This summer is only half over and already we have been treated to the film "Too Hot Not To Handle," Al Gore's "An Inconvenient Truth," as well as the news that the Supreme Court will get involved in the question of whether carbon dioxide (CO₂) should be considered a pollutant under the Clean Air Act. In an attempt to reinforce the idea that CO₂ is a pollutant, Gore and others often speak of "CO₂ pollution" in the global atmosphere.

Before you train yourself to add the "p" word to your CO₂ vocabulary, consider the following very convenient facts about this rather important constituent of atmospheric chemistry:

1. CO₂ comes from the Earth itself. The gas is produced naturally by hot spots in the crust that we see as volcanoes or fissures under the sea. The outgassing from the crust has occurred throughout the Earth's five billion year history and continues to this day. Had you visited the Earth 3.5 billion years ago, you would have found atmospheric concentrations of CO₂ around 70,000 parts per million (ppm) as opposed to the current value near 378 ppm. Over most of the history of the Earth, CO₂ levels were very much higher than the level we see today.

2. Obviously, something must have happened to reduce the levels of CO₂ from the large values seen billions of years ago. As it turns out, CO₂ combines with water in the atmosphere to form carbonic acid, and this naturally-occurring slightly acidic rainfall reacts chemically with silicates that make up the Earth's crust. At the timescale of billions of years, CO₂ fluctuated enormously with changes in the temperature of the Earth (and the oceans), the amount of land surface exposed to weathering, and geological activity.

3. The CO₂ story became even more complex as life evolved in the oceans. Creatures in the seas cleverly produced calcium carbonate shells from ions that were once floating around the atmosphere as CO₂. As the shells rained to the seafloor bottom, the CO₂ basically had been taken from the atmosphere and deposited as rock layers beneath the sea. Earth's ecosystem drew down the CO₂ levels to a few thousand parts per million.

4. Around one half billion years ago, a new player emerged in the CO₂ budget of the atmosphere. By that time, the ozone layer was sufficiently developed to block out much of the ultraviolet radiation from the Sun, and at long last, the vascular land plants spread throughout the Earth. In case you have forgotten your basic biology, plants survive via photosynthesis -- they are able to take CO₂ from the atmosphere, build their mass, and release oxygen. You guessed it -- animals evolved to eat the plants, and gloriously, the animals breathe in oxygen and give out CO₂.

5. Atmospheric CO₂ continued to fluctuate, largely due to the changes in the temperature of the Earth. During glacial times, often forced by changes in the Earth's orbit, the oceans would hold more CO₂, and the atmospheric levels of the gas would fall. During the most recent glaciation (the one that ended around 12,500 years ago), atmospheric CO₂ levels dropped to under 200 ppm which is perilously close to the 100 ppm level below which plants would no longer be capable of photosynthesis and the global ecosystem would suffocate. If someone is concerned about dangerous levels of atmospheric CO₂, going low is far more dangerous than going high! As the Earth warmed following the end of the most recent glaciation, atmospheric CO₂ levels moved up to around 250 ppm.

6. In just the right circumstances (e.g., swamps with little oxygen in the water), plants can die but not decay. They can be buried by geological processes and ultimately form coal, oil, and natural gas. As fate would have it, there were many such swamps in the past, and many plants were converted into fossil fuels. Humans evolved, discovered the various fossil fuels, and burned them to produce energy. Industrialization hit the streets, humans enormously increased their burning of fossil fuels, and CO₂ levels are up to around 387 ppm, and the concentrations are on the rise.

7. Plants all over the planet evolved when atmospheric CO₂ levels were very much higher than what we have today. Literally thousands of biological experiments show that when atmospheric CO₂ levels increase, plants grow faster, bigger, more resistant to any number of stresses, and far more water-use efficient. In many ways, plants must feel like they are going home to a world in which they evolved with CO₂ levels up to ten times what we have today. In order to make CO₂ more sinister, claims are made that ragweed and poison ivy will grow more vigorously in the future, and indeed they will. But so will every tree in the forest, grasses in our rangelands, and every agricultural crop.

There is no doubt that CO₂ is a greenhouse gas, and holding everything else constant, elevated CO₂ will act to warm the Earth. But as we see in the CO₂ story, the levels of this gas have fluctuated enormously over the history of the Earth, and the ecosystems of the planet have evolved to cope with these variations. To suddenly label CO₂ as a "pollutant" is a disservice to a gas that has played an enormous role in the development and sustainability of all life on this wonderful Earth. The Supreme Court ruling will be interesting, but Mother Earth has clearly ruled that CO₂ is not a pollutant.

Dr. Robert C. Balling Jr. is a professor in the climatology program at Arizona State University, specializing in climate change and the greenhouse effect.