In 2015, average global CO2 levels for the year surpassed 400 parts per million for the first time, the WMO revealed in its 2016 annual Greenhouse Gas Bulletin. At the time, any scientists regarded that ratio of carbon dioxide to other gases in the atmosphere as a “climate change touchstone.” Curiously, however, 400 ppm was not considered a tipping point. It was still possible to reverse the progression of the ratio—yet no one seems to ask how long that would take. In this regard, the ratio’s accelerating rate is particularly telling. Practically speaking, 400 ppm may in fact be a tipping point.
The full essay is at "CO2 Record-Level."