The keyword here is "Long term" I suppose, that can mean anything. I couldn't actually find where this claim comes from. The referenced paper doesn't seem to say anything about doubling CO2 concetrations.
If it "Short Term" becomes obvious that we are past the point of no return it doesn't really matter if "Long Term" units is 100 or 1000 years. Most future living things will be suffering. There could have been more future things than past things to enjoy this beautiful planet.
