Epistemic status: Fairly confident
Here are my thoughts on climate change, with EAs and EA-sympathetic people as the target audience:
1. Expected annual mortality in around 2100 is likely between 200,000 and 2 million.
2. Climate change by itself should not be considered a major near-term global catastrophic, >10% chance of causing >10% of human mortality or existential, non-negligible chance of ending human civilization as we know it, by 2100.
3. The effects of climate change on animal welfare is basically unstudied, and relies on crucial considerations we do not know the answers of.
4. It's rather unlikely that mainline climate change mitigation efforts are more cost-effective for improving quality of life or health outcomes for current living people than mainline global health or development spending.
5. The first 3 of the above points are relatively non-controversial among experts.
6. I consider myself fairly ignorant on this topic, but most educated laymen (including journalists and activists) who talk about climate change a lot on all sides seem even more ignorant than I am.
7. Depending on your moral preferences, other beliefs about the world, and general flexibility, it is probably wise to devote your altruistic energies to working on other cause areas.
8. EAs should be careful about messaging and turning off climate change-focused people, as there is a lot of strategic overlap including a quantitative focus, caring about “big problems” and frequently a longtermist outlook.
Meta: I'm experimenting with a new way to use the EA Forum. Instead of a top-level post explaining everything, I'll just have the eight main points I'm most confident in, and then add a bunch of side points /digressions in the comments, where upvotes/downvotes can help decide whether people end up reading those points.
Edit 2019/12/29: I bounded the timeframe of 2100 to points #1 and #2. I still personally believe the original (unbounded) claims, but I think my explicit evidence is too weak and I don't expect debating meta-level arguments to change many people's minds. I appreciate the pushback (public and private) trying to keep me honest.
Just emphasizing the value of prudence and nuance, I think that this^ is a bad and possibly false way to formulate things. Being the "marginal best thing to work on for most EA people with flexible career capital" is a high bar to scale, that most people are not aiming towards, and work to prevent climate change still seems like a good thing to do if the counterfactual is to do nothing. I'd only be tempted to call work on climate change "misguided" if the person in question believes that the risks from climate change are significantly bigger than they in fact are, and wouldn't be working on climate change if they knew better. While this is true for a lot of people, I (perhaps naively) think that people who've spent their life fighting climate change know a bit more. And indeed, someone who have spent their life fighting climate change probably has career capital that's pretty specialized towards that, so it might be correct for them to keep working on it.
I'm still happy to inform people (with extreme prudence, as noted) that other causes might be better, but I think that "X is super important, possibly even more important than Y" is a better way to do this than "work on Y is misguided, so maybe you want to check out X instead".