400ppm CO2 in our atmosphere is significant because we have 10 digits but it is a collective milestone nonetheless. But the climate change we see now is not caused by 400ppm: the oceans cause a lag of around 40 years in human driven climate change. Quite simply we will not see the fruits of our industry today until 2053. The changes that are measurable today [no computer models needed] are effectively driven by 330ppm.
Curiously there was no need to highlight the CO2 levels in history that are affecting us now as the data collection process changed 40 years ago.
bollocks2012 