Earth’s narrow escape from a big freeze
A subject of much debate is whether atmospheric levels of carbon dioxide were already significantly altered by emissions associated with human activities before the Industrial Revolution in the eighteenth century. One estimate suggests that the atmospheric concentration of CO2 would have been only 240 parts per million (ppm) in an agriculture-free world, rather than 280 ppm, as was measured just before the Industrial Revolution. On page 200 of this issue, Ganopolski et al. report modeling studies confirming that we would now be entering an ice age if the concentration had remained at 240 ppm By contrast, they report that glacial inception — the onset of an ice age — could not have occurred at CO2 concentrations that were typical of the eighteenth century.