"Scientific" American compares this event to the 9/11 attacks: CO2 crossing above 400 ppm again, down from thousands of ppm millions of years ago

As CO2 Concentrations Near Ominous Benchmark, Daily Updates Begin: Scientific American

Most people can mark their time on Earth by significant world events: the landing of a man on the moon, say; the dismantling of the Berlin Wall; or, more negatively, the 9/11 attacks.

Another significant event is impending. Scientist Ralph Keeling wants this generation to remember when atmospheric concentrations of carbon dioxide reached 400 parts per million, because of humans.

"I hope that many people out there in the decades to come will say, 'Gosh, I will remember when it crossed 400,'" he said.

Tom Nelson: 350: The most brain-dead campaign of your life

Take a careful look at the black line in this graph: