As per my comment on HaukeHillebrandt’s comment below—The trouble with these estimates is that I’m not convinced they do a good job of considering how costs change as a technology is scaled. For example, we’ve seen this with solar—http://solarsouthwest.co.uk/solar-panel-cost/. Do you have a recommended source which does somehow take account of these effects? If not, we’re not really comparing costs properly.
Also, I want to specifically comment on this. Unless you believe that very large scale CO2 air-capture is going to be economically/technologically/land-use viable, we don’t have time to wait for people to get richer. The CO2 being emitted today, is committing humanity to a particular temperature rise for centuries to come. The cheapest time to deal with that is right now, to avoid putting the CO2 in the atmosphere in the first place.
It is true we generally see reduction in costs as cumulative production increases (this is called learning in economics). But then this means it might be cheaper to reduce CO2 emissions in the future (at least at the margin for EA, and even for the world as a whole if some of the learning occurs in related fields that does not require spending money on CO2 mitigation now). It is possible that renewable energy will become less expensive than fossil fuels in the near future, though usually the comparison is made with fossil fuel electricity. It is much more difficult for renewable energy to be lower cost than fuels used directly. Furthermore, if we want to go back to 350 PPM, we would need to do some form of air capture, which I think will be expensive for quite a while. So overall, with learning, it would reduce the cost of solving the problem, but I think it is harder to imagine it being less than $1 trillion present value with low discounting. You are right that there is a trade off. If we spend money on saving lives at $3000 per life now with health interventions instead of reducing CO2 emissions, that means more CO2 in the atmosphere in 100 years. So the question is whether that harm to the relatively richer people in 100 years is greater than the harm you avert by spending money on global health now if your time horizon only extends about 100 years.
The final thing which makes this all more complex, is that climate change is something which we are on a very well defined trajectory towards—where inaction results in terrible consequences. However, things like nuclear war are risks which may never materialize. If we invest effort into averting credible but potential risks, we’ll never be sure whether that investment actually mattered. If we invest effort in averting climate change, we’ll be much more sure that the effort was worthwhile.
Full-scale nuclear war may very well not happen this century. However, when you include additional catastrophes such as extreme weather on multiple continents (which a UK government study estimated had an ~80% likelihood this century), regional nuclear war, etc., it appears to be more likely than not that we will have one of these catastrophes this century. But it is possible that we will not have one of these catastrophes. As I said in my 80,000 Hours interview, if you are someone who has paid for insurance where they have gotten no payout from it whether they have wasted their money, they say “no” because it makes sense to insure things we can’t afford. So I think of this as an insurance policy for the world. And actually in terms of probabilities, I would say one of these agricultural catastrophes is actually more likely than median or worse slow climate change, so the probability of the investment paying off is actually higher for alternate food preparedness.