. Without water life did not develop, thus Venus has no plants and no mechanism to remove carbon dioxide from the atmosphere.

Venus has no liquid water or oceans. Water has a huge specific heat capacity, nearly 5 times the specific heat capacity of the Venusian atmosphere. On the surface of the Earth, including both oceans and the atmosphere, 99.93% of the heat is in the oceans. In fact, if you add up all of the stored surface heat on the Earth, we have five times the amount of heat in our atmosphere/ocean system that Venus has in its atmosphere. Our temperature is lower because of the higher specific heat capacity of sea water. The oceans are not only a vast reservoir of heat; they also dampen atmospheric temperature changes. Venus has nothing this powerful to dampen their temperature swings.

The Venusian atmosphere is 96.5% carbon dioxide and as a result the greenhouse effect may be stronger on Venus than on Earth, but most of the reason for its higher surface temperature is probably pressure. The average surface temperature on Earth is 288K at one atmosphere. The same pressure on Venus is found at an altitude of about 50 km, there the temperature is roughly 339K (66°C). This is an increase of 51°C and could be due to an enhanced greenhouse effect. It is worth noting that the highest recorded air temperature on Earth is 70.7°C, so while 66°C is high, we have seen it on Earth. The remaining 398°C is probably due to the 92 atmosphere pressure at the Venusian surface.

Figure 2

The tropospheric lapse rate for Venus is about 7.7 K/km where observed. For the Earth the rate (including normal water vapor) is about 6.5 K/km. The rates are similar. Figure 2 plots altitude versus temperature for Mars, Earth and Venus. The Earth’s troposphere stops at about 12km, but the top of the Venusian troposphere is nearly 60 km. Notice that the slopes are nearly the same. If the Earth’s troposphere were as thick as the one on Venus our surface temperature would be much higher, but still less than on Venus. Presumably, the difference in projected temperature for a 60km thick troposphere is due to an enhanced greenhouse effect on Venus. Figure 3 also compares the tropospheric lapse rates for Venus and Earth and shows pressure logarithmically.

Figure 3

The Earth’s greenhouse effect is usually considered to be the blackbody temperature subtracted from the actual surface temperature or 288-254=34K. Minor fluctuations in the concentration of greenhouse gases, like carbon dioxide and water vapor, may change the temperature slightly over time. But, the enormous heat capacity of the oceans moderates and dampens any change. In addition, the oceans contain 60 times the carbon dioxide found in the atmosphere, they help regulate that as well. Further, plant growth is enhanced as temperature, water vapor and carbon dioxide increase. The biosphere currently contains 4 times the carbon found in the atmosphere. These vast reservoirs of heat, water and carbon dioxide help reduce the greenhouse effect.

Venus is different, it is a different planet after all. Its troposphere is very thick and it has no stratosphere. Immediately above the troposphere is the mesosphere. Normally we would expect the greenhouse effect to cool a “stratosphere.” But, look at Figure 1. Above the upper tropospheric boundary are highly reflective (very high albedo) clouds of sulfuric acid. These reflect solar radiation away from Venus, but they also reflect OLR downwards producing a greenhouse effect. This limits upper atmospheric cooling and raises the top of the troposphere.

There are many areas of uncertainty regarding the temperature and pressure in the Venusian atmosphere. In particular data between 30 km and the surface is sparse. For the most part we have to interpolate between 30 km and what measurements we have on the surface. Generally, these surface measurements were collected in very brief periods of time before the harsh environment crushed and destroyed the instruments. So, the data is suspect. Figure 4 shows the interpolation.

Figure 4

This profile and interpolation present a lot of problems. For one thing, carbon dioxide is a supercritical fluid above 73 atmospheres, see Figure 5.

Figure 5

A supercritical fluid is not a gas or liquid, but it has some properties of both. For one thing it causes the specific heat capacity of carbon dioxide to increase 41% from 841 J/kgK (gas at one atmosphere and 15°C) to 1185 J/kgK (supercritical fluid at 92 atmospheres and 464°C). This means it takes more heat to raise the temperature of carbon dioxide one degree on the surface of Venus than on the surface of the Earth. Another property of supercritical carbon dioxide is it can dissolve a number of gases and compounds that we would not normally suspect, like N2 and H2. Since our Venusian space probes typically have not operated on the surface of Venus very long, we really don’t know much about it.

