The Keeling Curve

The systematic measurement of atmospheric CO2 was one of the many data gathering efforts begun during the IGY and it has continued to the present time. The Weather Bureau's Harry Wexler (1911-62), who led the atmospheric science efforts for the United States, obtained funding to install an infrared gas analyzer to make continuous readings of CO2 at the Mauna Loa Observatory on the "Big Island" of Hawaii. The observatory was chosen because of its location in the middle of the Pacific, far removed from industrial pollution, and because its elevation of 11,140 feet (3,397 m) guaranteed the availability of clean air. The task of collecting and analyzing the CO2 data fell to a young atmospheric chemist, Charles D. Keeling (1928-2005) of the Scripps Institution of Oceanography in San Diego, California.

Keeling had been studying atmospheric CO2 since 1954. Before starting his project he had consulted Guy Stewart Callendar, obtaining data on CO2 levels extending back to the 19th century. Examining the combination of the continuous readout of CO2 data from the Mauna Loa Observatory and readings obtained in Antarctica during the IGY, Keeling realized that the percentage of CO2 in the atmosphere had increased from 1958 to 1959. In contrast to the commonly held assumption that once a good measurement had been made there would not be enough variation to warrant additional measurements, Keeling discovered that the CO2 level changed with the seasons—a significant 3 percent change in the Northern Hemisphere between spring and fall. Had the results shown no overall increase and no intraannual variation, the CO2 measuring


The Mauna Loa Observatory on the island of Hawaii (NOAA Photo Library)


program might have ceased at the end of the IGY. Since Keeling's initial findings confirmed Callendar's argument that the concentration of atmospheric CO2 had been steadily increasing since the industrialization of the early 19th century, the measurements continued. In 1961, Keeling published the first version of what came to be known as the Keeling curve. The now-familiar sawtoothed curve shown in the accompanying illustration indicates a steady increase in CO2 levels since 1958.

The intraannual variation in the Northern Hemisphere is due to the effect of plant life on CO2 levels. In the spring, when plants leaf out, they absorb large quantities of CO2 for photosynthesis and produce large quantities of oxygen. The CO2 level drops as this process continues until the fall. When leaves die and fall back to the ground, the process of decay releases the CO2 back to the atmosphere, and the level increases once again.

The overall increase in CO2 levels concerned scientists and spurred additional research into the climatic effects of CO2. In the almost 50 years since Charles Keeling made his first measurements at Mauna Loa, the Keeling curve has become one of the most recognizable symbols of the global warming debate.

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The Keeling curve shows the steady increase in atmospheric carbon dioxide since the beginning of the International Geophysical Year (IGY) collection program in 1958.

percent. Sea level was rising and schools of cod were moving north from their usual habitats. No one was really sure what was causing the change. Most meteorologists steadfastly refused to believe that CO2 could be the cause. They were convinced that Callendar had not taken into account the amount of CO2 that could and would be stored in the world's oceans. Some members of the general public connected the rising air temperature with the testing of nuclear weapons. Meteorologists quickly disputed that claim, but it was no longer possible to dismiss possibilities without finding one that would work.

By the mid-1950s, some meteorologists favored changes in the Sun's radiation as a cause, but there was really no evidence to back up this claim. Meteorologists did not accept climate change as being attributable to the rearrangement of Earth's continents and oceans proposed by Alfred Wegener and known as continental drift. Nor did they accept the

Some people mistakenly thought that nuclear bomb tests, such as this nuclear explosion during Operation Teapot at the Nevada Test Site in 1955, were changing the weather. (National Security Administration, Nevada Site Office)
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