Keeling Curve

  The Keeling Curve is a graph showing the variation in concentration of atmospheric carbon dioxide since 1958. It is based on continuous measurements taken at the Mauna Loa Observatory in Hawaii under the supervision of Charles David Keeling. Keeling's measurements showed the first significant evidence of rapidly increasing carbon dioxide levels in the atmosphere. Many scientists credit Keeling's graph with first bringing the world's attention to the effects that human activity were having on the Earth's atmosphere and climate.^[1]

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Charles Keeling of the Scripps Institution of Oceanography was the first person to make frequent regular measurements of the atmospheric carbon dioxide (CO₂) concentration, taking readings at the South Pole and in Hawaii from 1958 onwards.^[2] 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 Mauna Loa, which have continued to the present day.^[3]

The measurements collected at Mauna Loa show a steady increase in mean atmospheric CO₂ concentration from about 315 parts per million by volume (ppmv) in 1958 to over 380 ppmv by the year 2006.^[4] This increase in atmospheric CO₂ is considered to be largely due to the combustion of fossil fuels, and has been accelerating in recent years. This is supported by measurements of carbon dioxide concentration in ancient air bubbles trapped in polar ice cores, which show that mean atmospheric CO₂ concentration was between 275 and 280 ppmv for several thousand years but started rising sharply at the beginning of the nineteenth century. Since carbon dioxide is a greenhouse gas, this has significant implications for global warming.

Though Mauna Loa is an active volcano, Keeling and collaborators made measurements on the incoming ocean breeze and above the thermal inversion layer to minimize local contamination from volcanic vents. In addition, the data is normalized to negate any influence from local contamination.^[5] Measurements at many other isolated sites have confirmed the long-term trend shown by the Keeling Curve,^[6] though no sites have a record as long as Mauna Loa.^[7]

The Keeling Curve also shows a cyclic variation of about 5 ppmv in each year corresponding to the seasonal change in uptake of CO₂ by the world's land vegetation. Most of this vegetation is in the Northern hemisphere, since this is where the majority of the land is located. The level decreases from northern spring onwards as new plant growth takes carbon dioxide out of the atmosphere through photosynthesis and rises again in the northern fall as plants and leaves die off and decay to release the gas back into the atmosphere.

Due in part to the significance of Keeling's findings,^[3] the NOAA began monitoring CO₂ levels worldwide in the 1970s. Today, CO₂ levels are monitored at about 100 sites around the globe.^[1]

References

• Scripps Institution of Oceanography CO₂-Program: Home of the Keeling Curve
• Earthguide educational resource
• The Keeling Curve Turns 50 - Scripps Institution of Oceanography