Here is a simple point estimate illustrating how one could get the cost-effectiveness of decreasing GHG emissions from a longtermist perspective: “cost-effectiveness of decreasing GHG (1 kt/$)”*”increase in the value of the future if all risk from climate change was eliminated (0.1 bp)”/”reduction in GHG emissions to eliminate roughly all risk from climate change (4 Tt)” = 0.025 bp/G$, which would be half of OP’s conservative bar.
Here is a simple point estimate illustrating how one could get the cost-effectiveness of decreasing GHG emissions from a longtermist perspective: “cost-effectiveness of decreasing GHG (1 kt/$)”*”increase in the value of the future if all risk from climate change was eliminated (0.1 bp)”/”reduction in GHG emissions to eliminate roughly all risk from climate change (4 Tt)” = 0.025 bp/G$, which would be half of OP’s conservative bar.