Friday, June 01, 2007

So what about carbon dioxide? A first look

Next to water, carbon dioxide (CO2) is the most important of the infrared (IR)-active gases in the atmosphere and contributes to the opacity of clear air as water does. Kilo for kilo, the absorption/emission effect of CO2 is about 1/4 that of water. Molecule for molecule, taking into account CO2's larger mass (44 versus 18 for water), it's about 60% the effect of water.

For now, we'll only consider present and recent values of the CO2 concentration, postponing consideration of possible future trends. As a baseline, use the 1950 value of CO2 concentration for the Earth's atmosphere, which was about 300 parts per million (ppm) by volume.* The main sources of atmospheric CO2 are:
  • Volcanic and other geological burps (sporadic)
  • Burning of fossil fuels (anthropogenic, or man-made)
  • Oceanic release of dissolved CO2
  • Organic release of CO2
The first two are the main sources. The most important sinks (mechanisms that remove the gas from the atmosphere) of CO2 are:
  • Plant respiration
  • Oceanic dissolution and carbonate fixing, followed by deep ocean exchange and sedimentation
  • Photodissociation in the stratosphere
The first and third mechanisms convert the CO2 to C and O2, the latter remaining a gas. In the organic case, the C is "fixed" into organic tissue by photosynthesis. In the geological case, the CO2 gas diffuses into water (like carbonation in soda pop) and then dissolves by binding with H2O molecules. In dissolved form, it enters the oceanic carbonate cycle, then sinks to the deep ocean and sediments out. In the last case, the C floats around in the atmosphere and eventually precipitates out. The first sink is the most important; one way to see that is that there isn't a lot of CO2 in the atmosphere, but there's a lot of O2. Essentially all of the O2 we and other animals breathe comes from plant respiration, which per force must cycle a lot of CO2 to maintain that O2 level. Plant growing season is once or twice a solar year, so plants can respond fairly fast to changes in the CO2 level - it's their "air." Oceanic dissolution is slower, effective over decades; followed by sinking over centuries and sedimentation and crustal subduction over thousands of years. Photodissociation is the slowest, since there's almost no CO2 in the upper atmosphere.

What is man doing? Animal exhalation contributes a negligible amount to the total inflow of CO2 into the atmosphere. The only significant source of animal CO2 is our burning of fossil fuels. The increase is about 4/3 ppm per year. Thus in 2000, the concentration reached about 365 ppm. Everything else remaining the same, the atmospheric concentration of CO2 will double its 1950 value in 225 years, around the year 2175. Of course, evaporation, clouds, convection, plants, and oceans will respond, so that in reality, everything doesn't remain the same.

To fully analyze the effect of the baseline CO2 concentration (300 ppm), we'd need to look at all these ramifications. But for the present CO2 level, this is a moot point, and we'll analyze those effects in terms of future increases of CO2 concentration in subsequent postings. We'll leave it here by saying that the CO2 opacity contribution alone to the surface temperature is about +0.55 oC. The CO2 contributes 2% to the total IR opacity effect in the lower atmosphere, the other 98% being due to water vapor.**
* Compare water vapor at about 9,000 ppm.

** For the experts: the "total opacity effect" means the total optical depth.

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