It is not clear that the temperature interpolations have accounted for all of the uncertainty that arises from this phase change. The possible presence of a supercritical carbon dioxide “ocean” on the surface of Venus does help answer some nagging questions though. It is thought that the surface temperature on Venus is relatively uniform, even though the Venusian “day” is 117 Earth days long. Supercritical carbon dioxide is a good heat conductor and could keep the surface at close to a uniform temperature by conducting heat from the day side to the dark side of the planet. Although Table one shows that the heat content of the Earth’s surface is 5 times that of the Venusian surface; that is using the properties of carbon dioxide gas. If we assume the lower portion of the troposphere (where pressures exceed 73 atmospheres) is supercritical this difference will be reduced somewhat. But Earth will still contain more surface heat because of the very high heat capacity of our oceans.

Greenhouse Effect

The greenhouse effect theory suggests that certain “greenhouse” molecules in the air can cause atmospheric temperatures to increase as a function of their concentration. The Sun emits a lot of short wavelength visible and ultraviolet light energy and the Earth’s lower atmosphere is transparent to it, except for clouds. As surface temperatures go up, cloud feedback is more negative as Willis Eschenbach has suggested. The Earth’s surface is opaque, so it either absorbs these short wavelengths or reflects them. When the Earth absorbs the shorter wavelengths, it warms so it can emit an equivalent amount of  energy. Most of the atmosphere is transparent to these wavelengths also, so most of the emitted energy goes through it and into outer space. Water vapor, carbon dioxide and a few other molecules can absorb some of the infrared energy. Then they re-emit it, sometimes up toward outer space, sometimes back toward Earth. This slows the energy’s inevitable eventual passage to outer space and causes a small buildup of heat in the atmosphere. The greenhouse effect is real and can be demonstrated in the laboratory, but how much does it heat the atmosphere? How significant is it? No one knows. This is why the so-called ECS (equilibrium climate sensitivity of the atmosphere to a doubling of the CO2 concentration) has a huge range of 1.5 to 4.5°C according to the IPCC.

Currently, there is a vigorous debate on how much the Earth’s atmosphere can warm due to the carbon dioxide portion of the greenhouse effect. Clever experiments like those of Dr. Roy Spencer suggest an overall greenhouse effect in the atmosphere due to water vapor. But do changes in carbon dioxide warm the whole atmosphere in a measurable way? Connolly and Connolly, 2014 suggest the answer may be no. Others think carbon dioxide dominates the greenhouse effect and global warming. Dr. Spencer has suggested it may be impossible to measure. Most recent studies of satellite data suggest the carbon dioxide ECS is much lower than the IPCC has estimated. See the discussions of recent work here, here and here.

Greenhouse theory is one thing in the laboratory when we shine light at a tube filled with carbon dioxide and/or water vapor and quite another in the real world. In the Earth’s atmosphere we need to contend with air circulation, water vapor distribution, rain and evaporation, height, air density, varying temperatures and varying atmospheric composition. With all of these other factors considered what is the net impact of the greenhouse effect on our temperature? What is the net impact of the carbon dioxide component of the greenhouse effect? Water vapor is a stronger infrared absorber after all. Finally, what is the net impact of carbon dioxide from fossil fuels? Fossil fuels supply less than 4% of total carbon dioxide emissions. Most carbon dioxide generation is natural. While the overall greenhouse effect, including the effect of water vapor, is not in much doubt; the quantitative impact of man-made carbon dioxide concentration on the Earth’s climate is very much in doubt. Greenhouse theory is complex and often conflated with the probably mistaken idea that man-made carbon dioxide can raise atmospheric temperatures to dangerous levels.

