1 June 2012: Greenhouse gas levels pass symbolic 400ppm CO2 milestone “The world’s air has reached what scientists call a troubling new milestone for carbon dioxide, the main global warming pollutant.”
21 May 2013: NASA scientists react to 400 ppm carbon milestone “The global concentration of carbon dioxide in the atmosphere – the primary driver of recent climate change – has reached 400 parts per million (ppm) for the first time in recorded history…”
29 April 2014: April Becomes 1st Month With CO2 Levels Above 400 PPM “The end of April has arrived, and with it, the record for the first month in human history with an average carbon dioxide level in Earth’s atmosphere above 400 parts per million has been set.”
27 May 2014: Carbon Dioxide Passes Global 400 ppm Milestone “A new carbon dioxide milestone has been reached according to the World Meteorological Agency.”
2 July 2014: CO2 Levels above 400 PPM Threshold for Third Month in a Row “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.”
6 May 2015: Greenhouse gas benchmark reached “Global carbon dioxide concentrations surpass 400 parts per million for the first month since measurements began.”
20 Nov 2015: Carbon Dioxide Bidding Farewell to 400-ppm Benchmark “On an otherwise unremarkable day last week–November 11, 2015 –a crucial milestone in global climate was quietly transcended.”
19 May 2016: Atmospheric CO2 May Have Topped 400 PPM Permanently “Rising CO2 levels not only break a worrying threshold, ‘I think we’re essentially over for good,’ says Ralph Keeling.”
16 June 2016: Antarctic CO2 Hit 400 PPM for First Time in 4 Million Years “The most remote continent on Earth has caught up with its more populated counterparts”.
28 Sept 2016: The world passes 400ppm carbon dioxide threshold. Permanently “In the centuries to come, history books will likely look back on September 2016 as a major milestone for the world’s climate.”
24 Oct 2016: ‘New era of climate change reality’ as emissions hit symbolic threshold “CO2 in the atmosphere is expected to be above 400 parts per million on average over 2016 and will not dip below that mark for generations, UN says”.