Atmospheric concentrations of the greenhouse gas, which helps drive global warming, haven’t been this high in somewhere between 800,000 and 15 million years
April fell first. It lasted through May. Now June will be the third month in a row with average carbon dioxide levels above 400 parts per million.
And while the 400 ppm mark is somewhat symbolic (as the increase in warming between 399 ppm and 400 ppm is small), it serves to show how much carbon dioxide has been put into the atmosphere since preindustrial times, when concentrations were around 280 ppm. The increase in this and other greenhouse gases in the atmosphere has warmed Earth’s average temperature by 1.6°F since the beginning of the 20th century. World leaders agreed at a UN summit in 2009 to limit warming to 3.6°F, but prominent climate scientists like James Hansen have said that amount of warming will still be too much.