Keeling Curve

Charles David Keeling began systematic carbon dioxide measurements at Mauna Loa in March 1958 through Scripps Institution of Oceanography. The high-altitude site in the mid-Pacific was selected to sample air that is relatively representative of the broader atmosphere and less affected by nearby sources than measurements taken in cities. Keeling's work also included other locations; the Mauna Loa series became especially influential because of its continuity and the site's consistent observing programme. NOAA began its own measurements there in 1974, creating a complementary record alongside the Scripps series. The term Keeling Curve is commonly used for the graph of these observations and their trends.

The record has two visually distinct features. First, its long-term baseline rises as carbon dioxide accumulates in the atmosphere. Second, concentrations rise and fall each year: the seasonal cycle is especially pronounced in the Northern Hemisphere, where extensive land vegetation takes up carbon dioxide during the growing season and releases it through respiration and decay during other parts of the year. The seasonal swing does not reverse the long-term increase. Measurements from Mauna Loa represent a particular location and are not identical to a globally averaged concentration, though they provide an important record of atmospheric change.

The repeated measurements helped demonstrate that atmospheric carbon dioxide was increasing over time and made the seasonal exchange between the atmosphere and the biosphere observable. Their long duration is valuable: a sustained, carefully calibrated series can reveal gradual changes that short campaigns might miss. Together with measurements from other stations, the record is used to study the carbon cycle and assess changes in greenhouse-gas concentrations. The curve is an observation record, not by itself a complete explanation of climate change; it is interpreted alongside other evidence and measurements.

Scripps Institution of Oceanography and NOAA maintain distinct measurement programmes, and their methods, quality controls and published records should be consulted when using the data. Mauna Loa observations were interrupted by the 2022 volcanic eruption, which damaged access and power infrastructure; NOAA temporarily collected measurements at a nearby site, while observations at Mauna Loa later resumed. Recent measurements may be preliminary and subject to recalibration or quality checks. Data users should check the supplying programme's current notes, especially when quoting recent values or comparing a site record with global averages.