Discovery
| Parameter | Value |
|---|---|
| Start date | March 1958 |
| Location | Mauna Loa Observatory, Hawaii (3,397 m elevation) |
| Scientist | Charles David Keeling (Scripps Institution of Oceanography) |
| Instrument | Non-dispersive infrared analyzer (NDIR) |
| First measurement | 313 ppm (March 1958) |
| Precision | ±0.1 ppm |
Technical Explanation
1. NDIR measurement principle — CO₂ absorbs infrared radiation at 4.26 μm (asymmetric stretch absorption band ν₃). The analyzer sends an IR beam through a sample air cell and measures the attenuation relative to a reference cell. The Beer-Lambert law relates absorbance to concentration × optical path length.
2. Site selection — Mauna Loa is ideal: altitude 3,397 m (above the atmospheric inversion layer), isolated in the middle of the Pacific (4,000 km from the nearest continent), with prevailing westerly winds carrying uncontaminated oceanic air. It is precisely the nighttime and early-morning hours (~7 p.m.–9 a.m.) that are retained as background air; the daytime hours (11 a.m.–7 p.m.) are rejected, because anabatic winds then carry up air depleted by photosynthesis on the lower slopes. Volcanic contamination is handled separately, through a signal-variability criterion.
3. Seasonal oscillation — The amplitude is ~6 ppm between May (maximum) and September (minimum) in the Northern Hemisphere. Photosynthesis by boreal and temperate forests (spring–summer) absorbs CO₂; respiration and decomposition (autumn–winter) release it. This is the "breathing" of the terrestrial biosphere.
4. Secular trend — Superimposed on the oscillation, the concentration rises monotonically: ~+0.9 ppm/year through the 1960s, +1.5 ppm/year in 1990, +2.5 ppm/year in 2020. The acceleration is correlated (r² = 0.99) with cumulative fossil fuel emissions.
Why It Worked
Keeling insisted on absolute calibration using manometrically prepared reference gases — something no one else was doing at the time. His metrological rigor produced a time series with no instrumental breaks — a rare accomplishment in climate science. Mauna Loa's location guarantees a planetary "background signal," not a local measurement.
The near-loss of the program in 1964, when the Weather Bureau considered the data redundant after only a few years, underscores a deeper truth: the value of any continuous record grows with every additional year. Keeling fought for continuity — and was proven right.
Causal Chain
Industrial Revolution (coal, 1760) → Fossil CO₂ accumulation → Keeling installs NDIR at Mauna Loa (1958) → First proof of continuous rise → Charney Report (1979, climate sensitivity 1.5–4.5 °C) → IPCC First Assessment Report (1990) → Kyoto Protocol (1997) → Paris Agreement (2015, 1.5 °C target) → 427 ppm (2025)
Anecdote
Keeling nearly lost his funding in 1964. The Weather Bureau considered the measurement redundant after a few years of data. Keeling argued that the value of the series depended on its continuity — he was right. Each additional year makes the dataset more precious. To this day, it remains the longest continuous instrumental record in atmospheric chemistry.
Sources
References verified during the August 2026 fact-checking audit: these are the pages
against which this bulletin's claims were checked.