There’s a recent paper on carbon cycle uncertainties and the Paris agreement (Holden et al. 2018). It considers two mitigation pathways, one that keeps end of century warming to below ~2oC, and the other that keeps end of century warming to below ~1.5oC. The interesting thing about the paper is that it uses a climate-carbon-cycle model (i.e., it considers emissions, rather than concentrations) and it also considers scenarios that don’t include negative emissions. The reason I wanted to highlight the paper is that it says something that I think is worth repeating.
A widely held misconception is that given the approximately 1 °C warming to date, and considering the committed warming (warming that will inevitably happen) concealed by ocean thermal inertia, the 1.5 °C target of the Paris Agreement is already impossible. However, it is cumulative emissions that define peak warming. When carbon emissions cease, terrestrial and marine sinks are projected to draw down atmospheric carbon dioxide (CO2), approximately cancelling the lagging warming. Although the sign of this ‘zero emissions commitment’ is uncertain, its contribution can be neglected for low-CO2 scenarios. Therefore, at least when considering CO2 emissions in isolation, keeping below 1.5 °C of warming will remain physically achievable until the point that it is reached.
Essentially, it’s never actually impossible to keep warming below some level, until we’ve actually crossed that threshold. Of course, it’s not going to be possible – in reality – to immediately get emissions to zero, but I do think it’s worth recognising that our warming committment depends largely on how much we emit in future, and that the ocean thermal inertia does not mean that there is warming that we cannot avoid.
A couple of other interesting results in the paper. One was that the carbon cycle uncertainties contribute quite a lot to the peak warming uncertainty. The other is that in these rapid decarbonisation scenarios, decadal variability can dominate over the mean response in some critical regions. Something that I should probably make clear, though, is that even though it is never technically too late to avoid further warming, even this paper indicates that having a good chance of limiting warming to levels below about 2oC (relative to 1870) without negative emissions will require emitting no more – since 2017 – than about 300-400 gigatonnes of carbon. That’s about 30-40 years at current emissions. It might be possible, but it’s not going to be easy.
There is some criticism in the comments because I said “30-40 years at current emissions”. For clarity, I wasn’t suggesting that we simply carry on as we are for 30-40 years, and then instantaneously stop. I was simply trying to put the carbon budget into some kind of context. To limit warming to ~2oC is going to require (without negative emissions) that we emit from now (in total) no more than what we would emit were we to continue emitting at current levels for about 30-40 years. To do this will almost certainly require steep emission reductions starting very soon.
A bit more about committed warming (a post with some more details about our committed warming).
Emission reductions, negative emissions, and overshoots (a post about emission pathways that would satisfy the Paris goals).