400ppm [and a climate changed by 330ppm]

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.

Greenhouse gas - slash co2 emissions

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.