Giving What We Can has a section on climate charities[0]. One listed is Cool Earth, which I thought Givewell had recommended in the past, but I'm not seeing it in any of their archived recommendations.

(It's also worth noting that addressing global poverty is a pro-climate activity, in addition to be a moral imperative. Its climate-effects may be more long-term than is relevant, though.)

[0] https://www.givingwhatwecan.org/research/other-causes/climat...