Cycles of the Earth and Atmosphere — Module Overview

This on-line teaching module is for middle school science teachers. While some students may find portions of this site useful, the primary audience is classroom teachers and it has been developed with that target audience in mind. The site provides background information and supporting classroom teaching materials. The content focus is climate change and issues related to both stratospheric and tropospheric ozone.

There are seven sections, each with activities for classroom use. Each section provides background information, general learning concepts, and a list of activities.

  • Introduction to the Atmosphere
  • Introduction to Ozone
  • Introduction to Climate
  • Stratospheric Ozone
  • The "Greenhouse Effect"
  • Tropospheric Ozone
  • Global Climate Change

Before starting the activities, you are encouraged to read the Overview below and the background material provided in each section. It will give you a broad introduction to the science content found in this module.


It would be among the most profound discoveries in the history of our species if we could detect life on other planets circling other stars in the galaxy. Whether the life turns out to be intelligent, technological, or simply microbial, such a discovery would provide evidence that we are not alone, that life (in one form or another) is widespread in a universe that suddenly seems less stark and empty.

How might we detect such distant life? Not likely with fleets of starships. The energies required for star travel (barring a science-fiction style warp drive) are simply too great and travel times too long. Instead, sophisticated ground and space-based telescopes are being used to detect planets of nearby star systems. How planets block the light of their star as detected from Earth can reveal the chemical composition of the planet's atmosphere. Some scientists believe that the chemical composition of the planet's atmosphere will provide evidence of the presence of life. How is this possible? How can simple gases tell us about the presence or absence of life as we know it? To answer that, we must look at our own planet.

Earth has a radius of some 6400 km. Ninety-nine percent of the earth's atmosphere is contained within a layer approximately 50 km thick. Life on earth inhabits a layer no more than 9 km thick, extending from a bare few kilometers above sea level (airborne organisms and life on mountains) to a few kilometers below (deep ocean basin creatures and subterranean microbial communities).

The biosphere is the portion of earth in which all known life forms exist. It occupies a thin layer of air (atmosphere), water (hydrosphere), and land (lithosphere).


Just as we earthly organisms require a source of energy, water, and the chemical components of our bodies, so too does the entire global biosphere. These services are provided to the biosphere by global energy and chemical cycles.

Most people are familiar with the concepts of cycles—that certain substances move endlessly throughout the earth's biosphere, hydrosphere, atmosphere, and lithosphere, existing in different forms and being used by different organisms at different times, but always moving, always circulating. Most of your students are probably familiar with the water cycle. Water, in its different forms, cycles continuously through the lithosphere, hydrosphere, atmosphere, and biosphere. Water evaporates into the atmosphere from the land and the sea. Plants and animals use and reuse water and release water vapor into the air. Once in the air, water vapor circulates and can condense to form clouds and precipitation, which fall back to earth. At one time or another, all of the water molecules on earth have been in an ocean, a river, a plant, an animal, a cloud, a raindrop, a snowflake, or a glacier!

In addition to water, many other substances such as nitrogen, oxygen, and carbon cycle through the earth and atmosphere. These cycles are important to individual animals and plants and even to entire ecosystems. But we're less familiar with the notion that these cycles fundamentally influence the planet as a whole, dramatically and unmistakably altering the earth's atmosphere. When you think about it, this influence only makes sense. The atmosphere is the greatest, fastest, and most reliable global transport system we have.

Inject almost any stable gaseous material into the atmosphere and before long it is spread worldwide. For example, the graphic below shows how the smoke and ash plume from the eruption of Mt. Spurr in Alaska spread and moved over a 4-day period, as detected by satellite imagery. During that time, the original plume was carried rapidly eastward from Alaska and spread over an area many times its original size.

Because cycles require the movement of substances, what better conveyor belt to use than the atmosphere? To explain what we mean, let's look at several important components of the earth's atmosphere and see how they are influenced by these cycles.

Nitrogen comprises the bulk of the atmosphere (approximately 78%). Nitrogen cycles slowly through the earth's system. To most of the biosphere, nitrogen in the atmosphere is like the ocean to a thirsty person—amazingly abundant but not quite in the right chemical form. A molecule of nitrogen gas is made up of 2 atoms very tightly bound together. It takes tremendous amounts of energy, such as produced by lightning or fires, to break the bond. Amazingly, an assortment of bacterial species that specialize in taking nitrogen from the air can also convert nitrogen into different usable forms. These bacteria also release nitrogen from organic material back into the atmosphere. Nitrogen is the one element found almost entirely in the atmosphere—there's very little on land or in the sea. Nitrogen is essential to life, a key element in proteins and DNA.

Oxygen is found in the atmosphere at a stable concentration of approximately 21%. Because it is a very reactive element, it can quickly combine with other elements and disappear from the atmosphere. Yet it persists, and in high concentration. It is the cycling of oxygen through photosynthesis and respiration that accounts for its presence and stability. A world without cycles, without life, would retain little if any oxygen in its atmosphere.

Oxygen does more, however, than simply move between organisms and the atmosphere. Some of the atmospheric oxygen () finds itself lofted high into the upper reaches of the atmosphere called the stratosphere. There, in a series of reactions powered by solar radiation, it is converted into a new compound, ozone (). The presence of stratospheric ozone benefits us creatures of the biosphere tremendously! Ozone serves to absorb biologically damaging ultraviolet (UV) radiation from the sun. Without an ozone layer, the earth's surface would be bathed in high intensity UV radiation; with an ozone layer, the amount of UV radiation received at the surface is vanishingly small. Eventually, each ozone molecule will come apart, reform , and may either be carried elsewhere in the atmosphere or may take part again in the ozone-forming reactions. Over hundreds of millions of years (at least), these ozone-forming and destroying reactions have generally been in equilibrium, forming a balanced - cycle within the greater cycle of atmospheric oxygen.

The Atmosphere in Dynamic Equilibrium

Oxygen and nitrogen are not the only elements that cycle through the biosphere. Most elements critical to life constantly cycle. This is why earth's atmosphere can be described as being in a state of dynamic equilibrium. Things are constantly moving and changing—substances enter and leave the atmosphere, forming different compounds at different times and in different places.

On earth, the dynamic equilibrium changes with season. For example, in the spring and summer, growing plants take carbon from the soil and atmosphere. In the fall and winter, plants release carbon to the soil and the atmosphere. Because most of the plant life on earth is found in the Northern Hemisphere, there are global seasonal changes of carbon dioxide in the atmosphere. An atmosphere in static equilibrium (like that of Mars) indicates a dead world. All the reactions have taken place and the elements have found their most stable chemical form. The atmosphere of a living planet, like ours, is quite different. Unstable, interesting, and improbable reactions happen all the time, thanks to cycles!

Concluding Thoughts

We suggest you use this module to encourage your students to explore the cycles that make up their world. We also encourage you to challenge your students to look at the atmosphere and its influence on individuals, ecosystems, and the planetary biosphere as it moves, reacts, and changes under the influences of the lithosphere, hydrosphere, and biosphere. Cycles are the recurrent theme in the seven units:

Each unit has background information, concepts, goals, and relevant hands-on activities for classroom use.

Click on a unit in the navigational menu at the top of the page to proceed. Note that if you want to print any page in the module, click on the Print Button in the Browser header or select Print from the File Menu.