This is actually the 1000th post on this blog. I haven’t written them all, but have written many more than I had ever expected to write. I thought I might use this as an opportunity to write something about the recent paper on [t]rajectories of the Earth System in the Anthropocene that has been getting a lot of media coverage.The basic point of the paper is that, as we warm, there are various possible processes that could be triggered and that some of these might start at around 2oC. Examples of these processes are shown in the figure on the right, which also shows at what temperatures these processes might be triggered. The key point is that these processes would be expected to amplify the warming and that, if we do cross these thresholds, could lead us into some kind of “hothouse” state in which parts of the planet become essentially uninhabitable.
As I understand it, what is suggested in the paper is regarded as plausible. However, one should bear in mind that the paper is really a review of this topic and doesn’t really present any new, or original, research. Also, I don’t that think that there is much certainty about the temperatures at which we might expect these processes to be triggered; they’re mostly ballpark figures. Furthermore, the timescales associated with some of these processes are probably long; centuries to millenia. I don’t think we should necessarily ignore some of these potentially slow processes, but we should probably be clear about these timescales.
However, there was one aspect of the paper that I didn’t particularly like. There was a suggestion that we might cross some threshold beyond which the state to which we’d eventually settle would largely depend on the processes that had been triggered, rather than on how much we’ve perturbed the system through our emissions. However, I think there is reasonable evidence to indicate that we’d expect the system to respond roughly linearly to our emissions. Of course, the actual response will depend on whether we’re considering fast processes that occur on short timescales (decades), or slower processes that occur on much longer timescales (centuries to millenia), but we’d expect how much we warm to broadly depend on how much we emit.
There may be some processes that, once triggered, will carry on regardless of our emissions. However, I still think that our emissions are likely to dominate the perturbation to the system and will, therefore, mostly determine how much we warm. If we emit more, we’ll warm more. If we emit less, we’ll warm less. So, yes, I think it is quite likely that some of these processes could indeed amplify our warming, but we can still influence how much we warm overall, by limiting how much we emit. I realise that the paper isn’t really suggesting otherwise, but the framing does seem to imply that there might be thresholds beyond which there would be little that we could do. I think we can essentially always do something.
My goal was to keep this post short and let others give their views in the comments. As usual, I’ve failed. I wanted to end with something motivated by a tweet from Kate Marvel, and which has essentially also been highlighted by Richard Alley in the past. Science always has uncertainties. We can estimate the relationship between our emissions and how much we will warm, but we can’t be absolutely certain of this relationship. We might warm a little less than we expect, and the consequences would be slightly less severe. We might warm a little more than we expect, and the consequences would be slightly more severe. However, as this “hothouse” paper highlights, there are various processes, about which we’re quite uncertain, that could amplify our warming and make it much worse than we expect. As far as I’m aware, we don’t know of any that could largely cancel our warming and make things much better than we expect. The uncertainty is mostly on the bad side.
I’m sure others have thoughts about this paper, so feel free to present them in the comments.
Trajectories of the Earth System in the Anthropocene (Steffen et al. 2018).
Twitter threads by Bob Kopp, Doug McNeall, Chris Colose, and Richard Betts.