>Push as much co2 as possible into the air as quickly as possible to change global conditions enough to kill all plants at the same time
This makes no sense. How exactly an abundance of CO2 will "kill all plants" given that they are thriving on it? Let's say temperature rises further, like +10C rise. Desertification may kill some plants, sure. In the meantime, huge swaths of Siberia and Canada will stop being permafrost and transform back into thriving bogs, marshes and forests.
Moreover, there were times on Earth with widely higher temperatures. We are talking thousands of ppm of CO2 in the air and tropical flora on Antarctica. This didn't "end all life on Earth" at all.
A 10C rise would put us in the Cretaceous, the hottest time in the past 300 million years. It would melt all ice on earth and the globe would be unrecognizable due to sea rise. This has happened before. It hasn't happened in a couple centuries before.
A 3C rise will kill a lot of plants. A 5C rise will kill most of the plants, and if they can survive the change in biodiversity, the release of methane and CO2 from everything rotting, starving predators, and extreme weather and fire, life on earth might survive and eventually recover after a few hundred thousand or million years.
A 7-10C change over 300 years would, most likely, kill every single thing on the planet that was not transplanted by humans. There are only two ways to survive that without human intervention.
1: Be wide-ranging enough to spread very quickly to areas that become habitable as they open up. A few times a century the entire ecosphere will have to pack up and move to live in the changing climate as floods and deserts move things around. These plants would find it impossible to thrive over the long term and have very little chance of surviving.
2: Literally just tank the entire thing start to finish. For most plants, they will not be able to significantly change their range over this time. In ideal conditions yes, under the massively destabilized climate -disappearing coasts, halting ocean circulation, temperature swings from freezing to sweltering- no way.
The plants in northern Canada would have to be able to survive in tropical heat as well as they currently do in ice and snow. Plants in the tropics would have to survive 100 meters underwater. As the permafrost melts, plants will spread northerly- but the climate will catch up. Ironically it would be better if we killed 99% of plants right now so they could beat the climate. As it stands in a hundred years tropical vines will not be winding their way between New England evergreens- they will be dust in a desert. The pines outside my window will be choking in 40C weather. Boabab trees wouldn't survive in the soil here and Redwoods won't survive in Greenland.
NB: I don't mean to say I think the total collapse of life on earth is likely at this point in time. I think at present even humans are pretty likely to survive what's coming even if we stay our current course for 30 years. However I would give it better than 50% odds that a 7C+ warming would kill multicellular life. Most of the assumptions that you have to make to lead to warming that high necessarily kills everything, eg ice melting, circulation stopping, humans not giving a fuck, etc.
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.
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.
This makes no sense. How exactly an abundance of CO2 will "kill all plants" given that they are thriving on it? Let's say temperature rises further, like +10C rise. Desertification may kill some plants, sure. In the meantime, huge swaths of Siberia and Canada will stop being permafrost and transform back into thriving bogs, marshes and forests.
Moreover, there were times on Earth with widely higher temperatures. We are talking thousands of ppm of CO2 in the air and tropical flora on Antarctica. This didn't "end all life on Earth" at all.