A paper by Roger Pielke Jr, Matthew Burgess and Justin Ritchie has been submitted that suggests that the most plausible 2005-2040 emission scenarios project less than 2.5oC of warming by 2100. It’s generated a bit of debate on social media, so I thought I might write a post with some thoughts.
What the paper does is compare emission growth rates in IPCC scenarios with observed growth rates over the period 2005-2020, or with a combination of observed growth rates and those suggested by a set of IEA scenarios that go out to 2040. Those that most closely matched were then assumed to be the most plausible when it comes to projecting emissions, and warming, to 2100. The most plausible suggest a change in forcing of about 3.4 Wm-2 by 2100, only a small fraction are consistent with 6 Wm-2 by 2100, and the even higher emission scenarios (RCP7.0 and RCP8.5) lie far outside the envelope of plausible scenarios.
However, as Ken Caldeira pointed out on Twitter, would you expect grow rates in the early part of a century to be a good predictor of century-scale emissions? Imagine going back to the early 20th century, developing a set of emission scenarios for the 20th century, and then basing their plausibility on how well they match early 20th century grow rates. How well do you think you would have done?
On the other hand, given that there are active efforts to limit how much we emit and that the Paris agreement has the aim of limiting global warming to well below 2oC (with aspirations to limit it to below 1.5oC) we may well now be in a position where our current trajectory is taking us towards lower emission pathways and, hence, lower levels of global warming. So, my general sense is that the basic result in the paper is not unreasonable.
However, this is a very complex socio-economic system. How much we emit in future depends quite strongly on what we do in future. We could be stupid and find some way to extract hydrocarbons from clathrates, making the higher emission pathways suddenly more likely, or we could be clever and implement effective negative emission technologies that suddenly make the very low emission pathways plausible. I think both are unlikely, but neither is impossible.
Also, this is not an independent system. If we suddenly believe that we’ve done enough to limit warming to 2.5oC and relax efforts to limit future emissions, the higher emission pathways become more plausible. On the other hand, if we believe that we should re-double our efforts to limit future emisisons, then the lower emission pathways become more plausible. I do think we should be cautious of making probabilistic claims about complex socio-economic systems.
Also, it appears that there are a number of factors that this paper ignores. For example, it seems to be considering only CO2 emissions from fossil fuels and industry. It doesn’t consider emissions due to land use change, or (as far as I’m aware) consider the emission of non-CO2 greenhouse gases, such as methane. Both of these are factors that can influence how much we would need to emit to reach a certain change in forcing, and hence level of global warming. Similarly, it doesn’t include carbon released through thawing of the permafrost, which would act as an additional emission source. There’s also some uncertainty when it comes to associating emissions with concentrations. We could end up at a higher forcing level even if we follow an emission pathway typically associated with a lower forcing level.
I’m not suggesting that this implies that the basic conclusion is wrong; I do think that our current trajectory is taking us towards something like ~2.5oC (± ~1oC) but I’m less convinced that the confidence in this is warranted. I do think the paper would have benefitted from a broader discussion of the various caveats and uncertainties. In fact, given that two of the authors have also published a paper suggesting that the misuse of scenarios is a research integrity issue, you might expect this to have been more explicit. Of course, if you’ve been involved in the public climate debate for a while, you probably wouldn’t.
For some reason, Roger decided to challenge myself and Richard Betts to a bet on the basis of his claim that “In 2040 global CO2 emissions from energy will be closer to levels projected by SSP1-2.6 than to SSP4-6.0”. This seemed a bit odd given that there seemed little reason to bet on something that we would rather didn’t happen, and have never suggested is necessarily all that likely to happen. However, it seems that this may have been an attempt to somehow test our arguments. If we’re suggesting that something close to SSP4-6.0 is plausible, but won’t take a bet on this actually materialising, then that supposedly says something about the plausibility. One should probably, though, convolve this with the probability of someone betting on an outcome that they neither want to happen, nor think is all that likely to happen.