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The ‘Keeling Curve’ – Mauna Loa CO2 record, 1958–2015.
The ‘Keeling Curve’ – Mauna Loa CO2 record, 1958–2015.
One of the most important experiments of the twentieth century began in the late 1950s and is still running. It shows how our entire planet ‘breathes’ with the seasons, and revealed the build up of carbon dioxide in the atmosphere that threatens a potentially catastrophic global warming over the next hundred years. But it was another serendipitous discovery.
In the early 1950s, Charles David Keeling, a young researcher at the California Institute of Technology (Caltech), planned to study the balance of carbon dioxide among the air, the oceans, and in limestone rock. As a step towards this, he developed a very precise instrument, called a gas manometer, to measure the concentration of carbon dioxide in the air. Because the air in Pasadena, where Caltech is located, was polluted by human activities, he tested the instrument at Big Sur, near Monterey, where he found that the air contained slightly more carbon dioxide at night than in the day time, because of the respiration of plants. But the average was the same every afternoon – about 310 parts per million (ppm).
Measurements in other American forests showed the same pattern. Both meteorologists and oceanographers were intrigued by the observations, and in 1956 Keeling, by then based at Scripps Institution of Oceanography, obtained funding for further research as part of the International Geophysical Year. This was a worldwide programme of research into many aspects of planetary science, which officially ran from 1956 to 1957 but initiated many longer-term experiments. Keeling’s plan was to measure, with the aid of colleagues, atmospheric carbon dioxide concentrations at several locations around the globe, including on top of Mauna Loa in Hawaii. This volcanic mountain is in the middle of the Pacific Ocean, far from any forests or sources of industrial pollution. He began operating the Mauna Loa experiment himself, in March 1958, recording a concentration of 313 ppm on the first day.
To Keeling’s surprise, the concentration of carbon dioxide rose slightly through March, April and May, then declined until October, when it began to rise again until the following May, when the pattern was repeated. Indeed, it repeated every year. The cycle showed that the whole planet was breathing, driven by the fact that there is more land, and therefore more land-based vegetation, in the northern hemisphere than in the south. In northern hemisphere in summer, this vegetation grows, taking up carbon dioxide from the air. In winter, it dies back and decays, releasing carbon dioxide. This produces a cyclic variation of about 5 ppm each year.
After the seasonal effects were allowed for, however, the average concentration of carbon dioxide in the air was higher in 1959 than in 1958, and higher still in 1960. Keeling published scientific papers reporting this, regularly updating what became known as the ‘Keeling curve’, a graph showing the build up of carbon dioxide in the atmosphere with the seasonal cycles superimposed onto it. By the 1970s, with growing concern that this build up of carbon dioxide might contribute to global warming through the so-called greenhouse effect, it was becoming clear that the increase was due to human activities, including forest clearance and the burning of fossil fuels. This was confirmed in the 1970s, when a political crisis caused a large increase in crude oil prices and a consequent slowdown in burning oil-based fuel. The rise of the Keeling curve levelled off slightly to match the change in these anthropogenic sources of carbon dioxide.
By 2015, the concentration of carbon dioxide in the air had risen above 400 ppm, which represents an increase of roughly 28 per cent above the level in 1958. Comparison with the amount of fossil fuel being burnt around the world shows that 57 per cent of the carbon dioxide produced remains in the air, while the rest is absorbed by poorly understood natural ‘sinks’, including the oceans. In the longer-term context, studies of air bubbles trapped in ice cores from polar regions show that over the past 400,000 years, carbon dioxide concentrations were around 200 ppm during ice ages, and around 280 ppm during the warmer intervals known as interglacials. The concentration started rising further at the beginning of the nineteenth century. This suggests that even in 1958 the concentration was above the ‘natural’ level, thanks to human activities over the previous one and a half centuries. As carbon dioxide is known to trap heat near the surface of the Earth (see here), it is clear that this build up of carbon dioxide has contributed to, and is most probably the principal cause of, the global warming that the Earth has experienced since at least the middle of the twentieth century. Because of the importance of these implications, carbon-dioxide concentrations are now monitored at about a hundred sites around the world, but the Mauna Loa record is the longest and still has pride of place. These observations are now supervised by Keeling’s son Ralph, a professor at Scripps.