CO2 and Other Greenhouse Gases

Carbon dioxide (CO₂) is the most important of many human-generated “greenhouse gases”—gases that are contributing to a gradual warming of the planet. These gases, many of which have always existed in the atmosphere, contribute to a balance of heat flows that has given us a relatively stable climate. Since the beginning of the Industrial Revolution, however, the concentration of CO₂ in the atmosphere has grown from its historical average of 280 parts per million (ppm) to over 380 ppm and counting.

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The atmosphere helps keep a balance between visible light and other high-energy radiation arriving at our planet from the sun and much-lower-energy heat radiating outward from the Earth’s surface. The atmosphere reflects some of this radiation, absorbs some, and allows some to pass through. Exactly how much radiation of each type is reflected, absorbed, or transmitted depends on what’s in the atmosphere.

What defines CO₂ and certain other gases as greenhouse gases is that they let in more heat than they let out—absorbing and reflecting back more low-energy radiation from the Earth than they do high-energy radiation from the sun—warming the planet. Greenhouse gases absorb infrared (low-energy) radiation because of their ability to vibrate at correspondingly low energy levels. They tend to be molecules composed of three or more atoms—gases that exist as individual atoms, like helium, or in pairs, like oxygen or nitrogen, can’t absorb as much low-energy radiation.

CO₂ is the greenhouse gas that gets the most attention because it's relatively abundant and its balance in the atmosphere is a direct result of human activities. Water vapor is even more common but it doesn’t remain the atmosphere very long and human activities don’t have nearly as much effect—at least not directly—on how much is released. Methane, nitrous oxide, and many other greenhouse gases are much more powerful than CO₂ on a per-pound basis. Over a 100-year period, one pound of methane is estimated to have as much impact as 25 pounds of CO₂. For nitrous oxide the factor is 298 tons, and for sulfur hexafluoride it’s 22,800!

Several similar metrics are used to describe emissions of carbon dioxide and other greenhouse gases: pounds of carbon, of carbon dioxide, or of carbon dioxide equivalent. The most common thing to measure is the mass of carbon dioxide, in pounds in the U.S., and in kilograms or metric tons internationally. Until recently, U.S. government documents tended to express these metrics using just the carbon part of carbon dioxide. The carbon atom represents 27.3% (12/44) of the mass of a CO₂ molecule, so those measurements are smaller by that amount. Greenhouse gases are measured collectively in units of CO₂-equivalent, or CO2e, so the impact of one ton of methane emissions would be documented as 25 tons of CO2e.