CHARLES D. KEELING, mostly known as Dave, was a soft-spoken, somewhat courtly man who changed the way people and governments see the world. A slightly aimless chemistry graduate with an interest in projects that took him out into the wild, in 1956 he started to build instruments that could measure the proportion of carbon dioxide in the atmosphere, a scientific topic which, back then, was barely even a backwater. In 1958, looking for a place where the level of carbon dioxide would not be too severely influenced by local plants or industry, he installed some instruments high up on Mauna Loa, a Hawaiian volcano. He found that the level fluctuated markedly with the seasons, falling in northern summer as plants took up carbon dioxide and rising in northern winter as dead foliage rotted. And he found that the annual average was 315 parts per million (ppm). He continued his measurements, at Mauna Loa and elsewhere, for almost 50 years. Throughout that time the annually wobbling level rose, and the rate at which it was rising increased. The serrated sweep of the “Keeling curve” became an icon of climate-change science. And this week or next, eight years after Keeling’s death there will be a day when that curve breaks the 400ppm barrier for the first time (see article).

Like a birthday with a zero at the end of it, the 400ppm barrier has a psychological weight beyond any physiological significance. Although it may add a new rhetorical impetus to calls for carbon-dioxide emissions to be controlled, the argument that the risks of unfettered fossil-fuel use are too troubling to be borne was just as strong at 395ppm, or indeed 356ppm, the level at the time of the Rio “Earth Summit” in 1992; 400ppm is not a threshold. But the mere fact that the number is known—and that all the numbers for all the days for the past decades are known, too—carries with it an important lesson. To think about how to look after the planet, you need an objective measure of how it has been changing over time.

The unblinking gaze

You might think that, with automated buoys measuring the temperature of the oceans and gossamer balloons doing the same for the stratosphere, the world is being pretty thoroughly monitored. But it isn’t. Useful environmental measurements are made all over the world. But too few are made consistently across the decades; data are taken over a funding cycle, or the life of a satellite, and then not again. And when charting the course of a changing planet, it is consistency that matters.

Keeling knew this. Once he had shown that carbon-dioxide levels were rising he came under pressure from funding agencies to go and find something else to do (and to turn routine monitoring over to a bureaucracy). Merely monitoring something, it was suggested, was somehow infra dig for a researcher. Keeling stuck to his guns. To him, understanding the Earth meant looking at it unblinkingly with careful and consistent scientific eyes. His patient work proved him right.

Scientists involved in other measurements of the Earth, and those who pay for their work, need to build on his legacy. So does anyone taking a position on global-warming, where numbers as clear as Keeling’s are a rarity. Measurements of the temperature of the ocean depths and the acidity of its surface waters, of the volume of the planet’s forests and the mass of its ice sheets (see article), need to be made not just for the few years of a specific research project. Their ceaseless continuance needs to be built into the planet’s infrastructure. A world in which governments claim to be committed to spending trillions of dollars to change the shape of the Keeling curve decades hence, but do not find the funds to produce consistent records of the change going on today, is one that still has lessons to learn from the patient chemist.