400 ppm CO2 is not an "unrecoverable limit." That's a scare tactic. CO2 has been in the thousands of ppm in the past during periods of glacier growth. Compared to water vapor, it's greenhouse effect is negligible, in addition to water vapor being far more plentiful in the atmosphere than it. This renders any warming effect from rising CO2 several orders of magnitude lower than that of water vapor, meaning it doesn't actually impact temperature or the climate in any measurable way.