A continuous 65-year record of atmospheric CO2
Charles David Keeling began measuring atmospheric carbon dioxide at Mauna Loa, Hawaii in 1958 and continued until his death in 2005. His son Ralph continues the measurements today. The site was chosen for its elevation (3,397 meters) and remote location, minimizing local pollution contamination. Week after week, the instruments have recorded the steady climb from 315 ppm in 1958 to over 420 ppm today, creating the most direct and longest instrumental record of the atmosphere's chemical change.
The curve is not a simple upward line but a sawtooth pattern with a seasonal oscillation of roughly 5-7 ppm each year, driven by the Northern Hemisphere's summer plant growth (which absorbs CO2) and winter die-back (which releases it). The underlying trend, however, is unmistakably upward: CO2 has risen roughly 1.5 ppm per year on average.
What the rising trend reveals about fossil fuel burning
The Keeling Curve is not derived from models or estimates; it is direct measurement. The 105 ppm increase over six decades corresponds closely to the world's cumulative fossil-fuel consumption, cement production, and land-use changes over the same period. Isotopic analysis of the CO2 in the atmosphere confirms the carbon is primarily from combustion (not volcanic outgassing or ocean release) because the carbon-13 to carbon-12 ratio bears the signature of plant material.
The curve became iconic not because it was surprising, but because it quantified the atmosphere's transformation in real time. It anchored climate science in observational fact at the moment when climate models were still speculative.