(See Ben's comment below for the brief...)
Great read, although I think there are a couple of points, some already mentioned, that are worth highlighting further:
With regards details:
I would agree that the Stern report is now pretty old hat, it's over a decade old and there has been plenty of work since then, both on the climate science side (e.g. the discussions below on TCR vs. ECS) and on the economics side (e.g. what discount rate and damage function to use to work out impacts on global GDP).
I'd also say that it is remiss to leave out the IPCC's latest report on 1.5 degrees, given that it includes all of the most recent literature (and also given that it took on board a fair bit of previous criticisms of IPCC reports, regarding presentation of data, discussion of probabilities etc.).
Beyond this, I would go further and say that, to be honest, discussions about tail-risks of 6 degrees of warming, based off of uncertainties in ECS or TCR values, are of second-order importance. I think you can make a very convincing case for acting on climate change simply based off of the evidence presented in the IPCC SR1.5:
I checked some numbers, in order to illustrate the point about judging the impacts climate change. (The specific paper cited in the SR1.5 that is relevant for this is Byers et al., 2018)
Chapter 5 of the report cites a range of 62-457m fewer people that are 'exposed and vulnerable' at 1.5 versus 2 degrees of warming, based on three contrasting pathways out to 2100 (i.e. exposed to multi-sector climate risks and vulnerable to poverty, defined as income <10$/day).
Given the IPCC only backs these findings with 'middle confidence', if we take the lower range then we have ~62m fewer people exposed and vulnerable, at 1.5 degrees. If we spread that out over the 80 years out to 2100, it's about 800,000 people each year who are either vulnerable to poverty or susceptible to various risks. That's roughly twice as many people as die from malaria each year, one of the highest-priority sectors for GWWC. Obviously, the people affected by climate change aren't dying each year, but they are more susceptible to poverty, in worse living conditions, higher risk of extreme weather events etc. If we work on basis of reducing suffering then surely, even just the fact that, right now, you have comparable numbers of people being affected, should somehow be discussed and taken into account far more.
This is just one example, from one paper cited in the report - there are obviously far more, adressing a very broad variety of impacts. Another great reference is a paper by Pretis et al. (2018, Philosophical Transactions of the Royal Society) that highlights both the aggregate impacts on global GDP and the significant geographical disparities in the impacts on economic growth: the largest relative impact of 2C vs. 1.5C is on people on lower incomes in South-East Asia and Sub-Saharan Africa, predominantly living in coastal areas. This has been discussed in other comments as well, but I think that this specific paper is excellent.
In my opinion, there is little need to go into discussions about climate sensitivity, or tail-risks of 9 degrees, to make a strong case for weighting climate change more highly.
There is also the point about the adequacy of the ITN framework and the inclusion of 'urgency', which I think is a great one:
Beyond just the urgency of the problem with regards timescales of action, there is also the totality of the issue, as discussed in John's summary report and mentioned below - we have to get to net-zero, you can't just reduce GHG emissions by 50, 90 or even 99%.
This is a point that doesn't get discussed enough (beyond the fact that, for example, the report page on 80000 hours is mad old and out of date). Contrast this with, say, improving international discussions about nuclear disarmament, which could reduce the associated overall x-risk by 50% - a 50% reduction in GHG emissions will never be enough, because we'll still be warming things up. So, even though the marginal impact of a few keen EA people working on nuclear war, biohazards or AI might be very high, this neglects the fact that when it comes to climate change, the reductions have to be total, and we can't settle for less.
I think that this should also be taken into account in terms of 'urgency', or maybe it could somehow be factored into another part of the ITN framework. As you discuss in the post, I highly doubt that climate change is an x-risk, but I feel like this isn't really even the point: it doesn't need to be, nor does it even need to be a 'multiplier', in order to be taken seriously.
Your point about the bias towards intellectually stimulating problems is also fair, I think - there are always people who love to be 'bearish' and predict doom, but I think it is a bit of a form of escapism to choose to focus on other issues. I think there might be something in the fact that it is, frankly, fairly tedious work on continuous international discussions, the minutiae of national-level policies or working out how to influence financial flows for infrastructure. Still, I think this point was a really good one, and unfortunately one that a lot of people likely need to hear.
These last two points together, I think, tie in with what you said about the tractability of climate change: it is reasonably tractable, which probably makes it less sexy to work on, which means that EA people avoid it, and also means that we will never achieve the goal of totally reducing GHG emissions to net-zero.
Finally, I really do think that featuring interviews with a few prominent climate scientists or economists should be a big priority, as you could easily thrash out any misconceptions, worries or misunderstandings on either front - there are loads based in Oxford or London, where lots of UK EA people are. For the science, people like: Myles Allen, Friederike Otto, Sir Brian Hoskins, Joeri Rogelj, Tim Palmer (mentioned in John's report). For the economics: Cameron Hepburn, Simon Dietz, Joeri Rogelj again. Just get them on Rob's podcast, and clear this stuff up!
Thanks for the article, and for the great discussion it has already prompted, as seen in the comments.