I need an Earth where the atmosphere has become unbreathable for humans and animals because all plants have died. But everything else should stay pretty much the same.
So here are my questions:
What could kill all flora on Earth over a period of several years without humans being able to stop it? Maybe some very aggressive necrotrophic fungi, who can spread over the air and quickly mutate to attack different plant species?
What would happen then? Plants would stop absorbing CO2 and its concentration would rise. From my research there are huge quantities of O2 in the atmosphere, so O2-depletion won't be a problem. However, a rise of CO2 beyond certain level would make the air toxic for the fauna. How long do you think this process will take? Will something else happen - I read about ocean acidification and global warming? How severe would these be?