Following on from my previous post, Understanding Carbon Dioxide – CO₂, I will now look at how this molecule functions in the earth’s atmosphere. It is generally identified as a greenhouse gas contributing to the greenhouse effect. In this role CO₂ has been represented as being a harmful substance that causes global warming. To understand how this occurs it is worth looking at the processes involved.

The Greenhouse Effect

The word ‘greenhouse’ is only being used as a metaphor, but some people have taken the term literally. Greenhouse gases neither ‘trap’ nor ‘reflect’ heat as has previously been claimed. A greenhouse increases its interior temperature by physically trapping warm air through the use of glass panels. There is no such physical barrier in the atmosphere, however.

The sun is the Earth’s largest and most powerful source of energy. This energy reaches the planet in the form of electromagnetic radiation, which includes infrared, visible light, ultraviolet, X-rays, and gamma rays. The atmosphere serves to protect the surface of the planet from the more harmful forms of radiation transmitted by the sun. The thermosphere is the second highest level of the atmosphere, lying below the exosphere and above the mesosphere. It ranges from 80 to 700km (50 to 440 miles) in altitude. It absorbs harmful gamma rays and X-rays. The ozone layer is in the stratosphere and it absorbs most of the ultraviolet radiation. The final layer is the troposphere that climbs from 0 to 12 km, or 7 miles. It is the part of the atmosphere that physically touches both the land and the surface of the sea. Infrared radiation is absorbed in this layer mostly by CO_2, and also H₂O in the form of clouds.

Not all of the electromagnetic radiation is absorbed in the atmosphere and what gets through heats the planet. Without this solar radiation the surface would be some 32° C, 89.6° F, colder than it is. The warm temperature created by the Greenhouse effect is vital for life to continue to exist on Earth.

Greenhouse Gases

The most significant greenhouse gases are water vapour (H₂O), CO₂, methane (CH₄), and nitrous oxide (N₂O). They each absorb and reemit infrared radiation. All but H₂O are trace gases. The passage of radiation is not one way, however. Both the land and the sea emit a portion of the heat that they have absorbed back towards the atmosphere where it is absorbed by various greenhouse gases.

Absorption and Reemission of Infrared Radiation

CO₂ first encounters infrared radiation as it enters the atmosphere. On a molecular level the CO₂ absorbs the infrared radiation and then emits it in all directions. Some will head back upwards, some will be emitted sidewards, and the rest downwards toward the surface. If the infrared radiation emitted upwards does not encounter another greenhouse gas molecule capable of absorbing it then it will escape into space. If it is absorbed, however, it will again be reemitted in all directions by that molecule. The sum total of the amount of infrared radiation being absorbed and reemitted will decline each time it encounters a suitable greenhouse gas molecule. This applies to infrared radiation reemitted downwards towards the surface or sideways across the atmosphere.

H₂O in the form of water vapour actually absorbs and reemits more infrared radiation than CO₂. There is much more H₂O in the atmosphere. By mass over the full atmosphere water vapour is 0.25%, which would equal 2,500ppm approximately. In comparison CO₂ is measured at only 413ppm. The ability of H₂O to retain heat through the absorption and reemission of infrared radiation can be physically experienced. On cloudless nights the temperature gets noticeably colder.

How CO₂ warms the planet

To be honest I am not sure that I fully understand how the facts and processes described in these posts support the assertion that more CO₂ in the atmosphere leads to global warming. As stated, CO₂ does absorb and reemit infrared radiation, but it does not increase the amount of infrared radiation in the atmosphere. I have yet to find a concise explanation that not only supports CO₂’s role in that context but explains why it is more efficient than H₂O. As an example, this is from the Livescience website:

“When the molecules in carbon dioxide and other greenhouse gases re-emit this long-wave radiation back toward Earth’s surface, the result is warming.”

Carbon Dioxide is Warming the Planet (Here’s How)

However, as mentioned in the previous paragraph, CO₂ and other molecules reemit infrared radiation in all directions, not just downwards, therefore 100% of the absorbed radiation is not directed down to the surface. Some of it will be reabsorbed by molecules around the emitting CO₂ molecule and some of it will escape into space. Considering the comparative density of water vapour in the atmosphere and the scarcity of CO₂ the reemitted radiation is more likely to be absorbed by a H₂O molecule than another CO₂ molecule.

This suggests a problem with the role of CO₂ in climate change; no one appears to be clear on what it is. I will look further into this problem in my next post.