Ask a Scholar: What are the Effects of Carbon Dioxide on the Earth's Atmosphere?

Glenn Ricketts

Dear Ask a Scholar: How can CO2 in such a small amount measured in Parts per Million have any effect on the earth’s climate?

Answered by Dr. David Legates, Professor in the College of Earth, Ocean and Environment at the University of Delaware:

Thanks for your question.  Yes, the normal assumption is that the effect of a substance should be proportional to its concentration.  Thus, Nitrogen and Oxygen should have the biggest effect on the Earth’s climate because they make up 99% of the atmosphere by volume (about 78% and 21%, respectively) whereas Carbon Dioxide should have minimal effect since it comprises only about 0.04% of the atmosphere by volume.  But despite its abundance, Nitrogen is not very important.  Here's why:

Some substances are particularly effective at absorbing certain types of energy (as measured by their wavelength or frequency).  If we consider visible light, very few of the molecules that comprise the atmosphere absorb energy in this portion of the energy spectrum.  Rayleigh scattering (caused by visible light energy bouncing off the particles in the atmosphere) is significant in attenuating light reaching the Earth’s surface as is the effect of liquid or solid water in the air (note that visibility is very much lowered in fog or that clouds block sunlight) or pollution, which can significantly affect the transmission of light.  Thus, we conclude that the atmosphere is relatively transparent with respect to visible light, meaning that most of the visible light that enters the Earth’s atmosphere from the Sun passes through the atmosphere unimpeded.

By contrast, most of the energy given off by objects at temperatures we find on the Earth exists in a band of infrared energy we often refer to as “heat” or thermal energy.  In this part of the energy spectrum, numerous gases exist which absorb energy at these wavelengths.  Water vapor has the biggest effect (thus, it is the most important “greenhouse” gas) with carbon dioxide and methane following behind as well as many forms of pollution.  Methane, for example, is more efficient at absorbing thermal energy on a per molecule basis but its concentrations are measured in parts per thousand.  Nevertheless, there are many compounds and elements in the atmosphere that absorb thermal energy and so we say that the atmosphere is relatively opaque to thermal energy.

We are fortunate here on Earth where the high oxygen concentration allows life as we know it to exist.  The gas giants are comprised largely of methane while the atmospheres of Venus and Mars are largely carbon dioxide.  Thus the importance of the gas is not just in how much of it there is, but also in the types of wavelengths of energy that it absorbs.  Carbon dioxide is important in the energy budget not because of its abundance; but rather, because of its influence despite its low concentration.

Image: Pixabay, Public Domain

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