Climate Change Irony
This probably won't get all macroscopic life, life is pretty resilient. But it could come close.
I'm going to say this isn't technically anthropogenic because it would be a result of people stopping their interference in the planet's biosphere.
The level of CO2 in our atmosphere has gone up and down over the millennia, but the general trend is down. Between organic matter getting trapped under sedimentary rock, and the whole pile of sea creatures that bond carbon into their shells so tightly that it pretty well won't get released until it's been compressed into limestone and subducted into the mantle, carbon gets sequestered, on average, faster than it gets released.
The estimated CO2 concentration in the pre-industrial era was 280ppm.
The level at which all "C3" type photosynthesizers cease to function and die is 220ppm.
Now that's not all plants. But it is the vast majority of them. C3 photosynthesis is so much more efficient that it has outcompeted C4 photosynthesis pretty much everywhere except deserts. (C4 has the advantage that it can store CO2, so in hot, dry, climates the plants can do their gas exchange at night to reduce water loss, and then photosynthesize during the day.)
C3 is also the type of nearly all the algae in the oceans which, despite what you may have heard about the rainforest, is what produces something like 2/3rds of our oxygen. Not to mention that practically all of the rainforest is going to die too...
So, the climate change activists get everything they could possibly dream of. Humanity stops all use of fossil fuels and, via a massive carbon sequestration effort, returns CO2 levels to their pre-industrial levels. "We're going to return the planet to Mother Nature" they say. (The deliberate carbon sequestration is optional, it'll get there eventually regardless, just a question of how far in the future you want the disaster to occur.)
Well, what Mother Nature had originally planned was another mass extinction, which she cheerfully carries out. The few who notice the upcoming problem are shouted down as "climate change deniers" and "barbarians" and "pro-oil" and "wanting to return to the dark ages of non-renewable energy".
Then, one day, most of the photosynthesis on the planet... stops. Hardly anybody notices. The leaves don't turn brown and fall off the plant or anything. They stay a nice, vibrant green! But that's because the chlorophyl is not being consumed. Which means no sugar production, which means the plant can't pump water up its stem. After a few days the leaves curl up from dehydration and the plants all die, still beautifully green.
There goes most of the planet's food production.
Meanwhile, oxygen production drops off drastically. Weeks to months later CO2 levels spike. Between lightning strikes igniting the vast swaths of dead, dehydrated vegetation, and normal animal respiration, the air starts to get a little... thick... The problem might correct itself in time, there are probably C3 plant seeds still surviving -- for now -- and the C4s will eventually move in if not, but that will take decades, if not centuries, and right now the atmosphere is rapidly becoming decidedly toxic to animal life. Oxygen concentrators and CO2 scrubbers become precious commodities. Depending on how bad the fires are, there might be enough smoke in the air to trigger off the next ice age, further slowing the plant recovery and making the surface even more hostile. In order to not wear breather masks 24/7, people with access to the necessary resources construct sealed shelters to live in. Those without... Struggle along with what life support equipment they can manage to produce with the resources they have... Most of them die, but there will probably be at least some nomads. The technology isn't too horribly complicated if you could find the raw resources for it before the collapse.
Like I said, this probably won't kill all macroscopic life, but depending on how you tune the ice-age, the wars over access to critical life-support supplies, the opportunistic wars over territory, etc. etc. you could certainly turn the surface into pretty much whatever degree of toxic wasteland you please. And, on top of that, it's the kind of apocalypse that will clear up on its own if the survivors can just hang on long enough. So you'll have all the politics of hope, and the people who prey upon the hope of others.
Left to nature it'll probably cycle for a half million years at least worth of burn-off, regrowth of surviving C3 plant types in the increased CO2, followed by another collapse and burn-off. If humanity can manage to free up some carbon during the regrowth cycle they could stop it. If they fall to internal squabbles and lose the technology and/or population level to dig up significant quantities of coal, etc. then the alternative is that it eventually stabilizes again at a much lower O2 level and much lower animal carrying capacity since the world will have only much slower-growing plants.
