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The Keeling Curve Explained

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The Keeling Curve is one of the most compelling pieces of scientific evidence that shows that carbon dioxide (CO2) is accumulating in our atmosphere. CO2 is a greenhouse gas. Greenhouse gases trap heat in the atmosphere and help keep the planet warm. More greenhouse gas molecules in the air means more heat is trapped, leading to an overall warming of the planet.

Read the explanations in the video above to learn more about the Keeling Curve, which plots CO2 in the Earth’s atmosphere over time.

COis a molecule made up of one carbon atom and two oxygen atoms, and it moves in and out of the atmosphere through various process known as the carbon cycle. In the carbon cycle, natural processes like human respiration and volcanic activity release CO2. Plants absorb CO2 during photosynthesis and the ocean absorbs it when CO2 from the air mixes with seawater. However, human activities, such as the burning of fossil fuels, like coal, oil and natural gas, also release CO2 into the atmosphere.

Since the start of the Industrial Revolution in the late 1700s, humans have been emitting more and more CO2. At the same time, forests all over the world are being cleared for agriculture and development. Deforestation not only leaves fewer plants to absorb the increasing amounts of CO2, but also adds CO2 to the air when trees are burned or left to decay.


Until about the mid-20th century many scientists thought that the oceans would easily absorb the excess CO2 emitted from fossil fuel burning, so there wasn't a great concern over the possible effects of increased emissions in the atmosphere. However, there wasn't a lot of data on the actual concentrations of CO2 in the atmosphere. In 1958, Charles Keeling of the Scripps Institute of Oceanography began measuring CO2 levels in the air at a weather station in Hawaii and at the South Pole. After a few years, Keeling observed a steady increase in atmospheric CO2, an indication that fossil-fuel emissions were building up in the atmosphere. Due to funding cuts, Keeling had to stop measurements at the South Pole, but the weather station in Hawaii continues to measure concentrations of CO2. The data from these measurements are plotted on a graph known as the Keeling Curve.

Keeling's findings were so profound that they prompted the National Oceanic and Atmospheric Administration (NOAA) to start monitoring CO2 concentrations in other locations around the world. And, in July 2014, NASA launched the Orbiting Carbon Observatory, a satellite designed to gather the most precise data ever for tracking the carbon cycle on Earth. All of this data indicates that CO2 is building up in Earth's atmosphere.

Discussion Questions

  • What does the Keeling Curve show about the concentrations of CO2 in the Earth's atmosphere?
  • Why does the concentration of CO2 go up and down throughout the year?
  • Why do you think Keeling chose Mauna Loa and Antarctica to measure atmospheric CO2?
  • Why does the concentration of CO2 increase over time?
  • What does an increase in CO2 levels in the atmosphere mean for global warming?

This video explainer is part of our Clue into Climate collection of resources.

An interactive version of this explainer is featured in our Clue into Climate e-book series. Click on the tabs below to download our free e-books or subscribe to our iTunes U course. You can also visit our e-books page to view our other offerings.

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Investigate shifts in the distribution of plant and animal species due to climate change, and the effects of increased carbon dioxide emissions on the ocean.

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