One of the talking points of COP27 is giving up, or compromising, on the 1.5oC target. The main reason is simply that the remaining carbon budget is very small. Essentially, to stop global warming requires getting human emissions to (net) zero. Having a good chance of limiting warming to < 1.5oC would therefore require doing so while also staying within what is now a very small carbon budget.
Even though I understand why we should maybe accept that meeting the 1.5oC target is probably no longer possible, I’m uncomfortable with doing so. One reason is simply that the current evidence makes clear that future warming depends mostly future emissions. Since we haven’t crossed the 1.5oC threshold yet means that it is still technically possible to avoid doing so; the constraints are mostly societal/political/technological/economic etc.
You could argue that these constraints are as real as the constraints associated with the climate system itself, but I’m a physicist, so I’m biased towards seeing the latter as more fundamental than the former. Even if it would be very challenging to limit future emissions so that we have a good chance of keeping warming below 1.5oC, it’s not actually impossible to do so.
Another issue is that the emissions that will likely drive warming well past 1.5oC are probably going to be from infrastructure that is not yet in place. According to this paper by Tong et al., the best estimate of cumulative emissions, starting in 2018, from existing infrastructure is 658 GtCO2. At the same time, the carbon budget for having a 50% change of keeping warming below 1.5oC was about 580 GtCO2.
I realise that the carbon budget has been revised down somewhat and that cumulative emissions from existing infrastructure could be much higher if their actual lifetimes exceed what is planned. However, they are of a similar magnitude. This does suggest that emissions from infrastructure that is planned, but not yet in place, or not yet planned but likely to be implemented, will play a key role in determining how close we get to meeting the 1.50C target, or how badly we miss it. Maybe I’m just being a naive physicist, but this does seem to be something over which humanity has some control.
A final point I wanted to make is that carbon budgets typically give a 50%, or 66%, chance of meeting the target. Hence, not staying within the carbon budget doesn’t immediately mean that the target will be missed. It just means that it is less likely. Hence, if we try to stay within the budget and just fail, there is still a chance that the target will be met. Also, if it isn’t, it will probably be missed by a much smaller margin that if we fail to try because it was decided that it was no longer possible to meet the target.
Links:COP27: Fears of compromise on key 1.5C global temperature issue – BBC article.
Guest post: What the tiny remaining 1.5C carbon budget means for climate policy – Carbon Brief post about the remaining carbon budget.
Committed emissions from existing energy infrastructure jeopardize 1.5 °C climate target – Nature paper by Tong et al.