CO2 has historically been higher, but life on earth, and ocean life in particular, was completely different that what we have today. The rate of ocean acidifica

CO2 has historically been higher, but life on earth, and ocean life in particular, was completely different that what we have today. The rate of ocean acidification is such that species do not have millennia to evolve to handle increased CO2 levels and are experiencing massive die-offs. An extremely well-understood effect is that ocean acidification thins the shells of shellfish. This may seem like a minor effect, but rapidly upsetting the food chain of the ocean seems like an absurdly risky experiment to run on the planet we inhabit.