Global warming, as many know, can be a fairly contentious subject with some believing it’s not happening at all, others believing that it is but is not a consequence of human activity, and the rest believing it is indeed a consequence of human activity. Personally, I believe that the planet is warming and that it is a direct consequence of what we are doing. The main reason that I believe this is that although the carbon dioxide (CO2) concentration in the atmosphere has varied quite a lot over the last 400000 years, until about 1800 it had never been higher than 300 parts per million per volume (ppmv). It is now 385 ppmv. This rise in CO2 concentration also correlates with a rise in average temperature with, importantly, the rise in CO2 leading the rise in temperature.
Whatever anyone believes, I think it would be useful for people to have some understanding of the planet Venus. First, however, I should talk about planets in general, and the Earth in particular. It is fairly straightforward to show that the average temperature of a planet, in Kelvin (K), in the Solar System should be Tavg = 279 (1 – A)1/4rp-1/2, where rp is the distance of the planet from the Sun in Astronomical Units (AU – the distance of the Earth from the Sun), and A is the fraction of the incident radiation that is reflected and not involved in heating the planet.
For the Earth, rp = 1 and A = 0.29 (i.e., the Earth reflects 29 % of the incident radiation). The average temperature of the Earth should therefore be 256 K. The conversion from Kelvin to oC is 273 K = 0oC, so the average temperature of the Earth should be about -16oC. This clearly isn’t the case and in fact, the average temperature of the Earth is more like +15oC. Why is this? The Earth has an atmosphere that contains various molecules, in particular CO2 and water vapour. The radiation from the Sun is mainly in the optical, most of which passes easily through the atmosphere to be absorbed by the planet’s surface. The Earth, however, is much cooler than the Sun and so reemits radiation at longer wavelengths – mainly in the infrared. The atmosphere is not particularly transparent at these wavelengths, trapping them and causing the Earth to heat up. As the Earth gets hotter, the radiation it reemits moves to shorter and shorter wavelengths, allowing more and more energy to escape back into space. Eventually the Earth reaches a temperature where as much radiant energy escapes back into space as is absorbed from the Sun. The Earth has then reached it’s equilibrium temperature which, fortunately for us, produces an average temperature of 15oC. This is a form of greenhouse warning that is clearly beneficial to us.
Now, what about Venus. Venus is a planet that is very similar in size to the Earth and probably formed at about the same time as the Earth. It is quite likely that at some point in the distant past it may have had an atmosphere similar to the Earth’s. If we assume that it also would have reflected about 29% of the incident radiation, and knowing that for Venus rp = 0.723, then we would expect it’s average temperature to be about 301 K or 28oC. As with the Earth, global warming would cause Venus to heat up to a higher average temperature, but one might naively expect this to have less of an effect than on the Earth since Venus might already be hot enough to be reemiting radiation at a wavelength for which the atmosphere is reasonably transparent.
So, what is Venus’s actual average temperature. Today Venus has an average temperature of 480oC, more than 10 times hotter than we would expect based on the above calculation. The reason is that to prevent greenhouse warming, greenhouse gases like CO2 and water vapour need to be kept out of the atmosphere. On Earth, water is mainly liquid (oceans) and most of the CO2 has dissolved in this liquid water ultimately forming carbonate rocks. Because Venus started with a higher average temperature than the Earth, there would be more water vapour in the atmosphere and less liquid water. Since water vapour is also a greenhouse gas this would actually act to also cause Venus to heat up adding more water vapour into the atmosphere. The lack of liquid water also means that more of the CO2 would be in the atmosphere (rather than in the form of carbonate rocks) also causing more greenhouse warming. In the case of Venus this lead to a runaway process in which more and more water vapour and CO2 was added to the atmosphere causing the planet to heat up until it eventually reaches it’s current equilibrium temperature of 480oC. The water vapour would also be dissociated by UV photons allowing the constituents elements (hydrogen and oxygen) to escape into space. Ultimately Venus is a planet with almost no (if any) water and an atmosphere that is primarily composed of CO2.
Is this relevant to the Earth and to us. in some sense no, because what happened on Venus was almost certainly natural and occurred because Venus was closer to the Sun and hence had an initially higher average temperature. However, it is clear that it has undergone a runaway greenhouse process that increased it’s temperature from a value where liquid water should be able to exist to one where there is no water at all and no life could possibly survive. It also seems that an Earth-like planet with an average temperature of 30 – 40oC could easily undergo this process. Although the Earth’s average temperature is somewhat lower than this, it’s not quite as far away as we might like. We also don’t really know at what temperature this runaway process actually starts. Currently it is predicted that the Earth’s average temperature will rise by more than 1o C per century. If this continues, this means it will take only a thousand years or so to reach a temperature at which the runaway greenhouse process should start. However, whatever anyone actually believes is happening at the moment, the fact that the runaway greenhouse process has actually happened on a planet that initially was not significantly different to our own should at least, in my view, give us pause for thought.