The ABCs of CO2

Tens of billions of metric tons of carbon are exchanged every year among the Earth's plants, animals, land, oceans and air in complex cycles. In the most familiar cycle, people exhale carbon dioxide as a byproduct of turning food into energy for their bodies. Green plants take CO2 in from the air to build their cells. Most scientists are convinced that human activities are upsetting the balance of those cycles and increasing the amount of heat-trapping carbon dioxide in the atmosphere. This illustration shows why the planet's natural cleansing cycle may not be able to compensate for burning fossil fuels. A car that consumes 600 gallons of gasoline a year, by traveling 15,000 miles at an average of 25 mpg, puts about 12,000 pounds of carbon dioxide into the air. In contrast, a typical tree takes in an estimated 50 pounds a year for its own growth. Photo credit: KYLE RAETZ
Among the great achievements of the past 120 years was learning how to burn gasoline and diesel fuel to propel cars and trucks. But scientists who study global warming say there's a downside, and here's how it works:

In simple terms, combustion tears apart the hydrogen and carbon atoms in gasoline molecules and unites them with oxygen from the air.

Although only a fraction of the energy from these chemical reactions - 15 to 20 percent, under the best of circumstances - makes it to a vehicle's wheels, burning a gallon is sufficient to send a 5,000-pound Ford F-150 about 17 miles down the road at a mile a minute or more.

But now one of the natural byproducts, carbon dioxide, is becoming a problem.

People are burning gasoline and other fuels at a rate that puts more carbon dioxide into the atmosphere than the planet can recycle through the normal exchanges among plants, animals, land, water and the air, say climate scientists.

Some carbon dioxide is needed in the atmosphere - not just because plants use it to produce nutrients for themselves and oxygen as a byproduct, but also to maintain moderate temperatures. Without the heat-trapping quality of CO2 and other greenhouse gases, the planet would be much colder.

But when concentrations of CO2 go too high, too much heat is trapped, climate scientists say.

U.S. car and truck drivers are spewing out carbon dioxide from about 140 billion gallons of gasoline a year.

Each of those gallons of gasoline puts an astounding 20 pounds of carbon dioxide into the atmosphere. The CO2 weighs more than the original fuel because, in combustion, each carbon atom has combined with two heavy oxygen atoms instead of very light hydrogen atoms.

The only fixes: burn less fuel containing carbon or find ways to take carbon dioxide out of the air.

You may e-mail Harry Stoffer at

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