It's not just climate change, it's the combined effects of a lot of different things, not the least of which is overfishing, deforestation, loss of habitat, and selective destruction of ecosystems.

So far we've really only scratched the surface, but there have been hundreds of local mass die-offs (billions of individual animals each). https://news.nationalgeographic.com/news/2015/01/150113-mass...

The Great Barrier Reef is about half dead now. In two years. https://www.theatlantic.com/science/archive/2018/04/since-20...

But look at what happened at the end of the Permian age. 96% of species were lost. It was due to CO2/methane causing climate change. https://cosmosmagazine.com/palaeontology/big-five-extinction...

(btw we still have no idea what caused the extinction at the end of the Triassic).

We're basically on the way to combining all of these causes of mass extinctions in a super-short timeframe that may mushroom into one super-massive extinction.

And as far as what-if's go, if climate change sets off a global conflict, and there's a nuclear war, forget it.

All those things are horrible. None of them have the remotest chance of wiping out all life larger than a few cm.

Phytoplankton seems to decline due to warming[1]. If this trend continues we're going to have a bad time. Phytoplankton produces at least half of the oxygen we breathe and is key to the oceanic food web.

[1] https://www.nature.com/news/2010/100728/full/news.2010.379.h...