Martin.Dlugo/Shutterstock by Jack Marley, The Conversation A graph I saw in high school appeared to show the Earth breathing. It was a graph that plotted carbon dioxide in the atmosphere over the course of the 20th century and into the 21st. CO₂ had risen steadily, and then more rapidly, but it hadn’t gone up in a straight line. Each year it had fallen sharply before rising to a new peak, increasing over time in an upwards zig-zag.