Carbon dioxide's 400ppm milestone shows humans are rewriting the planet's history «

Levels of CO2 are pushing beyond 400 parts per million in the atmosphere. The last time they were there, 15 million years ago, the world was very different

Round numbers can trigger all sorts of weird and sometimes irrational responses.

For example, in about 19 years time when I turn 40 there'll be some sort of celebration at which I'm told I have reached a milestone. The number can also trigger denial in those afflicted (I honestly wouldn't know*).