In 1958 Charles David Keeling began measuring carbon dioxide on Mauna Loa in Hawaii, far from cities and forests. NOAA continues the record today. It is the most famous graph in climate science, and reading it uses average rate of change and the carbon budget.
Key idea
The Mauna Loa record shows two things. A yearly wiggle of about 6 ppm, highest in May and lowest in September or October, from the growth and decay of northern plants. And a steady rise beneath it, from about 316 ppm in 1959 to about 425 ppm in 2024, rising faster over time: under 1 ppm a year in the 1960s, about 2.4 ppm a year in the 2010s. Before industry, the air held about 280 ppm.
Why it works
Most land, and so most plants, is in the Northern Hemisphere. In the northern spring and summer, plants take in more carbon dioxide than they release, and the level falls; in fall and winter, decay and respiration win, and it climbs. The swing repeats every year. Underneath, the burning of fossil fuels adds more each year than the sinks remove, so each year's average is higher than the last. The rise speeds up because emissions grew over the decades.
Worked example: Rate of change
Mauna Loa measured about 390 ppm in 2010 and about 414 ppm in 2020. What was the average rate of rise?
- Change: $414 - 390 = 24$ ppm over $2020 - 2010 = 10$ years.
- Rate: $24 \div 10 = 2.4$ ppm per year.
Answer: 2.4 ppm per year
Worked example: Compared with before industry
By what percent had CO₂ risen above the pre-industrial 280 ppm when it reached 425 ppm?
- Increase: $425 - 280 = 145$ ppm.
- Percent: $145 \div 280 \approx 0.52$, about a 52% increase.
Answer: About 52%
Common mistake
Thinking the dip each spring and summer means the rise is reversing. The dip comes every year as northern plants grow, and it is followed by a rise to a new high. Compare the same month in different years, or yearly averages, to see the trend. Check: every yearly average in the Mauna Loa record is higher than the year before.