Ed Hawkins and colleagues have a new paper called Estimating changes in global temperature since pre-industrial times, which Ed also discusses in this post. The basic suggestion seems to be that we should probably be defining pre-industrial as the period 1720-1800, rather than as the period 1850-1900, which is often what it is assumed to be. As I understand it this is largely because our emissions probably started in the mid-1700s, rather than the mid-1800s, and that, consequently, there is a some warming that is missed if you define the baseline as being 1850-1900, rather than as 1720-1800.
This has, however, cause some confusion because some are suggesting that this new baseline means that we’re closer to the 2oC limit than we realised, with others claiming that this is not the cause because the limit was defined with respect to a known baseline, and so doesn’t change. However, it almost appears as though noone is entirely clear with respect to what baseline the 2oC limit was defined.
Sometimes, however, a temperature target is also associated with a carbon budget, which is the total mount of carbon we can emit while still giving us a certain chance of remaining below that temperature. As far as I can tell, this is clearly determined relative to when we started emitting, so if the temperature target is relative to a different period, then this would seem to suggest that the temperature target and the carbon budget are not entirely consistent. However, given that the difference is of order 0.1oC and we’re unlikely to set a target more finely than 0.5oC, it’s not clear that we would necessarily change anything. Would we really change the target to 2.1oC if we think that 2oC from 1850-1900 is the “right” target and that there was 0.1oC warming between when we started emitting and this baseline period. I can’t see why.
Something to bear in mind is how much of the carbon budget we have left. As this Carbon Brief post shows, we have about 25 years left at current emissions if we want a 50% chance of staying below 2oC. This seems like rather a tough task and it does seem that a large number of people think it extremely unlikely that we will avoid 2oC of warming. If so, why are people arguing about what the correct baseline should be when we are unlikely to meet the target whatever baseline we use? It would seem to me that what we should be doing now does not depend on whether we use 1850-1900, or 1720-1800.
This isn’t to suggest that the paper isn’t an interesting paper and that being clearer about the baseline wouldn’t be a good idea. I’m just not clear as to what difference it really makes. If we’re already at a stage where we’re going to miss a target however we define the baseline, then apart from some extra clarity, I really can’t see what overall difference this makes. I could, of course, be missing something, so feel free to point it out in the comments if I am.