This is technically true, but it does not comprehend the threat. Crab populations (etc) can survive in a broad range of CO2 levels, but there is no reason to th

This is technically true, but it does not comprehend the threat. Crab populations (etc) can survive in a broad range of CO2 levels, but there is no reason to think that they can survive a rapid CO2 and acidity transition like the one that is underway right now. Corals are among the threatened organisms, and they can't migrate very quickly. Coral reefs, in turn, are a habitat for organisms like crabs (and 25% of marine species overall) and if they die off quickly, the underlying ecosystem is obviously under threat. In other words: geological-scale change allows time for species and ecosystems to adapt and migrate. Rapid ecological change is correlated with loss of biomass, biodiversity, and species. The source of risk is not the CO2 level, but the rate at which it has changed.