4.4: Atmospheric Carbon Dioxide measurements

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Page ID: 50826

Neel Desai and Alicia Mullens
De Anza College

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The Keeling Curve

While there are numerous greenhouse gases present in the atmosphere, with all of them contributing somewhat to Earth’s Greenhouse Effect, by far the most influential and concerning greenhouse gas is Carbon Dioxide (we’ll explore why in more depth when we explore Radiative Forcing). Since the mid-1800’s, approximately the same time as the Industrial Revolution, which resulted in significant advancements in the quality of life across many developed countries, Carbon Dioxide has seen a notable rise, as seen in Figure \(\PageIndex{1}\).

Carbon dioxide concentration curve shows a hockey stick shape with significant increase after 1950. — Figure \(\PageIndex{1}\): Carbon Dioxide concentrations in the atmosphere since 1700. (CC BY 4.0; Ralph Keeling via Scripps Institution of Oceanography) Alternative description of the image.

Figure \(\PageIndex{1}\) shows Carbon Dioxide concentrations in the atmosphere since 1700. However, since 1958, Carbon Dioxide concentrations have been directly measured and monitored at the Mauna Loa Observatory on the Big Island of Hawaii. Today, a graph of these remarkable measurements versus time is known as the Keeling Curve, and it is shown in Figure \(\PageIndex{2}\). On the curve, the unit for Carbon Dioxide concentrations is parts per million (ppm), which is the average number of carbon dioxide molecules in a sample of 1,000,000 molecules of atmospheric air. It was named after the monitoring station’s founder, David Keeling, who showed a steady increase in atmospheric Carbon Dioxide since 1958. Prior to 1958, ice core data were used to estimate atmospheric Carbon Dioxide concentrations, as shown in Figure \(\PageIndex{1}\).

Carbon dioxide concentration increasing steadily since 1958. — Figure \(\PageIndex{2}\): Carbon Dioxide concentrations in the atmosphere measured by the Keeling Curve since 1958. (CC BY 4.0; Ralph Keeling via Scripps Institution of Oceanography) Alternative description of the image.

Based on this information, let's try to answer the following questions:

Based on Figure \(\PageIndex{1}\), Carbon Dioxide concentrations were generally stable from 1700 until approximately 1850. During this time, they had a concentration of approximately:
1. 210 ppm
2. 280 ppm
3. 315 ppm
4. 420 ppm
On the other hand, based on Figure \(\PageIndex{2}\), the current Carbon Dioxide concentrations are approximately: (CO₂ concentration in 2025-2026)
1. 315 ppm
2. 295 ppm
3. 395 ppm
4. 430 ppm
Also based on Figure \(\PageIndex{2}\), while there is some variation in annual Carbon Dioxide concentrations (the zig-zags in the curve), the overall trend in the concentrations is:
1. decreasing
2. leveling off
3. increasing

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