A recent paper about [e]mission budgets and pathways consistent with limiting warming to 1.5 °C essentially argues that it is still possible to follow an emission pathway that will give us a good chance of keeping warming below 1.5oC. More specifically, if we can keep total emissions from 2015 to below 200 – 250GtC (depending on what we do with regards to non-CO 2 emissions) then we will have about a 66% chance of keeping warming to below 1.5oC.

Unfortunately, this has been interpreted in some circles as suggesting that we can relax a bit because we have more time than we had previously thought. This was mainly based on previous analyses suggesting that the carbon budget that would keep warming below 1.5oC was only about 50GtC (from 2015). Let’s be clear about something; we’re currently emitting 10GtC per year. Whether the budget is 50GtC, or 250GtC, we pretty much have to start reducing emissions now, and get them to zero as soon as we realistically can. Of course, if it is 250GtC, then that will be easier to achieve than if it is 50GtC, but it doesn’t really change what we should do, assuming that we do want to achieve this target. Personally, I think the correct framing is: if this study is correct, and if we keep total emissions from 2015 to below 200-250GtC, we might keep warming to below 1.5C.

However, I think there are some potential issues with this paper. One is that they’re assuming an 1861-1880 baseline from which they’re estimating the observed temperature change. There are arguments (here, for example) suggesting that to properly capture the warming we should use an earlier baseline, which would lead to us having warmed more than if we use a late 1800s baseline. Hence, we may already be closer to 1.5oC than this paper suggests.

Another potential issue is that a key factor in their analysis is a potential mismatch between the model warming and the observed warming (see figure on right). Their argument is that after emitting as much as we have to date, the models predict more warming than has been observed. Hence, the models are predicting a smaller carbon budget than may actually be the case.

One problem is that there have been a number of recent studies reconciling the supposed model/observation discrepancy. These include updating the forcings and doing a proper apples-to-apples comparison by using blended temperatures (i.e., climate model output is typically air temperatures, while observations are a combination of air and sea-surface). So, there may not even be as big a discrepancy as suggested in this paper.

However, even if there is a discrepancy between the models and the observations, we still don’t know if this is because the models are really running too hot, or because some internal process (the pattern of sea surface warming, for example) has suppressed some of the forced warming. If the latter, then we’d expect the observations to catch up to the models at some point in the future and, hence, the initial model estimates for the carbon budget may not be too low.

So, I would certainly be cautious about assuming that the carbon budget is indeed as high as suggested by this new paper. However, in some sense it doesn’t make a great deal of difference. Even if the carbon budget that would give us a 66% chance of staying below 1.5C is 250GtC (or ~400GtC for 2oC) achieving this is going to require pretty drastic emission cuts starting as soon as we possibly can. It certainly doesn’t, in my view, imply that we can now sit back, relax, and wait a few more years before seriously thinking about how to reduce our emissions.

Links:

Ambitious 1.5C Paris climate target is still possible, new analysis shows – Guardian.

Guest post: Why the 1.5C warming limit is not yet a geophysical impossibility – Carbon Brief.

Study saying climate change poses less of a threat than first thought ‘has been dangerously misinterpreted,’ academics warn – Evening Standard.

Did limiting global warming to 1.5C just get easier – Climate Home.

Did 1.5C suddenly get easier? – Glen Peters.

Have scientists really admitted climate change sceptics are right? – The Independent.