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Carbon Dioxide: Friend or Foe?

Dateline: 02/02/98

By Alan Bruzel

Once thought an innocuous gaseous by-product of animal metabolism, carbon dioxide (CO₂) now pits environmentalist against industrialist. Can the substance shunned for its role in man-made global warming be embraced as a replacement for more potent pollutants? Read on.

There is no doubt that average global temperature has risen over the past century. There is also no doubt that atmospheric carbon dioxide concentration has similarly increased. How are the two related? Should CO₂ alone bear the stigma for the rise in global temperature if, as claimed, 98% of the greenhouse effect is due to water vapor and clouds? Can so small a tail wag such a large dog? (Also, see this site's article, Chlorofluorocarbons and Ozone.)

Atmospheric carbon dioxide concentrations rise and fall. Antarctic ice core samples correlate low atmospheric CO₂ levels (190 to 200 parts per million [ppm]) with periods of glaciation and high CO₂ levels (260 to 280 ppm) with glacial recessions. Today's atmospheric CO₂ concentration is 355 ppm (and climbing). Is this simply a normal recovery from the Little Ice Age (1550 - 1850 AD) or an abnormal recovery exacerbated by burning fossil fuels?

Panic is an inappropriate response. Saving America's glaciers – they will melt by the year 2030 if "nothing is done to curb global warming" – is impracticable if causes are of planetary origin. Normal geological processes are beyond the control of state and federal legislatures. Otherwise, through popular fiat, Wisconsin's brawny Ice Age inhabitants would have preserved that state's ice cap for their descendants. Why daintily curd cheese when you can lustily herd mammoth?

Also inappropriate is blithely burning fossil fuels without removing the resultant CO₂. One proposed remedy is the H₂ - Methanol - CO₂ project. Here, CO₂ from industrial emissions combines with hydrogen to form methanol. Loaded aboard tankers, the methanol ships out to off-shore platforms and is reformed back into CO₂ and hydrogen. The CO₂ is buried at sea; hydrogen is sent to the mainland through pipelines.

Carbon dioxide does have a good side. It provides the carbon source for green plants in light and stimulates their growth (even in the presence of ozone). Beans and cucumbers show more resistance to cold weather when grown in higher CO₂ concentrations. In the dark, however, high CO₂ levels impair soybean seedlings' oxygen respiration. And it would be remiss to omit carbon dioxide's deleterious effects as exemplified by Lake Nyos, Horseshoe Lake, scuba diving, and stunning pigs.

Solid, gaseous, and liquid carbon dioxide furnish products familiar to all. Solid carbon dioxide (Dry Ice) cleans (blasts) a variety of materials, cools food and drink, launches toy rockets, and smokes and fogs. Gaseous carbon dioxide is the star player in CO₂ lasers, fire extinguishers, carbonated beverages, oil-recovery from fractured reservoirs, and intra-abdominal insufflation (ask anyone who recently had gall bladder surgery).

When heated under pressure, gaseous carbon dioxide enters the supercritical fluid state, exhibiting properties of both gas and liquid. Like a gas, supercritical CO₂ permeates solid substrates; like a liquid it acts as a solvent. It extracts caffeine from coffee beans (to prepare decaffeinated coffee) and removes pesticide residues from soil. Its new uses in chromatography will interest the analytical chemist and its ability to substitute for traditional degreasing agents and paint solvents will appeal to the manufacturing chemist.

The non-polarity of carbon dioxide limits its usability as a solvent. It will only dissolve other non-polar entities (caffeine, pesticides, grease, paint). Fortunately, a newly invented technique adds polarity, giving CO₂ the potential to eliminate both non-polar and polar pollutants from contaminated areas.

It is indeed ironic that the substance linked with the perils of global warming also holds the promise of soil remediation and offers an alternative to chlorinated solvents. Only a diehard glacier-hugger would ignore the olive branch being held out by this wonderful, suffocating gas.

Recommended Web resource for additional information:

Organochlorine and Metal Pollution
An article from this Web site describing bioaccumulation of these toxins, plus a calculation of breathing's contribution to atmospheric carbon dioxide levels.

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