Climate Science 1960

The Concentration and Isotopic Abundances of Carbon Dioxide in the Atmosphere

Charles David Keeling

One careful instrument caught the air's carbon dioxide breathing each year — and rising every year.

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In depth · the introduction

A young chemist set out to measure something almost no one thought worth measuring — exactly how much carbon dioxide is in the air — and found it climbing, year after year.

The idea, unpacked

Carbon dioxide is a tiny fraction of the air — about four parts in ten thousand. For a long time nobody had measured it carefully enough to say whether that fraction was steady or changing. Charles Keeling built an instrument and a routine careful enough to settle it.

His record showed two things at once. The air breathes: CO₂ dips each summer as northern forests grow and take it up, and rises again each winter. And underneath that yearly breath, the whole curve drifts upward — a little more carbon dioxide every single year, in step with the coal, oil and gas the world burns.

The man who wouldn't round off

Keeling was almost obsessive about getting the number exactly right — he had spent years just perfecting how to measure CO₂ in clean air. At Scripps Institution of Oceanography, the oceanographer Roger Revelle saw the value of it and, with funding from the International Geophysical Year, put an analyzer on Mauna Loa, a remote Hawaiian volcano far from cities and trees, in 1958. The very first readings caught the seasonal rise and fall. By 1960, with a few years of data, Keeling could write that the gas was not just cycling but accumulating. He kept the measurement going for the rest of his life, fighting for funding the whole way.

Why it mattered

Sixty years earlier, the chemist Svante Arrhenius had calculated that adding CO₂ to the air would warm the planet. But no one knew whether human emissions were actually piling up in the atmosphere, or whether the oceans and plants simply soaked them up. Keeling's curve answered that: the carbon dioxide really is accumulating, and the extra carbon carries the chemical fingerprint of burned fossil fuel. It turned global warming from a theory about the future into a measurement of the present — the bedrock under all the climate science that followed.

A bathtub filling

Picture a bathtub with the tap running and the drain open. The drain — oceans and plants soaking up carbon — is real, but the tap is running faster, so the water level creeps up. The yearly seasonal dip and rise is like small waves sloshing back and forth on the surface; the slow rise of the whole water line is the part that matters, and it doesn't reverse as long as the tap stays ahead of the drain.

Before and after

Arrhenius (1896) supplied the theory that CO₂ warms the Earth; Guy Callendar argued in 1938 that it was rising, but his data were too scattered to convince. Keeling supplied the proof. Everything downstream — the IPCC assessments, the 350 and 450 ppm targets, the very phrase “carbon footprint” — rests on the unbroken line he started in 1958, now past 420 ppm and still climbing. In this Library it sits beside Arrhenius's greenhouse calculation: the prediction, and the measurement that confirmed it.

The original document

Original source text

Charles David Keeling (1928–2005) · Tellus 12(2):200–203 (1960) · measurements at Mauna Loa Observatory (Hawaii, from March 1958) and in Antarctica (from 1957)

What was measured

Keeling reports continuous and flask measurements of the concentration of atmospheric carbon dioxide, together with its carbon and oxygen isotopic abundances, made at remote baseline sites chosen so the air sampled would represent the free atmosphere rather than nearby sources. The Mauna Loa and Antarctic records, only a few years long at the time of writing, already show a consistent pattern.

A seasonal rhythm

In the northern hemisphere the concentration varies systematically through the year — falling in summer and recovering in winter — a swing of several parts per million that Keeling reads as the seasonal uptake and release of CO₂ by land vegetation. The isotopic data move in step, consistent with plant matter as the agent.

And a rise

Beyond the seasonal swing, the averages do not return to where they began: where the data extend over more than a year, the second year reads higher than the first. The increase is small but persistent, and its size is what fossil-fuel combustion would predict.

…at the South Pole the observed rate of increase is nearly that to be expected from the combustion of fossil fuel.

(From the paper's discussion of the year-on-year change. The carbon released by burning coal and oil is depleted in the heavy isotope ¹³C, so the added CO₂ carries an isotopic signature — the thread Keeling's isotope measurements were built to follow.)

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Scripps Institution of Oceanography · La Jolla, California · 1960