400 ppm is of course only a symbolic threshold but as the guys at Mountain Beltway suggest it should give us reason to pause and contemplate what a momentous change it is in terms of the planet. Something we do in thefosterlab is reconstruct CO2 in the geological past using the boron isotope proxy and this allows a little bit more perspective on this issue. The image above is a plot compiling the state of the art when it comes to CO2 over the last 3 million years or so, including some data for the Pliocene (2.6 to 5.3 Million years ago that we published this year in Nature). What is clear is that the last time CO2 was 400 ppm was around 2.5 to 3 million years ago. The world was a different place back then – global temperatures around 3 C warmer than the pre-industrial and sea-levels were around 20 m higher (due to retreat/collapse of the West Antarctic and Greenland ice sheets). With global temperatures likely to top 1 C above pre-industrial levels this year and CO2 now likely to stay above 400 pppm it appears we are well on the way to a Pliocene-like future.