What mechanism for "killing all plants" through sudden atmospheric CO2 increase do you have in mind?

I'm just saying that the biggest, most sudden, longest lasting change in habitability would be made with CO2. The temperature change is what kills the plants, not CO2. It's a bigger temperature change than an asteroid impact and lasts longer.

It's like setting up a bomb, and it has multiple stages. You load co2 into the air until the bomb goes off, causing enough temperature change to cause large desertification. That releases even more stored carbon. That triggers the polar melt feedback, which triggers the permafrost melts, which triggers even more deforestation, which triggers wildfires and weather, which trigger even more polar melting, which wrecks ocean circulation, which fires the clathrate gun, and suddenly there are a handful of species in an earth that is set to random and they will inevitably die out.

We're already rather close to "as quickly as possible" :(

But plant life? Are you thinking phytoplankton?

Well, for why specifically you'd do it like that: Loading up CO2 in the atmosphere gives you a delayed action on all of this. It's not the CO2 directly, its the fact that you can just start investing into a sudden, massive change easily and then have it all take off exponentially.

If you instead pumped a toxin into the air, you would kill a great deal of things right away but as the concentration of poison rose they would develop a resistance. Thats the slow, traumatic burn, which is global but survivable with enough biodiversity.

If you nuked everything, that would kill almost everything on earth but after a couple decades it'll be relatively survivable again. Life only has to stick around a little bit- spores and seeds is safe places, that by some freak coincidence managed to survive the radiation. Also survivable.

With CO2, nothing happens until suddenly it does. There is a massive change in temperature that kills off biodiversity at a rate higher than ever before- we haven't even hit significant changes and we're experiencing this already. Then, once the biosphere is vulnerable, it gets hit with millenia of instability and extreme problems. There might be pockets that are shielded from this- if they sustained large, diverse populations through the massive initial changes.

It's like the KT extinction on steroids. KT was ~10,000 years- a CO2 bomb would happen even faster and last even longer.