Plants survive just fine on what we have now. But, to be clear, photosynthesis as a biochemical process is simply more efficient at higher concentrations of carbon. I've seen anywhere from 1000-2500 ppm being "optimal" though of course that varies by source and target species. Photosynthesis is not just the one textbook pathway. There are plenty of variants across plants, single-celled eukaryotes, and bacteria.
Regardless, in terms of geologic time, we are at a low point. Biological processes have locked up carbon in rocks (limestone) with very high efficiency for over a billion years. We are at a relative low point in terms of Earth's longer history in terms of atmospheric carbon dioxide. Pre-industrial concentrations were ~280-300 ppm. We're at ~420ish now. When the dinosaurs were doing their thing, the Earth was warmer. There were no polar ice caps. Oxygen concentrations were closer to 33% than the 21% we have now and carbon dioxide was 5x higher than now. Go the opposite direction, say, down to 180ish and plants start to starve, some species will die off. By 150 ppm, photosynthesis stops, and then so does all other life dependent on it. The only thing that will be left is the chemosynthesizers like the critters that live around deep sea hydrothermal vents.
Whether we "need" more CO2 is debatable. Rapid changes in anything will mess up a homeostatic system, creating sometimes wild and unpredictable changes within complex systems, so we should take that into account. I'm not sure I'd go so far as to say human should be actively trying to perturb the system. The question is whether what we're doing is intrinsically harmful. As I said before, jarring a homeostatic system can be unpredictable, but right now, we're simply not seeing evidence of the kinds of harm we keep being told about. The corals aren't dying. The polar bears are fine now that we stopped humans from hunting them. The oceans aren't acidifying any more than the rain was in the 80s. There's far more fossil fuels in the ground than anyone thought in the 60s, and since fracking showed up in the 2010s, we've gotten really good at pulling clean-burning methane (natural gas) out of the ground. No, it's not perfect, but we're not all going to die.
Plants survive just fine on what we have now. But, to be clear, photosynthesis as a biochemical process is simply more efficient at higher concentrations of carbon. I've seen anywhere from 1000-2500 ppm being "optimal" though of course that varies by source and target species. Photosynthesis is not just the one textbook pathway. There are plenty of variants across plants, single-celled eukaryotes, and bacteria.
Regardless, in terms of geologic time, we are at a low point. Biological processes have locked up carbon in rocks (limestone) with very high efficiency for over a billion years. We are at a relative low point in terms of Earth's longer history in terms of atmospheric carbon dioxide. Pre-industrial concentrations were ~280-300 ppm. We're at ~420ish now. When the dinosaurs were doing their thing, the Earth was warmer. There were no polar ice caps. Oxygen concentrations were closer to 33% than the 21% we have now and carbon dioxide was 5x higher than now. Go the opposite direction, say, down to 180ish and plants start to starve, some species will die off. By 150 ppm, photosynthesis stops, and then so does all other life dependent on it. The only thing that will be left is the chemosynthesizers like the critters that live around deep sea hydrothermal vents.
Whether we "need" more CO2 is debatable. Rapid changes in anything will mess up a homeostatic system, creating sometimes wild and unpredictable changes within complex systems, so we should take that into account. I'm not sure I'd go so far as to say human should be actively trying to perturb the system. The question is whether what we're doing is intrinsically harmful. As I said before, jarring a homeostatic system can be unpredictable, but right now, we're simply not seeing evidence of the kinds of harm we keep being told about. The corals aren't dying. The polar bears are fine now that we stopped humans from hunting them. The oceans aren't acidifying any more than the rain was in the 80s. There's far more fossil fuels in the ground than anyone thought in the 60s, and since fracking showed up in the 2010s, we've gotten really good at pulling clean-burning methane (natural gas) out of the ground. No, it's not perfect, but we're not all going to die.