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

Mar 23, 2016 |  Glenn Ricketts

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

Mar 23, 2016 | 

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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Ron Hooper

| March 23, 2016 - 5:07 PM

Thanks.  But it seems to me that such a very small amount of CO2 can’t have much, if any, effect on the climate.

Antonio Chaves

| March 25, 2016 - 8:33 AM

I do not know of any scientists who doubt that carbon dioxide is a greenhouse gas. The legitimate debate is whether or not the burning of fossil fuels is producing enough CO2 to change the climate. Unfortunately, the issue is so thoroughly politicized that I do not know who to trust. Furthermore, water vapor plays the wild card in any projections that are being made because it is both a greenhouse gas AND increases the Earth’s albedo (clouds that reflect sunlight away from the Earth).

Vic Wieland

| May 03, 2016 - 12:36 AM

The IPCC do not say CO2 can cause any heating worth worrying about - but they claim a “Forcing ” by claiming the small theoretical heating will cause more water vapour which will then cause more heating .
This is all “modelled” and all models are noticeably failing against actual empirical measurements as the models could not account for clouds ( water vapour causes clouds) which have a net cooling effect thanks to albedo and rain .
Suffice to say there’s no proof that co2 will result in runaway warming and in fact as co2 levels were way way higher in the past and we went into an ice age it seems the proof is the exact opposite . Like Polar Bears , extreme weather , the North Pole vanishing , “our kids will never know snow” and all the other fraudulent alarmist claims - a complete and utter hoax .

Chris Schoneveld

| May 03, 2016 - 2:55 AM

Vic Wieland says: “but they claim a “Forcing ” by claiming the small theoretical heating will cause more water vapour which will then cause more heating”

This incorrect. This phenomenon they claim is called “Positive Feedback”