I think it's more brutal than that. How many humans are we willing to kill (or watch them die) in order to do much about it?

Most humans care some about animals, but they care more about humans.

(Yes, I am well aware that the developed world could accept a lower standard of living, and that would help. I question whether it's enough. And yes, I am aware that that is not an excuse for the developed world to do nothing.)

This is a good point as while there’s not an agreed upon formula for how many nonhumans are worth a human, I’m certain that the vast majority will prioritize human life over animals.

Fortunately, the human growth rate is stabilizing so hopefully we can figure out how to have humans grow to a natural equilibrium AND keep diverse ecosystem. If the question is framed as either humans or animals, society will end up with only zoos, farms, and humans.

I dislike this line of thinking. There's plenty we can do to reduce our impact on the Earth. Things like eating less beef, walking/bicycling when traveling short distances, reducing single-use plastic consumption, etc. go a long way. In the U.S., we think plastic takeout containers, driving 1/2 mi to the convenience store, eating meat every meal are normal. They're not; these are all absurd things the future will look back at with contempt.

If the world's population lived more like an average Indian, the human population could still roughly double without issue. If the world's population lived like an average American, we'd need 3.1 additional Earths to sustain us. [1] We're kind of jerks.

[1] http://www.bbc.com/news/magazine-33133712

> Things like eating less beef, walking/bicycling when traveling short distances, reducing single-use plastic consumption, etc. go a long way

I think the above things are useful, but I doubt that they would really go far -- the population growth of the past several decades is unlikely to be balanced by a few relatively minor adjustments you cite. I suspect technological advances like efficient vehicles (or synthetic beef as you put big emphasis on beef consumption) will help more than biking.

I am all for biking, but you are unlikely to reduce car use by more than 20-30% by it (bad weather, aging) while increasing miles per gallon (or a similar metric) has more upside potential.

In the end though I think we have too many people already and will have way more in 20 years; unless we address this somehow all of the above could be irrelevant. Just my 2c.