Climate change

I was at a party last night with a couple of geophysicists, one of whom had a particular interest in climate change. What was interesting was that one of them didn’t seem particularly convinced that we should necessarily worry about climate change. I asked about the relevance of Venus (which I blogged about here) and she seemed to think that even though Venus has a carbon dioxide rich atmosphere and has an equilibrium temperature of 700 K, we don’t know what the Earth’s equilibrium temperature would be if it’s atmosphere were much more carbon dioxide rich than it is now. She seemed to think that it may be the case that if we added as much carbon dioxide as we possibly could to the Earth’s atmosphere, the Earth might just heat up a little and then stabilise at some temperature slightly higher than it is today.

In fairness this wasn’t an argument against changing to renewable energy sources, but simply a suggestion that science hasn’t yet proven convincingly that adding lots of carbon dioxide to the Earth’s atmosphere could lead to runaway global warming. Although she may be strictly correct (i.e., that science hasn’t yet provided convincing prove), I still think that Venus’s properties suggests that it is reasonable to be worried (very?) about putting too much carbon dioxide into the Earth’s atmosphere.

As I discussed in an earlier post Venus being closer to the Sun resulted in more water vapour and carbon dioxide in the atmosphere which lead to a runaway greenhouse process, resulting in Venus having an equilibrium temperature of 700 K (500^oC). What I didn’t mention in that post is that Venus reflects about 75% of the incident radiation from the Sun (the Earth reflects about 30%). This means that the amount of energy reaching the surface of Venus is actually less than the amount of energy reaching the surface of the Earth, even though Venus is closer to the Sun than the Earth.

So what does this tell us? It tells us that a planet that started off similar to the Earth but closer to the Sun (and hence receiving more Solar energy) has ended up with a surface temperature of 700 K, but now absorbs less energy from the Sun than the Earth (since the carbon dioxide rich atmosphere now reflects a large fraction of the incident radiation). Of course the Earth can’t undergo exactly the same process as Venus because for the same atmospheric conditions, the Earth’s surface has to receive less Solar Energy than the Sun.

However, the expected ratio of the temperature of Venus to the temperature of the Earth is 1.17:1 (i.e., Venus should be about 17 % hotter than the Earth). The difference is of course because Venus reached a tipping point where the process ran away, the temperature rose uncontrollably, water vapour and oxygen escaped from the atmosphere and the process only stabilised once essentially all the carbon dioxide was in the atmosphere and all the water vapour and oxygen had escaped. I guess this person I was speaking to was technically correct in that we don’t know where this tipping point is. What I think we do know is that if we reach it, we would expect the Earth’s temperature to stabilise at something like 630 K (357^oC). The actual transmission properties of the atmosphere will influence this slightly, but it’s hard to see how it could be such as to produce a stable temperature significantly below this.

Ultimately this person’s main gripe seemed to be about how alarmist some of the climate change rhetoric can be. In principle I would tend to agree that any form of extremism is generally misguided and that with an educated society we should be able to approach this openly and honestly. Ultimately, however, this doesn’t seem to have worked very well so far. I happen to believe that climate change might be the greatest threat we’ve ever faced and if we are going to be extreme and alarmist about something, this might be it. The real problem seems to be that our leaders and politicians haven’t had the courage to do what really needs to be done and until we can rely on them to make the difficult decisions, I’m not sure what else we can do.

Global warming and the planet Venus

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 T_avg = 279 (1 – A)^1/4r_p^-1/2, where r_p 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, r_p = 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 = 0^oC, so the average temperature of the Earth should be about -16^oC. This clearly isn’t the case and in fact, the average temperature of the Earth is more like +15^oC. 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 15^oC. 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 r_p = 0.723, then we would expect it’s average temperature to be about 301 K or 28^oC. 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 480^oC, 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 480^oC. 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 – 40^oC 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 1^o 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.