n 10 May 2013, 400 parts per million of CO2 concentration in the atmosphere was recorded at the Mauna Loa Observatory, Hawaii. This seemingly innocuous number failed to grab the attention of mainstream media, but it inadvertently signaled a quiet earth revolution – a geohistoric moment of “wild destratification”, as Deleuze and Guattari would have it. At the start of the Industrial Revolution (and the designated origins of the Anthropocene), atmospheric CO2 was at 280 ppm. The last time CO2 levels hit 400 ppm was in the Pilocene (between 2.6-5.3 million years ago). In light of these potentially seismic changes in the atmosphere and society, Society and Space invited some interdisciplinary reflections on 400 ppm.