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.