I know David well, and David, if you're reading this, apologies if it comes across as a bit uncharitable. But as far as I've ever been able to see, every important argument he makes in any of his papers against longtermism or the astronomical value of x-risk reduction was refuted pretty unambiguously before it was written. An unfortunate feature of an objection that comes after its own rebuttal is that sometimes people familiar with the arguments will skim it and say "weird, nothing new here" and move on, and people encountering it for the first time will think no response has been made.

For example,^[1] I think the standard response to his arguments in "The Scope of Longtermism" would just be the Greaves and MacAskill "Case for Strong Longtermism".^[2] The Case, in a nutshell, is that by giving to the Planetary Society or B612 Foundation to improve our asteroid/comet monitoring, we do more than 2x as much good in the long term, even on relatively low estimates of the value of the future, than giving to the top GiveWell charity does in the short term. So if you think GiveWell tells us the most cost-effective way to improve the short term, you have to think that, whenever your decision problem is "where to give a dollar", the overall best action does more good in the long term than in the short term.

You can certainly disagree with this argument on various grounds--e.g. you can think that non-GiveWell charities do much more good in the short term, or that the value of preventing extinction by asteroid is negative, or for that matter that the Planetary Society or B612 Foundation will just steal the money--but not with the arguments David offers in "The Scope of Longtermism".

His argument [again, in a nutshell] is that there are three common "scope-limiting phenomena", i.e. phenomena that make it the case that the overall best action does more good in the long term than in the short term in relatively few decision situations. These are

1. rapid diminution (the positive impact of the action per unit time quickly falls to 0),
2. washing out (the long-term impact of the action has positive and negative features which are hard to predict and cancel out in expectation), and
3. option unawareness (there's an action that would empirically have large long-term impact but we don't know what it is).

He grants that when Congress was deciding what to do with the money that originally went into an asteroid monitoring program called the Space Guard Survey, longtermism seems to have held. So he's explicitly not relying on an argument that there isn't much value to trying to prevent x-risk from asteroids. Nevertheless, he never addresses the natural follow-up regarding contributing to improved asteroid monitoring today.

Re (1), he cites Kelly (2019) and Sevilla (2021) as reasons to be skeptical of claims from the "persistence" literature about various distant cultural, technological, or military developments having had long-term effects on the arc of history. Granting this doesn't affect the Case that whenever your decision problem is "where to give a dollar", the overall best action does more good in the long term than in the short term.^[3]

Re (2), he says that we often have only weak evidence about a given action's impact on the long-term future. He defends this by pointing out (a) that attempts to forecast actions' impacts on a >20 year timescale have a mixed track record, (b) that professional forecasters are often skeptical of the ability to make such forecasts, and (c) that the overall impact of an action on the value of world is typically composed of its impacts on various other variables (e.g. the number of people and how well-off they are), and since it's hard to forecast any of these components, it's typically even harder to forecast the action's impact on value itself. None of this applies to the Case. We can grant that most actions have hard-to-predict long-term consequences, and that forecasters would recognize this, without denying that in most decision-situations (including all those where the question is where to give a dollar), there is one action that has long-term benefits more than 2x as great as the short-term benefits of giving to the top GiveWell charity: namely the action of giving to the Planetary Society or B612 Foundation. There is not a mixed track record of forecasting the >20 year impact of asteroid/comet monitoring, and no evidence that professional forecasters are skeptical of making such forecasts, and he implicitly grants that the complexity of forecasting its long-term impact on value isn't an issue in this case when it comes to the Space Guard Survey.

Re (3), again, the claim the Case makes is that we have identified one such action.

1. ^{^}

   I also emailed him about an objection to his "Existential Risk Pessimism and the Time of Perils" in November and followed up in February, but he's responded only to say that he's been too busy to consider it.

2. ^{^}

   Which he cites! Note that Greaves and MacAskill defend a stronger view than the one I'm presenting here, in particular that all near-best actions do much more good in the long term than in the short term. But what David argues against is the weaker view I lay out here.

3. ^{^}

   Incidentally, he cites the fact that "Hiroshima and Nagasaki returned to their pre-war population levels by the mid-1950s" as an especially striking illustration of lack of persistence. But as I mentioned to him at the time, it's compatible with the possibility that those regions have some population path, and we "jumped back in time" on it, such that from now on the cities always have about as many people at t as they would have had at t+10. If so, bombing them could still have most of its effects in the future.