Outside the window, 7 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 are immersed in Earth’s invisible atmosphere all of the time.
We cannot live without air for more than a couple of minutes, yet we seldom give it a thought until the wind blows too hard or the air is not moving on a muggy September evening.
Earth has had an atmosphere since early in its history, but not the air to which we are accustomed. 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 have 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 7 miles. There is no clearly defined upper limit. It simply becomes increasingly 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 does not react with anything and so stays in the atmosphere for long periods.
Nitrogen is almost inert. It is an important nutrient for plants, but they cannot use it in gaseous form. They rely upon chemical and biological processes to convert it into usable forms.
Ultraviolet sunlight converts some oxygen to ozone in the stratosphere and thus shields the surface by absorbing some of those potentially damaging rays.
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, such as the comet impact that most likely caused the demise of the dinosaurs. There are many other, more serious mass extinctions in the geologic record where as many as 90 percent of existing species were wiped out in environmental disasters.
Human activities have affected the systems with effects that we do not completely understand. We do know that burning fossil fuels adds carbon dioxide at a high rate while deforestation and oceanic pollution reduce oxygen production. How harmful these activities are in the long term or what level of ecological change they might cause remain to be seen.
One thing is for sure: We will never use all of the air, but it is likely we could change its composition enough to make ourselves uncomfortable or, worse, our planet unlivable.
Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
