I’ve never really written about the 2 degree threshold before, largely because I don’t really have particularly strong views about it. Scientifically, it’s a little odd to try and characterise what is clearly a very complex system with a single number. A single number can’t really tell us much about other fatcors (like ocean acidification) that are also relevant. My personal, clearly naive, ideal is that we should be able to discuss the complexities of something like climate change in more detail and shouldn’t need to focus on a single number.
On the other hand, I can see that from a policy perspective it’s beneficial to have some kind of target, so that we can then focus our policy goals on achieving that target. Even that has its drawbacks though. Some argue that the 2 degree threshold isn’t really a dividing line between safe and dangerous. This is probably true, but – in my opinion – rather irrelevant. I suspect most would agree that there is some level of warming that would probably be catastrophic. I don’t know precisely what it would be, but somewhere between 4 and 5 degrees is probably a reasonable estimate. Given that we don’t want to get close to this, a target of staying below (or close to) 2 degrees seems reasonable.
There are others who argue that even 2 degrees is too high and that we should be doing all we can to keep the warming at an absolute minimum. I have quite a lot of sympathy with this view. The problem I have, though, is that the evidence suggests that there’s little we can do now to avoid reaching 2 degrees. It’s possible that we can still do so, but quite likely that we’ll not achieve this. Arguing that we should be staying well below 2 degrees when the best we can do is – maybe – to not miss it by too much, seems a little counter-productive.
At the end of the day, I don’t have particularly strong views. I certainly see climate change as presenting risks that we should be doing our utmost to avoid. Done properly this would achieve the goal of preventing the temperature rise from reaching levels where it becomes more likely that climate change has extreme consequences. Of course, I realise that realistic policy probably requires some kind of target and so I have no issue with the 2 degree limit, I just don’t have any particularly strong reason to defend it or attack it. One reason I thought I would write this was as a prompt to get others to express their views through the comments. As usual, I might learn something.
For other views, there are a couple of interesting RealClimate posts. In the latter post, I tried to engage in a discussion with Roger Pielke Sr about determining the radiative imbalance using ocean heat content data (he hasn’t responded yet). Unless I misunderstand him, he appears to not understand the difference between an average rate of change and an instantaneous rate of change.
I thought I’d end this post with a video of Marc Hudson interviewing Kevin Anderson. Kevin Anderson makes, in my view, a number of very valid points.