Is Carbon Dioxide a Greenhouse Gas?

By: by Tom Schlatter
By: by Tom Schlatter

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The ultimate source of energy for driving the atmospheric winds is the Sun. Of the energy from the Sun that reaches the top of the atmosphere, a little less than half is absorbed at the Earth’s surface, and another 30 percent is reflected back to space by the Earth’s surface, clouds, or air. Almost all of the Sun’s energy is transmitted at wavelengths less than 4.0 micrometers (μm). The Earth’s surface, warmed during the day by the Sun, is continually radiating energy upward, almost all of it at infrared wavelengths greater than 4.0 μm. For this reason, 4.0 μm is a convenient dividing line for separating solar radiation from terrestrial radiation. A greenhouse gas is one that is fairly transparent to solar radiation but efficient at absorbing infrared radiation. In other words, greenhouse gases let the Sun’s radiant energy pass through the atmosphere with little depletion (absorption), but they strongly absorb ground radiation trying to escape to outer space.

Different greenhouse gases absorb infrared radiation at different wavelengths, but all keep the Earth and its atmosphere considerably warmer than it would be without them. They are effective despite very low concentrations in the atmosphere. The most plentiful greenhouse gas by far, and also the most highly variable, is water vapor (H2O). Even in a warm and very moist atmosphere, water vapor comprises at most 3 percent of the mass of air. The other important greenhouse gases are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halocarbon gases, principally chloroflourocarbons, and ozone (O3).

Because your question referred specifically to carbon dioxide, we’ll concentrate on this gas. The diagram shows carbon dioxide measurements taken near the top of Mauna Loa in Hawaii. This site was chosen because it is far removed from sources of pollution, and the exceptionally clean air allows for accurate measurements of background values of atmospheric CO2. The undulating red curve shows seasonal variations of CO2 superposed upon the smoothed black curve. The steady rise in concentration is obvious, but if you put a ruler to the black curve, you will quickly be convinced that the increase is accelerating with each passing decade. In 2005, the concentration stood at 379 parts per million by volume (ppm). At the beginning of the industrial era (1750), it was only about 280 ppm.

Carbon dioxide has been a component gas in the atmosphere for much of the Earth’s geological history. Natural sources are volcanoes, decaying plant life, respiration by mammals, and hot springs and geysers. Anthropogenic (human-generated) sources—in particular, the burning of fossil fuels, which accounts for about two-thirds of the total—are the main cause of the increase in CO2 concentrations, especially in the past century. During the years 2000-2005, an average of 7 billion tons of carbon per year were emitted as a result of fossil fuel burning throughout the world. Since 1980, about half of the anthropogenic emissions have been taken up by the oceans, which are growing slowly more acidic, and by plant life.

The evidence is incontrovertible that the concentration of CO2 in the atmosphere is rising at an accelerating rate, and virtually no one questions any more whether the Earth’s surface and its atmosphere are warming. Only a small and shrinking minority are skeptical about the connection between the two. The Intergovernmental Panel on Climate Change (IPCC) issued a report last year titled “Climate Change 2007: The Physical Science Basis,” that, in my view, is an invaluable and authoritative compendium of what is known and not known about climate change.

This report was compiled by 152 lead authors from more than 30 countries and reviewed by more than 600 experts. A summary was approved by officials from 113 countries. A major conclusion of this report with regard to greenhouse gases is that “it is extremely likely that human activities have exerted a substantial net warming influence on climate since 1750.”

In the parlance of the IPCC reports, “extremely likely” means with a probability greater than 95 percent.

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