Prior to the 1950s, measurements of atmospheric concentrations had been taken on an ad hoc basis at a variety of locations. In 1938, engineer and amateur meteorologist Guy Stewart Callendar compared datasets of atmospheric from Kew in 1898–1901, which averaged 274 parts per million by volume, and from the eastern United States in 1936–1938, which averaged 310 ppmv, and concluded that concentrations were rising due to anthropogenic emissions. However, Callendar's findings were not widely accepted by the scientific community due to the patchy nature of the measurements. Charles David Keeling, of Scripps Institution of Oceanography at UC San Diego, was the first person to make frequent regular measurements of atmospheric concentrations in Antarctica, and on Mauna Loa, Hawaii from March 1958 onwards. Keeling had previously tested and employed measurement techniques at locations including Big Sur near Monterey, rain forests of the Olympic Peninsula in Washington state, and high mountain forests in Arizona. He observed strong diurnal behavior of, with excess at night due to respiration by plants and soils, and afternoon values representative of the "free atmosphere" over the Northern hemisphere.
In 1957–1958, the International Geophysical Year, Keeling obtained funding from the Weather Bureau to install infrared gas analyzers at remote locations, including the South Pole and on the volcano of Mauna Loa on the island of Hawaii. Mauna Loa was chosen as a long-term monitoring site due to its remote location far from continents and its lack of vegetation. Keeling and his collaborators measured the incoming ocean breeze above the thermal inversion layer to minimize local contamination from volcanic vents. The data were normalized to remove any influence from local contamination. Due to funding cuts in the mid-1960s, Keeling was forced to abandon continuous monitoring efforts at the South Pole, but he scraped together enough money to maintain operations at the Mauna Loa Observatory, which have continued to the present day. Keeling's Tellus article of 1960 presented the first monthly records from Mauna Loa and Antarctica, finding a “distinct seasonal cycle…and possibly, a worldwide rise in CO2 from year to year.” By the 1970s, it was well established that the increase of atmospheric carbon dioxide was ongoing and due to anthropogenic emissions. Carbon dioxide measurements at the Mauna Loa Observatory in Hawaii are made with a type of infrared spectrophotometer, now known as a nondispersive infrared sensor, that is calibrated using World Meteorological Organization standards. This type of instrument, originally called a capnograph, was first invented by John Tyndall in 1864, and recorded by pen traces on a strip chart recorder. Currently, several laser-based sensors are being added to run concurrently with the infrared spectrophotometer at Scripps Institute of Oceanography, while NOAA measurements at Mauna Loa still use the nondispersive infrared sensor.
Results and interpretation
The measurements collected at Mauna Loa Observatory show a steady increase in mean atmospheric concentration from 313 parts per million by volume in March 1958 to 406 ppmv in November 2018, with a current increase of 2.48 ± 0.26 ppmv per year. This increase in atmospheric is due to the combustion of fossil fuels, and has been accelerating in recent years. Since is a greenhouse gas, this has significant implications for global warming. Measurements of concentration in ancient air bubbles trapped in polar ice cores show that mean atmospheric concentration was between 275 and 285 ppmv during the Holocene epoch, but started rising sharply at the beginning of the nineteenth century. The Keeling Curve also shows a cyclic variation of about 5 ppmv each year corresponding to the seasonal change in uptake of by the world's land vegetation. Most of this vegetation is in the Northern hemisphere where most of the land is located. From a maximum in May, the level decreases during the northern spring and summer as new plant growth takes out of the atmosphere through photosynthesis. After reaching a minimum in September, the level rises again in the northern fall and winter as plants and leaves die off and decay, releasing back into the atmosphere.
Legacy
Global monitoring
Due in part to the significance of Keeling's findings, NOAA began monitoring levels worldwide in the 1970s. Today, atmospheric levels are monitored at about 100 sites around the globe through the Global Greenhouse Gas Reference Network. Measurements at many other isolated sites have confirmed the long-term trend shown by the Keeling Curve, although no sites have as long a record as Mauna Loa.
Since Charles David Keeling's death in 2005, responsibility and oversight of the project was transferred to Keeling's son, Ralph Keeling. On the fiftieth anniversary of the beginning of the project, the younger Keeling wrote an article in Science magazine describing his father's life and work, along with how the project has grown and evolved over time. Along with more precise measurement materials and funds for the project of monitoring of the Earth's levels, Keeling wrote his pride for his father's work and how he has continued it in his memory.
On May 9, 2013, the daily mean concentration of in the atmosphere measured at Mauna Loa surpassed 400 parts per million. Estimates of during previous geologic eras suggest that has not reached this level since the mid-Pliocene, 2 to 4 million years ago.
