Notice that the curve is jagged. As humans keep burning fossil fuels, the amount of carbon dioxide in the atmosphere has trended upward over time. But there are still seasonal fluctuations. When trees in the Northern Hemisphere bloom in the spring and summer, they absorb some of that carbon. When the leaves wilt in the winter, carbon returns to the air and readings spike. The curve is a record of the planet’s breathing.
Since 1956, the Mauna Loa Observatory in Hawaii has been gathering data on how much carbon dioxide is in the atmosphere — a very basic measure of how humans are transforming the planet and setting the stage for future climate change.
The so-called Keeling Curve is attracting even more attention than usual this month, as the amount of carbon in the atmosphere is on the verge of hitting 400 parts per million, a new milestone (the readings hit 399.71 on Tuesday):