> Did you mean to qualify this somehow, like “non-livestock mammalian biomass”? That would still be incorrect, but at least might not be off by orders of magnitude.

Not necessary. The loss in biomass in fish[0][1] and insects[2][3] and tropical rainforests[4] can account for that. Birds have also taken a deep dive[6]. The most visible I think it is for fish. There alone we've lost an approximate 80% since the 1950ies. Imagine that. 80% of fish biomass, just gone. Similar for insects. German nature reserves have seen a decline of insect population by 75%. Really; non-livestock mammalian biomass is the least of our problems.

> Not really. Even desalination is cheap enough at this point (on the order of 1kL/$, last I checked) that we’d be fine with modest cost increases.

That only covers populations near bodies of water. Ironically these are the most vulnerable to climate change and threatened by the rising sea level.
And while it might be possible to build and maintain the desalination infrastructure for the 9M people in Israel, good look trying the same thing in countries like India[8] or Balgladesh[8a]?

Water crises have been ranked in the top five of the World Economic Forum's Global Risks by Impact list nearly every year since 2012[9].

A quarter of the world's population faces severe water shortage[10]. Are you going to provide water to 3 Billion people using desalination plants? In many of these places, even water treatment plants are too expensive to operate. What are you going to power these plants with? Coal? We don't even have a plan to migrate the already existing energy infrastructure to renewables.

> What exactly are you predicting here? This sounds very apocalyptic but doesn’t match any plausible model I’m aware of for ecological failure modes

Here's a helpful link[11].

The norther African coast of the Mediterranean used to be woodland. The continued depletion and over exploitation of the has led to it being the arid region it is today [12]. Now imagine that, but on a global scale. Because the last 250 years have been harsh for our ecosystem and it's now as threadbare as it can be. Ecosystem fragmentation is already a dire issue. The strings in the web are breaking, one by one [13].

[0] https://www.aaas.org/news/researcher-reports-stunning-losses...

[1] https://www.researchgate.net/publication/266684781_A_century...

[2] https://journals.plos.org/plosone/article?id=10.1371/journal...

[3] https://en.wikipedia.org/wiki/Decline_in_insect_populations

[4] https://www.researchgate.net/publication/326657472_Loss_of_b...

[5] https://www.sciencedaily.com/releases/2021/02/210225082525.h...

[6] https://abcbirds.org/wp-content/uploads/2019/10/Bird-Decline...

[7] https://www.researchgate.net/publication/309200743_Defaunati...

[8] https://qz.com/india/1114843/chinas-grand-plan-for-the-brahm...

[8a] https://undark.org/2021/08/04/the-water-crisis-in-climate-vu...

[9] http://www3.weforum.org/docs/WEF_The_Global_Risks_Report_202...

[10] https://news.trust.org/item/20200902202142-ku0o2

[11] https://en.wikipedia.org/wiki/Ecological_collapse

[12] https://en.wikipedia.org/wiki/Deforestation_during_the_Roman...

[13] https://theecologist.org/2012/feb/27/humanity-has-already-ha...