While longtermism is an interesting ethical principle, I believe the consequence of the extent of uncertainty involved with the impact of current decisions on future outcomes has not been fully explored. Specifically, while the expected value may seem reasonable, the magnitude of uncertainty is likely to dwarf it. I wrote a post on it and as far as I can tell, I have not seen a good argument addressing these issues.
https://medium.com/@venky.physics/the-fundamental-problem-with-longtermism-33c9cfbbe7a5
To be clear, I understand the argument of risk-reward tradeoff and how one is often irrationally risk-averse but I am not talking about that here.
One way to think of this is the following: if the impact of an intervention at present to influence long term future is characterized as a random variable X(t) , then, while the expectation value could be positive:
the standard deviation as a measure of uncertainty ,
could be so large that the coefficient of variation is very small:
Further if the probability of a large downside, is not negligible, where , then I don't think that the intervention is very effective.
Perhaps I have missed something here or there have been some good arguments against this perspective that I am not aware. I'd happy to hear about these.
I think that the case for longtermism gets stronger if you consider truly irreversible catastrophic risks, for example human extinction. Lets say that there is a chance of 10% for the extinction of humankind. Suppose you suggest some policy that reduces this risk by 2%, but introduces a new extinction risk with a probability of 1%. Then it would be wise to enact this policy.
This kind of reasoning would be probably wrong if you have a chance of 2% for a very good outcome such as unlimited cheap energy, but an additional extinction risk of 1%.
Moreover, you cannot argue that everything will be OK several thousand years in the future if humankind is eradicated instead of "just" reduced to a much smaller population size.
Your forum and your blog post contain many interesting thoughts and I think that the role of high variations in longtermism is indeed underexplored. Nevertheless, I think that even if everything that you have written is correct, it would still be sensible to limit global warming and care for extinction risks.