I used to be a physicist. I even used to be pretty good at Physics. I sometimes therefore get asked my opinion on climate change. This is roughly it.
One of the more difficult bits of physics is the study of complex systems, of which global climate is undoubtedly one - in fact a lot of the field was invented as a result of research by a weather forecaster called Lorenz, who found that if he changed the numbers he put into his computer simulation very very slightly, it gave a big difference in the results, which was famously explained as a butterfly flapping its wings in Asia being able to affect the course of a hurricane in the Atlantic. (The impression I get is that modern weather forecasting still hasn't really caught up with it - what they should do is put in a range of different scenarios which fit the available data, parallel process them and then come out with statistical results like "70% chance of rain" and not claim to know the shape of the clouds or whatever. My friend Chris, who is a weather forecaster, may of course correct me on this...). Basically, the upshot is that if anyone says they know what is going to happen with the weather, they're way too confident.
Anyhow, one shape that keeps coming up in the study of complex systems is the Lorenz Attractor. Basically, it seems to be good at describing how these systems work, and it's a pretty good guess to say that the global weather system will follow some kind of multi-dimensional Lorenz attractor. Features of the Lorenz Attractor are roughly as follows:
- Generally, systems appear to stay stable
- Every so often, but it's pretty much impossible to predict when, the system will "flip" to a different sort of stable. This flipping might take a long time, or it might be quite quick. In very complex systems, like global weather, they may be a very large number of different possible fairly stable states, so it's almost impossible to predict exactly what it will flip to.
- A slight change in initial conditions can make a big difference to how long a system takes to flip, or which direction it flips in.
We can apply this to the global climate:
- The global climate appears to have been fairly stable over the last 100 years or so
- It appears to be less stable now
- Over the last 150 years, we have put large quantities of carbon dioxide, heat, etc into the atmosphere.
- If the climate is flipping now, it is impossible to say for sure whether it is due to human activity, but it seems likely that human activity is at least a factor.
- We can't know precisely what the global climate will flip to
- We're used to the climate the way it was. Cities are built assuming the shoreline is where it was, agriculture assumes the climate was the way it was, etc. Any big change in climate is therefore going to be very disruptive and cause a lot of suffering, especially to the poor, because they're always the ones who get it.
- Because it is virtually impossible to do accurate long term predictions, it is very difficult to know exactly what to do about it. Would reducing the carbon dioxide emissions stop the current changes? If anyone says they know the answer to that one, they're far too confident.