Interesting first point, but I disagree. To me, the increased salience of climate change in recent years can be traced back to the 2018 Special Report on Global Warming of 1.5 °C (SR15), and in particular the meme '12 years to save the world'. Seems to have contributed to the start of School Strike for Climate, Extinction Rebellion and the Green New Deal. Another big new scary IPCC report on catastrophic climate change would further raise the salience of this issue-area.

I was thinking that $100m would be for all four of these topics, and that we'd  get cause-prioritisation VOI across all four of these areas. $100m for impact and VOI across all four seems pretty good to me (however I'm a researcher not a funder!)

On solar geo, I'm not an expert on it and am not arguing for it myself, merely reporting that its top of the 'asks' list for orgs like Silver Lining.

I actually rather like the framing in Xu & Ram - I don't think we know enough about >5 °C scenarios, so describing them as "unknown, implying beyond catastrophic, including existential threats" seems pretty reasonable to me. In any case, I cited that more to demonstrate the lack of research thats been done on these scenarios.

I think its a really good point that there's something very different between research/policy orgs and orgs that deliver products and services at scale. I basically agree, but I'd slightly tweak this to
"It is very hard for a charity to scale to more than $100 million per year without delivering a physical product or service."

Because  digital orgs/companies who deliver a digital service (GiveDirectly, Facebook/Google/etc) obviously can scale to $100 million per year. 

Hell yeah! Get JGL to star - https://www.eaglobal.org/speakers/joseph-gordon-levitt/

Do you mean just the fourth bullet, or do you think this about all four? 

The 1980s nuclear winter and asteroid papers (I'm thinking especially Sagan et al, and Alvarez et al) were very influential in changing political behaviour - Gorbachev and Reagan explicitly acknowledged that on nuclear, the asteroid evidence contributed to the 90s asteroid films and the (hugely successful!) NASA effort to track all 'dino-killers'. On the margin now, I think more scary stuff would be motivating. There's also VOI in resolving how big a concern nuclear winter is (eg some recent papers are skeptical) - if it turned out to not be as existential as we thought, that would change cause prioritisation for GCRs.

On geoengineering (sorry 'climate interventions'(!)), note 'getting more climate modelling' is a key aim for e.g. Silver Lining. 

On the fourth one, on the margin, I think more research - especially if it were the basis for an IPCC special report - would be influential. There's also VOI for our cause priotisation. It just is really remarkable how understudied it is!
https://www.pnas.org/content/114/39/10315
https://forum.effectivealtruism.org/posts/HaXxEtx4QdykBjJi7/betting-on-the-best-case-higher-end-warming-is

Megaprojects cost $1 billion or more. Ben Todd was using the (admittedly somewhat confusing) term 'EA megaproject' by which he meant a new project that could usefully spend $100m a year. So these concerns about megaprojects don't apply.
How about we use the term '$100m-scale project'? (I considered 'kiloproject' but that's really niche.)
 

Here's the interesting, frustrating evaluation report:  https://www.macfound.org/media/article_pdfs/nuclear-challenges-synthesis-report_public-final-1.29.21.pdf[16].pdf
Looks to me like a classic hits-based giving bet - you mostly don't make much impact, then occassionaly (Nixon arms control, H.W. Bush's START and Nunn-Lugar, maybe Obama JCPOA/New START) get a home run.