Compare five seriously bad things:
(1) Unjust discrimination along the lines of racism, sexism, etc., in Western countries.
(2) War and terrorism
(3) Global poverty
(4) Animal suffering (from factory farming)
(5) Global catastrophic (i.e. civilization-ending) risks
Just how bad is each of these, in the world as we find it today? If you could prevent just one of them, which would it be? (What would your rank ordering be if you weren't sure how many philanthropic wishes the genie was going to give you?)
It can be hard to know where to start with this question of moral prioritization. But one way to get a grip on the question is to consider how great a cost it would take (in the constant metric of, say, human lives lost) to balance out the gains of protecting against these various evils.
To avoid deontological qualms, do not ask yourself how many people you'd be willing to kill to achieve world peace (or whatever). Maybe you would not be willing to kill anyone. Instead ask: if a natural disaster struck a foreign country, at the same time as one of the above evils was magically banished (for a century, say), how bad could the loss of lives from the disaster be while still making the day seem to you one that was, all things considered, more good for the world than bad?
My above presentation lists the evils in descending order of (how it seems to me) roughly how much public attention each cause receives, e.g. on social media. (Strikingly, this seems more or less precisely the opposite of what I think the correct ranking would be in terms of actual importance.)
To consider the magnitude of each problem in turn:
(1) Unjust discrimination: A few individuals suffer greatly (or are even killed) due to this, and far greater numbers suffer more moderate harms and indignities.
It's extraordinarily difficult to attempt any sort of estimate of the total magnitude of harms here, but perhaps something in the ballpark of 500 - 1500 lives per year would be a reasonable estimate? If so, it would take around 100k deaths to balance out the gain of a century without such discrimination. (Note: this would plausibly be at least an order of magnitude greater if we were to include non-Western countries, due to how extremely badly women are treated in many parts of the world.)
(2) War, in recent decades, seems to kill around 50k - 100k people per year, and has been in general decline since the end of WWII. It also causes significant non-fatal casualties and other (e.g. economic) forms of disruption. (Terrorism, by contrast, is fairly trivial, except insofar as it affects political behaviour, but I'll bracket such indirect effects here.) So I'm going to guess that a century without war or terrorism would be worth around ten million deaths.
(3) Global poverty is obviously hugely harmful. Roughly 10% of the world's population lives in extreme poverty (less than $1.90 per day), with another 2 billion or so living on less than $3.10 per day. I assume this makes a huge difference to one's quality of life (which I take to not just reduce to momentary felt happiness, but also self-actualization through meaningful projects, etc.). It also causes many millions of preventable deaths each year, not to mention other forms of ill health (e.g. intestinal parasites, blindness due to malnutrition, etc.).
If global poverty were entirely abolished -- with everyone in the world attaining, say, the (purchasing power adjusted) level of the current U.S. poverty line -- this would vastly improve many billions of lives. It would plausibly take more than a billion deaths (or perhaps more, e.g. 50 million deaths per year over the course of the century) to outweigh these gains.
(4) "Almost 60 billion animals and bred and killed for food each year worldwide". A large portion of those experience lives of extreme suffering in factory farms. Chickens make up the largest numbers, and their lives can be quite short, so I don't have a good sense of how many life-years of suffering this corresponds to. But the total amount of suffering could plausibly exceed the badness of 50 million human deaths per year. (It gets even worse if you consider wild animal suffering too...)
(5) Global catastrophic risks: "The expected value of reducing existential risk by a mere one millionth of one percentage point is at least ten times the value of a billion human lives." Many experts estimate a 10 - 20% chance of global extinction over the next century. Hopefully that risk will decline over future centuries: there's a strong case to be made that the near future is especially volatile, with artificial superintelligence plausibly eventuating in the next century or two. If we manage to navigate that risk successfully, friendly AI could greatly improve our capacity to deal with future problems. If that's right, then miraculous protection against catastrophic risks for the next century could plausibly be worth losing a large fraction of the world's current population (at least until the point where the population loss is so great that this itself risks civilizational collapse).
Of course this depends on certain assumptions about value, such that the loss of future lives from extinction counts as a bad thing. But I think it's pretty obviously true that this is so, even if many philosophers do deny it. (Denying obvious truths is, alas, an occupational hazard.)
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So, that's a (very!) rough first attempt to quantify the magnitudes of these various problems. I should flag that this question of "how bad?" alone does not decisively settle where we should focus, as some of these problems may be more tractable than others. But it is at least suggestive, and I think a worthwhile exercise to consider.
Suggestions welcome, of course, for refining any of these estimates. I assign pretty low confidence to my estimates: any or all of them could well be completely off-track, in which case I welcome friendly corrections!