The data go back 800,000 years: that's the age of the oldest fossil air bubbles extracted from Dome C, an ice-bound summit in the high Antarctic. And throughout that time there has been nothing like this. At no point in the preindustrial record have concentrations of carbon dioxide in the air risen above 300 parts per million (ppm). 400ppm is a figure that belongs to a different era.
The difference between 399ppm and 400ppm is small, in terms of its impacts on the world's living systems. But this is a moment of symbolic significance, a station on the Via Dolorosaof environmental destruction. It is symbolic of our failure to put the long-term prospects of the natural world and the people it supports above immediate self-interest.