by Paul Braterman
In 1957, Charles Keeling of Scripps Institution of Oceanography began regular measurements of carbon dioxide concentration at Mauna Loa, Hawaii. By 1960, he was already in a position to report a steady increase, together with seasonal variations. In the northern atmosphere, CO2 concentration falls during the spring and summer growing season, but recovers during autumn and winter as vegetable matter decays. This sawtooth pattern is superposed, however, on a steady overall increase.
Above, R: Scripps Institution of Oceanography (Invertzoo via Wikipedia)
The Keeling curve and beyond
Charles Keeling died in 2005, but the work is being continued by his son Ralph. When I visited Scripps in 1995, I saw Charles Keeling's original curve, ink on graph paper, on the wall in the corridor outside his office. That curve has now been designated a National Historic Chemical Landmark, and there are commemorative plaques both at Scripps and at the Mauna Loa Observatory. Charles Keeling's original paper, freely available here, goes into meticulous detail regarding sample collection, calibration, precautions taken to prevent local contamination, and comparisons between the Mauna Loa data and those that numerous other sites, including the Antarctic and samples collected from an aircraft.
L: Atmospheric CO2, 1700 – 2014; NASA via Forbes. Click to enlarge. Note that the zigzags for atmospheric data are not error bars, but annual fluctuations.
By 1985, the record had been extended backwards in time by analysis of air bubbles trapped in ice cores, with dates ranging from the 1980s to the 1600s and earlier. These dates overlap Keeling's data, and take us back to pre-industrial times. Before long, the ice core record had been extended to an 160,000 years, taking us into the Ice Ages, while further work has pushed it back to 800,000 years. We have estimates going back far beyond that, but employing indirect methods and with higher uncertainty.
During the Ice Ages, carbon dioxide played a dual role, as product and as agent. The temperature oscillations at this time were driven primarily by subtle changes in the Earth's motion (so-called Milankovitch cycles). But carbon dioxide is less soluble at higher temperatures (which is why your carbonated drink fizzes inside your mouth). And so in the first place the rise and fall of temperature led to a rise and fall of carbon dioxide in the atmosphere, as the oceans released and reabsorbed the gas. But then, the changes in carbon dioxide concentration amplified the original effect, since more carbon dioxide acting as a greenhouse gas makes the Earth lose heat less efficiently into space.
To summarise the results, current levels of CO2 are the highest they have been for over twenty million years. In the centuries leading up to 1800, levels were steady at 280 parts per million (ppm); a slow but steady increase took place throughout the nineteenth and early twentieth century, so that levels had reached over 310 ppm when Charles Keeling began his studies; this increase has accelerated steadily since then; the present value is over 400 ppm; and the current rate of increase appears to be unprecedented in the geological record.