Charles David Keeling started measuring atmospheric carbon dioxide at Mauna Loa in Hawaii in 1958. The resulting line climbs steadily upward — and zigzags as it goes.
Why the zigzag
Most of the world's land, and therefore most of its forests, sits in the northern hemisphere. During the northern spring and summer, plants photosynthesise and pull CO₂ out of the air, so the concentration falls. In autumn and winter, growth slows while decomposition continues, and CO₂ rises again.
The result is an annual cycle of several parts per million, superimposed on the long-term trend. Its amplitude is largest at high northern latitudes and nearly vanishes near the South Pole, exactly as a land-vegetation explanation predicts.
Why this makes the record more convincing, not less
A measurement that reproduces an independently known seasonal signal, with the right amplitude and the right phase at every latitude, is a measurement that is working. The wobble is a continuous calibration check on the instrument and the site.
Mauna Loa itself was chosen for the same reason: it sits far from cities and forests, high on a bare volcanic slope, sampling air that has mixed across the Pacific.
The part that matters
Strip the seasonal cycle out and the underlying line is monotonic. When the record began, CO₂ was around 315 parts per million. It has passed 420. Ice cores show that for the previous 800,000 years it stayed between roughly 180 and 300.
The breathing is annual. The trend is not.