2.3 Measuring the atmospheric carbon dioxide concentration

In late 1957 the scientist Charles Keeling began a continuous series of measurements of the atmospheric carbon dioxide concentration at the Mauna Loa Observatory in Hawaii. This is situated high up at the top of a mountain. The results (the Keeling curve) are shown in Figure 8.

It shows a continuously rising trend from about 315 parts per million (ppm) of CO₂ in the atmosphere in 1958 and reaching over 420 ppm in 2023. Measurements were taken simultaneously from other parts of the planet, including from Antarctica. These only differed very slightly, so the Mauna Loa figures can be taken to be representative of the global atmosphere.

Note that there is an annual variation, showing that peak levels are reached in spring in the northern hemisphere, before plants on the northern landmasses and continental shelves start growing strongly again and absorbing carbon dioxide.

This curve is important because it shows, very clearly, the steady increase of carbon dioxide in the atmosphere, one of the first major signs of global environmental change. This extra CO₂ is what most environmental scientists consider to be a major cause of climate change and the additional warming of the whole planet.