Even Antarctica has hit 400 ppm of carbon dioxide. The 400 ppm threshold is symbolic, but important. Despite our studies of the planet’s climate, we don’t know how the real CO2 threshold that will trigger unavoidable greenhouse conditions. Considering the 13th month of record heat, we may have already crossed the threshold. When the first stations reported hitting 400 ppm, some considered it a local phenomenon. With this data from Antarctica, every continent has now crossed that line. It is doubtful that we can dip back below that concentration within the next few decades. It is more likely that the trend will continue to grow as we continue to pollute and as a warmer planet releases more greenhouse gases.
(Click on the graph for the link.)