The atmosphere just passed another doom threshold — there are now more than 400 parts per million of CO2 up there. “It was only a matter of time that we would average 400 parts per million globally,” said Pieter Tans, lead scientist of NOAA’s Global Greenhouse Gas Reference Network. “We first reported 400 ppm when all of our Arctic sites reached that value in the spring of 2012. In 2013 the record at NOAA’s Mauna Loa Observatory first crossed the 400 ppm threshold. Reaching 400 parts per million as a global average is a significant milestone.
For reference, the pre-Industrial levels of CO2 were around 280 ppm, and the first measurement made in 1959 at Mauna Loa was 313 ppm. The number has been growing since then, at an average rate of more than 1 ppm per year since 1977 (some years the increase was well above 2 ppm). Scientists think we need to reduce atmospheric CO2 concentration to 350 ppm if we are to avoid the worst of climate chaos — to which pessimists say, fat chance.
— source grist.org