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.”[1] 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."
1. Lydia O’Connor, “The
Planet Just Crossed Another Major Carbon Milestone,” The Huffington Post,
October 25, 2016.