Mauna Loa Observatory

A station for clean air

The US Weather Bureau founded the Mauna Loa Observatory on 28 June 1956, on the north flank of the volcano above the Pacific trade-wind inversion, where the air is clean and well mixed. Charles David Keeling of the Scripps Institution of Oceanography, who had shown that carbon dioxide in unpolluted air is nearly uniform, began continuous measurements there in 1958 using an infrared gas analyser.

The first years of his record showed two things at once: a regular seasonal cycle, as northern forests grew and decayed, and a steady upward trend.

The Keeling Curve

The record at Mauna Loa has continued without interruption except for brief gaps ever since, and the curve, which began at about 315 parts per million, passed a daily average of 400 parts per million for the first time in May 2013. In 1972 the observatory became part of NOAA's baseline observatory network, which runs matched stations at Barrow in Alaska, Samoa and the South Pole.

Both NOAA and Scripps take measurements, and their independent analysers give a check on each other. A calibration chain based on reference gases keeps the record consistent across instruments and decades.

The volcano takes a hand

Mauna Loa erupted in November 2022, for the first time in 38 years, and lava cut the access road and power lines to the observatory. Measurements were moved temporarily to Maunakea, and a temporary access road to the observatory was completed on 26 March 2026. The observatory is on a volcano, which itself emits carbon dioxide, so the sampling is arranged to exclude that signal, using wind direction and statistical filters.

A curve and its checks

The Keeling curve is simple to read and hard to produce. Behind it are calibration gases, frequent comparisons with flask samples, corrections for the volcano's own emissions and independent measurements by NOAA and Scripps that agree to a fraction of a part per million. Critics sometimes ask whether a station on a volcano can be trusted, and the answer is in the procedures: air from the volcano's vents is recognised by wind direction and by its shape in the data and removed. The same curve is matched by stations at the South Pole, Samoa and Alaska, which makes the global rise unmistakable.

Instruments and technologies

• Non-dispersive infrared analyser A gas cell through which infrared light passes; carbon dioxide absorbs a specific band, and the loss of light is measured against a reference cell.
• Calibration gases A set of reference gases of known concentration, certified against a primary scale, are run through the analysers regularly.
• Flask sampling Air is collected in glass flasks and analysed in a central laboratory, as an independent check.

The people

• Charles David Keeling Chemist who began the continuous measurements in 1958.

Timeline

• 1956 Founded on 28 June by the U.S. Weather Bureau.
• 1958 Keeling's continuous CO2 measurements begin.
• 1972 Becomes part of NOAA's baseline observatory network.
• 2015 Daily CO2 reading reaches 400 ppm.
• 2022 Mauna Loa eruption cuts the access road and power lines.
• 2026 Temporary access road completed on 26 March.

Key lessons for observation today

• Continuity beats novelty The curve is valuable because it was never allowed to stop. Funding a measurement for decades is a scientific choice.
• Calibrate to a scale, not to an instrument Reference gases tied to a primary scale make instruments and stations comparable. Traceability is the heart of a climate record.
• Plan for access and power An eruption cut the road. Remote stations need redundant access and power.

Sources

• NOAA Global Monitoring Laboratory, Mauna Loa Observatory
• Wikipedia, Mauna Loa Observatory
• Wikipedia, Charles David Keeling