Runaway Greenhouse Effect

The so-called runaway greenhouse effect is very poorly understood. Both Earth and Venus may have been runaway in their early history due to bolide impacts. When bolides strike a planet they increase the heat input and can temporarily initiate a runaway temperature increase. But, at some point the planet will reach a temperature where its radiant heat can escape. Then it can return to normal, whatever that may be. Just being hot, with an atmosphere of carbon dioxide does not make a planet runaway. Another common misconception is that the runaway greenhouse effect is simply water vapor feedback gone wild, this is not true. To be in the runaway state a planet must have reached an outgoing longwave radiation (OLR) limit, for example the “Komabayashi-Ingersoll” OLR limit, and its temperature must be increasing. This is not currently true for either the Earth or Venus. Further, reaching this OLR limit and going “runaway” is independent of carbon dioxide concentration. It requires an increase in external heating. It could be possible on Venus if the planets albedo were somehow reduced from 0.77 to much less than 0.4, but this is unlikely. Other options for increasing external heating are discussed in Goldblatt and Watson. The following quote is from, which can hardly be called a “denier website.”

“Because Earth is well under the absorbed solar radiation threshold for a runaway, water is in a regime where it condenses rather than accumulating indefinitely in the atmosphere. The opposite is true for CO2, which builds up indefinitely unless checked by silicate weathering or ocean/biosphere removal processes. … Note the traditional runaway greenhouse threshold is largely independent of CO2 … since the IR opacity is swamped by the water vapor effect. This makes it very difficult to justify concerns over an anthropogenic-induced runaway.”

Here is another quote from Scientific American:

“James Kasting, a geoscientist at The Pennsylvania State University, suspects that even in theory an anthropogenic runaway remains out of reach of humanity. Kasting performed many of the earlier seminal studies that seemed to rule out a present-day runaway, and with his student Ramses Ramirez is currently polishing a new study that reinforces those conclusions. No matter how much carbon dioxide is pumped into the present-day Earth’s atmosphere in Kasting’s models, the resulting heating is insufficient to cause the planet to rapidly boil off its oceans. “The bottom line,” Kasting says, “is that we do not get a runaway.”


Attempts to use the gas law to predict the pressure contribution to the high Venusian surface temperatures always seem to fail to produce reasonable numbers. Normally one would expect a pressure of 92 bars (92 times the pressure of the Earth atmosphere at sea level) would result in an increase in temperature of 92 times. But, we do not know the starting point and we don’t even know if we are dealing with a gas. These factors make a huge difference. So, bottom line, we don’t know how much difference pressure makes, we just know it makes some difference, probably large.

The Venusian greenhouse effect may well be larger than the greenhouse effect on the Earth. How much larger? One atmosphere is reached at an altitude of 49.5 km where the temperature is roughly 66°C. This is within the Venusian troposphere. We do not expect a linear lapse rate on Venus due to the transition to a supercritical state at some altitude above the surface. In Earth’s troposphere the dry lapse rate is approximately linear. Is the additional greenhouse effect just the 51°C difference at one atmosphere? Maybe, certainly it is unlikely to be larger than 51°C. The rest of the temperature increase could be due to pressure and the phase change, perhaps other factors are involved, but we don’t know. Since the very reflective clouds are quite high in the atmosphere and the troposphere is very thick, this author leans toward a maximum additional greenhouse effect of 51°C and a density and phase change effect of 398°C or more, but this is very speculative.

Neither Venus nor Earth are close to being in the “runaway” state. Nor is it likely that either will enter the runaway state unless solar input increases dramatically. A planet cannot go runaway due to increases in carbon dioxide alone. The whole idea that man could cause Earth to go runaway with fossil fuel emissions is implausible, even if we burned all of the fossil fuels on Earth.

The Earth and Venus are very different places and any comparisons of the two planets must be made with caution. The Earth has a strong magnetic field, no electrical field, oceans cover 70% of the Earth’s surface, the Earth’s atmosphere has a stratosphere and plants (including phytoplankton) that use carbon dioxide cover most of the Earth’s surface. Venus has almost no water and it appears that what water it generates escapes from the planet rapidly. The atmosphere has no stratosphere, the planet has no protective magnetic field to speak of and the upper atmosphere has a very high albedo. It is likely that at lower altitudes the atmosphere is supercritical. These factors will have a huge climatic affect, but how much and which direction? All we are faced with is enormous uncertainty. Comparisons are very dangerous and not recommended.

We acknowledge the invaluable help and advice from Dr. Ronan Connolly, but all errors are the author’s alone.