Charity Evaluator "Giving What We Can" has determined that Cool Earth is very effective, based on their direct impacts: https://www.givingwhatwecan.org/report/cool-earth/ For example, they are able to save as much CO2 as you emit in a year (20 tons) with a donation of only $27.

Other organizations work on political change, which may be much more effective but is harder to measure. https://350.org/ is the biggest organization working on that. http://catf.us/ is focussing on carbon-free energy.