Fingers crossed that it will never happen but it's a good question, “what happens if all the plants on Earth died?”
Well, there are two major problems (see below) that this would cause in terms of the make-up of the atmosphere.
As well, there's the not-so-trivial problem of what would we eat.
Let's examine the scientific ramifications in greater detail.
What If All The Plants Died?
Problem One: Carbon Dioxide
The first major problem that such a die-off would cause is that plants consume carbon dioxide.
The current atmospheric concentration of carbon dioxide is already on the high side and without plants, we would continue producing carbon dioxide but nothing would remove it from the air.
Over time all the animals including people would eventually be poisoned by this rising level of CO2 in the atmosphere.
This is a major concern as lethal concentrations of CO2 might build up in as little as two months depending on how fast other animals died out. Certainly, we'd find breathing difficult fairly quickly.
Problem Two: Plant Photosynthesis Generates Oxygen
While plants consumed CO2, humans consumed oxygen and that won't change if there are only animals left on the planet.
In fact, plant and animal life are neatly balanced and thus, as we exhale carbon dioxide, plants turn that back into oxygen.
In a world where only humans or animals were around, eventually, all the oxygen in the Earth's atmosphere would be used up and we would suffocate.
However, this isn't a huge concern either – it would take 100 years or more to run out of oxygen and thus, the biggest problem of them all, is what would really hurt us.
Problem Three: What Does Everything Eat?
The food chain depends on plants. Even carnivores need plant-eating animals in order to survive.
If those animals eat nothing then very quickly, they starve to death, and over a very rapid period, everything would starve.
Sure, a few humans might have access to stored food but eventually, it would run out. Worldwide extinction is the only possible outcome if all other organisms died off. Of course, there might be a few specialized bacteria or viruses kicking around even after all the animals died but that would be it even just a few months after the plants went.
How Long Would We Live If All Plants Died?
Nick Canning, reported in the New Scientist that we would probably starve before either oxygen deficiency or CO2 could reach lethal concentrations.
He estimates that based on known quantities of biomass that it would take just under 14 years for everything on Earth to die off.
How Long Would The World Last Without Plants?
Forever. Well, the several billion years that it's likely to take for our sun to expand and swallow the Earth.
Dead vegetation is a serious problem for life on Earth but it is not an issue for the planet itself.
In fact, it might well be that the remaining stuff on the planet would eventually evolve into other plants and other life and the place would be completely invigorated.
Humans run the planet now but it doesn't have to remain like that.
Final Thoughts On Why We Need Green Plants
The gaia hypothesis, suggests James Lovelock, says that the entire planet is connected as a single organism and everything on earth is dependent on everything else to a greater or lesser extent.
By envisioning a world without plants we can quickly see that on a macro scale, at least, this is entirely true.