CO2’s role in climate hard to prove

earthI am often asked how I could question the greenhouse explanation of global warming, when so many climate scientists believe the theory is sound. One possible explanation is I am not a climate scientist, so I may look at the physics of the atmosphere with more emphasis on the physics. The whole idea of greenhouse-gas warming is based on the “observation” that planets with atmospheres are 33 degrees C warmer than planets without atmospheres.

Now, planets satisfying these criteria are not exactly jumping out of the sky. This would mean to me that this atmospheric-effect theory never actually has been experimentally tested according to the scientific method. This may be acceptable to 97 percent of climate scientists, but it tends to make me itch.

Let’s say, however, that there is something to it. Having established that our atmosphere warms the planet by 33 degrees C (notice, I didn’t say how), what gases are the major components of today’s atmosphere? There is nitrogen (78 percent by volume), oxygen (21 percent), water vapor (about 0.2 percent; it varies), and carbon dioxide (0.04 percent by volume). Other gases are at much lower concentrations and will not be considered.

My first inclination on how this works is by an insulating-blanket effect. Oxygen and nitrogen gases near the surface absorb heat energy from the warm surface by colliding with it during the day, and then hold on to this energy by passing it back and forth through molecular collisions as they slowly rise to higher altitudes. During the night, the gases near the surface pass the energy back to the surface, which then radiates it as infrared light to space. Increasing CO2 levels should have little effect on this mechanism, since the concentration is so low. Since warming of the planet seems to be nearer to the surface, this is a plausible, but untested, theory.

A second possibility is the greenhouse effect. Oxygen and nitrogen gases are not greenhouse gases, meaning they cannot absorb infrared light. So we are left with water vapor and CO2. Now, greenhouse gases can’t just absorb any arbitrary IR radiation. Like a radio or TV, they must be tuned to a specific wavelength of light. Carbon dioxide only can absorb Earth’s IR radiation in a narrow band of wavelengths centered around 15 micrometers. But the Earth does not emit a lot of radiation at 15 micrometers. According to infrared astronomers, Earth radiates strongly in a band that peaks around 10 micrometers (infrared astronomers refer to this as N-band radiation). CO2 does not absorb strongly in the N-band, because 15-micrometer radiation falls in the tail of this band at very low intensity. It does not take a lot of CO2 in the atmosphere to absorb essentially 100 percent of the 15 micrometer radiation in this band. Adding more CO2 to the atmosphere once this radiation is gone has no effect.

Colder areas of Earth’s surface (minus 80 degrees C, Antarctica temperatures) do shift Earth’s radiation toward 15 micrometers where CO2 absorbs more strongly, but these cold regions of the Earth’s surface are very small compared with the warmer areas near the equator.

Which of these theories is correct, and could there be others? Who knows, but until any are tested experimentally, they all are just chalk dust.

James Barrante of Cheshire is a retired college professor of physical chemistry.