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 (500oC). 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 (357oC). 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.