Facts of the Matter
RICHARD BRILL
Sunday, August 5, 2001
Outside the window, seven miles up, the wings of the jet airliner slice through the invisible air that seems magically to keep it aloft. It reminds me that we're immersed in Earth's invisible atmosphere all of the time. We can't live without air for more than a couple of minutes, yet we don't give it a thought until the wind blows too hard or when the air isn't moving on a muggy September evening. Why is there air?
Earth has had an atmosphere since its early history, but not the air we are accustomed to. Much of the early atmosphere came from volcanic activity as Earth cooled from a ball of hot gas and metal 4.5 billion years ago. Today's air is here because biological, chemical and physical processes modified it and kept its composition relatively constant for 2 billion years or so.
The atmosphere completely surrounds the earth, held close to the planet by gravity. It gets thinner upward, with half of it lying below seven miles. There is no clearly defined upper limit. It simply becomes more tenuous and gradually merges with the interplanetary gases.
Air is a mixture of several different gaseous elements, a little less than four-fifths nitrogen and about one-fifth oxygen. The remainder is mostly an inert gas known as argon. There are small amounts of carbon dioxide and trace amounts of other gases such as water vapor, chlorine, nitrogen oxides, sulfur oxides and assorted other substances including small particles of smoke, salt and dust.
Oxygen, carbon dioxide and water vapor are the most important gases. Argon is chemically inert, meaning that it doesn't react with anything and so stays in the atmosphere for long periods of time. Nitrogen is almost inert. It is an important nutrient for plants but can not be used in gaseous form. They rely upon chemical and biological processes to convert it into usable forms. Some oxygen is converted to ozone in the stratosphere by ultraviolet sunlight and thus shields the surface from some of those potentially damaging rays.
The earth has evolved many complex and interrelated systems of checks and balances that regulate atmospheric composition and climate. One such system regulates the amounts of oxygen and carbon dioxide. For example, accelerated plant growth pumps more oxygen into the system. With increased oxygen the atmosphere becomes more reactive, so forest fires increase and burn longer. Combustion consumes oxygen and releases carbon dioxide.
The time scale of natural regulations is on the order of tens of thousands of years, long in human time but extremely short in geological time. Short-term disruptions to the system can cause ecological disaster, and there are many examples in the earth's history, like the comet impact thought to have caused the dinosaurs' demise.
Human activities have affected the systems in ways that are not completely understood. We do know that burning fossil fuels adds carbon dioxide at a high rate, while deforestation and oceanic pollution reduce oxygen production.
One thing is for sure: We won't use all of the air, but it is likely that we may be capable of changing its composition enough to make ourselves uncomfortable or, even worse, to make our planet unlivable.
We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at rickb@hcc.hawaii.edu
