The Atmosphere

STOP - You must read Chapter 14 before doing this lesson

Introduction

The modern atmosphere consists of a mixture of gases so perfectly and consistently diffused among each other. By volume, the composition of dry air can be broken down as follows: Nitrogen (N2) - 78%, Oxygen (O2) - 21%, Argon (Ar) - 0.9%, Carbon Dioxide (CO2) - 0.03%, and trace amounts of neon, helium, krypton, xenon, hydrogen, methane, and nitrous oxide. Of the four major atmospheric components, only two, oxygen and carbon dioxide, directly enter into biological process. Oxygen is required by most living organisms for the production of energy, a process known as aerobic respiration; carbon dioxide constitutes the carbon source for photosynthesis.

Regions of the Atmosphere

Troposphere - Extending from sea level to an altitude about 8-9 miles above the earth. Virtually all life activities occur within this region and most weather and climatic phenomena occur here. Within the troposphere temperature steadily falls with increasing altitude.

Stratosphere - This region is distinguished by a temperature gradient reversal, meaning that the temperature slowly rises with increasing altitude. The stratosphere contains the ozone layer.

Mesophere - It is above the stratosphere, temperature once again begins to fall with increasing altitude.

In addition to providing the major source of certain chemical elements necessary for life, the atmosphere performs a vital role in controlling the earth's surface environment by regulating both the quality and quantity of solar radion that enters and leaves the biosphere. The source of all energy on earth is the sun.

The major causes of human-induced atmospheric change are: 1. introduction into the atmosphere of pollutant gases and particles not usually found there in significant amounts, and 2. changes in the concentration of natural atmospheric components. Two of the most difficult issues cause by the impact of humans activities on the earth-atmosphere system are depletion of the ozone layer and rising levels of atmospheric CO2 (global warming).

Depletion of the Ozone (O3) Layer

The presence of ozone in the stratosphere is of vital importance in protecting life on earth from the damaging effects of solar ultraviolet radiation.
The ozone depletion process begins when chloroflurocarbons (CFCs), also known as Freon, and other ozone-depleting substances (ODS) leak from equipment (Step 1 figure). CFCs are extremely stable, and they do not dissolve in rain. After a period of several years, ODS molecules reach the stratosphere (Step 2). Strong ultraviolet (UV) light breaks apart the ODS molecule. CFCs release chlorine atoms, and halons release bromine atoms (Step 3). It is these atoms that actually destroy ozone, not the intact ODS molecule. It is estimated that one chlorine atom can destroy over 100,000 ozone atoms before finally being removed from the stratosphere (Step 4). Since ozone filters out harmful UV light radiation, less ozone means higher UV levels at the surface. The more depletion, the larger the increase in incoming UV light (step 5). UV light has been linked to skin cancer, cataracts, damage to materials like plastic, and harm to certain crops and marine organisms. Although some UV light reaches the surface of the earth even without ozone depletion, its harmful effects will increase as a result of this problem (Step 6).

CLick here and go to the EPA web site. Follow the story of the missing ozone and learn more about the causes, effects and solutions to ozone depletion.

Global Warming

The greenhouse effect is a natural process by which the earth absorbs and reflects solar radiation. It is usually carbon dioxide and water vapor in the atmosphere which play a vital role in this process. However, man have been changing the atmosphere equilibrium by increasing the concentrations of CO2. Levels of CO2 have been gradually rising ever since the dawn of the Industrial Revolution as a result of the ever-increasing use of coal, oil, and gas to power the world's factories and vehicles. When fossil fuels are burned, one of the primary combustion products is carbon dioxide; the combustion of one ton of coal, for example, releases three tons of CO2. The equilibrium that has prevailed for millennia has been disrupted, with the result that atmospheric CO2 levels are now sharply increasing. As CO2 levels increase, more infrared radiation will be absorbed and over time average global temperatures will increase causing global warming.

CLick here and go to the EPA global warming site.

Greenhouse Gases -
The primary greenhouse gas is CO2. However there are other gases in the atmosphere that are further enhancing the grenhouse effect by capturing wavelengths of infrared radiation not readilly absorbed by CO2. These are methane, nitrous oxide (N2O), and chlorofluorocarbons. The concentration of these gases is far below that of CO2, however these gases are much more efficient than CO2 in absorbing outgoing heat waves. For example, a molecule of methane can trap 20-30 times more infrared radiation than does a molecule of CO2. Each molecule of CFCs has a direct warming effect up to 20,000 times greater than that of a CO2 molecule.

Impacts of Global Warming

The major impact of global warming trend would not be felt in terms of human physical discomfort but rather in a possibly drastic alteration of worldwide rainfall and temperature patterns. The greatest temperature changes are predicted to be at high latitudes and in the middle of continents. It is also predicted that there will be changing in precipitation patters. All these changes can lead to:

Diminishing Crop Yields
Loss of Biodiversity
Rising Sea levels - due to both the thermal expansion of water as temperatures in the ocean rise and to the melting of polar ice caps.

CLick here goes to Global Warming: Early Warming Signs.

Global Warming

Climate Change

Global Warming - Natural Resources Defense Council web site.