| Glossary
In discussing the issue of climate change, it is useful to know the definitions of various terms and acronyms that are commonly used. Some of these are provided here.
anthropogenic global warming (AGW): AGW is the warming of Earth due to the activities of man. Most of this is thought to be due to the combustion of fossil fuels and the resulting emissions of CO2 into the atmosphere. Other anthrogogenic contributions include the emissions of other greenhouse gases and the reduction of vegetation that removes CO2 from the atmosphere.
albedo: This is the fraction of incoming solar radiation that is reflected by Earth's surfaces back into the universe. Snow-covered surfaces and clouds contribute major portions of this effect. For example, the solar radiation that strikes the snow-covered polar regions is largely reflected, making the albedo of such regions very high, about 80 percent. The albedo of water surfaces, on the other hand, is very low, less that 10 percent. The average albedo of Earth is thought to be about 30 percent. A decrease in the average albedo of Earth caused, for example, by the melting of the Arctic Ocean's ice cap would lead to an increase in Earth's average temperature.
chlorofluorocarbons (CFCs): Also known by their trade name, the Freons, these compounds were produced for a variety of commercial and industrial purposes. They are no longer made in large quantities because they were shown to cause the destruction of our protective layer of ozone in Earth's stratosphere. They also act as greenhouse gases, and because of their exceedingly long atmospheric lifetimes, will remain in the atmosphere for several centuries to come.
greenhouse gases (GHGs): Greenhouse gases are components of the atmosphere that absorb infrared radiation emitted from Earth, thereby inhibiting Earth's means of cooling itself. As a result of this greenhouse effect, the temperature of Earth must increase so that the amount of infrared energy it emits into the universe is equal to the amount of energy it absorbs from the Sun. Without the naturally occurring greenhouse gases, the average temperature of Earth would be about 60 oF colder than it is, much too cold to support many existing forms of life.
electromagnetic radiation (EMR): EMR is a form of energy emitted by all heated objects. The most familiar example of EMR is ordinary visible light, so named because our eyes can detect it. Visible light is emitted by the surface of the sun and any other object that is heated to a temperature of about 10,000 oF. Another example of EMR is infrared radiation, which is emitted by moderately warm objects such as a stove, your hand, or the Earth. The emission of EMR is how heated objects, including the Earth, attempt to cool themselves.
energy balance: The Earth will continuously attempt to maintain a balance between the input of energy it receives from the sun and its output of energy due to its own emission of infrared radiation into outer space. If additional insolvation (that is, additional infrared-absorbing greenhouse gasses) is added to our atmosphere, then the temperature at the surface of the Earth will increase (that is, warming will occur) until the Earth's output of energy becomes equal to its input.
forcing agent: This refers to a specific change in the existing conditions on Earth that will force its average temperature to either increase or decrease. For example, increased levels of carbon dioxide in the atmosphere will cause positive forcing by increasing the absorption of the infrared radiation coming from the Earth. Increased cloud formation can cause both negative forcing (by increasing the reflection of incoming solar radiation) and positive forcing (by absorbing the infrared radiation emitted by the Earth.
global dimming: This is a term used to describe a cooling effect caused by the reflection of incoming solar radiation by particulate matter (often referred to as aerosols) suspended in the atmosphere. Major sources of particulates include volcanoes, coal-fired power plants, and vehicles.
hydrofluorocarbons (HFCs): These man-made compounds have replaced the CFCs in many commercial and industrial applications because they do not destroy stratospheric ozone. They are greenhouse gases, however, and are presently accumulating in the atmosphere.
hydroxyl radical: This is a simple molecule (OH) consisting of one oxygen and one hydrogen atom that initiates the breakdown and removal of hydrocarbons in the troposphere. It is formed in the lower atmosphere by the interaction of the small amount of UV light that manages to penetrate through the stratosphere with tropospheric ozone and water vapor. Once made, hydroxyl radicals will quickly react with any molecule having a carbon-hydrogen bond (note that the CFCs do not have such bonds). The steady-state concentration of OH in the troposphere is exceedingly low (only about 0.01 pptr), and thereby, it provides a spectacular example of how even an ultra-trace component of the atmosphere can provide a vital atmospheric function; in this case, that of keeping it clean. It literally is the "janitor of the atmosphere."
ice ages: This term if often used to refer to what would be more rigorously called the "glacial periods" that have occurred during the last major ice age that we have been in for the last 2.5 million years. The warm periods between these glacial periods are rigorously called the "interglacial periods" of the last major ice age. Geologists tell us that there have been several major ice ages with the first occurring about 2.5 billion years ago.
infrared radiation (IR): IR radiation has wavelengths (1 to 50 microns) that are longer than those of visible light, and is emitted by all objects having moderate temperatures, (such as the Earth, itself). The emission of IR radiation is how the Earth attempts to cool itself. Infrared radiation is readily absorbed, however, by all molecules in the atmosphere that have 3 or more atoms (the greenhouse gases). Therefore, in order to emit enough IR radiation into the universe so as to maintain its total energy balance (solar energy "in" must equal IR energy "out"), the average temperature of the Earth must increase.
Intergovernmental Panel on Climate Change (IPCC): This is a scientific organization sponsored by the United Nations. It consists of the leading scientists in the world whose research areas involve climate change. They meet regularly and provide updated reports every three years. The contents of their last report, provided in 2007, are often referred to here.
lifetimes: This is a term commonly used to describe how long a given compound persists in the atmosphere. Stated more rigorously, this actually refers to the "half-life" of a given compound; that is, the time required for the concentration of that compound to be reduced to one-half its present value if further emissions of that compound were stopped.
microns: This is the unit of length used here to indicate the wavelength of the electromagnetic radiation. One micron is equal to one millionth of a meter, and one meter is just slightly longer (by 3.3 inches) than one yard. The width of a fine human hair is about 30 microns.
Milankovitch cycle: This term describes in detail how the Earth revolves around the sun. Over relatively long periods of time, small changes occur in the position and the tilt of the Earth relative to the sun. With respect to causing cold and warm periods of Earth, the most important factor is how much solar radiation strikes the Northern Hemisphere during its summer season. This is because the northern half of our planet has more land-versus-sea exposure than its southern half. If, due to the changes within Milankovitch cycles, the northern half gets less than its usual amount of irradiation during its summer season, a net increase in northern glaciations will occur, and this will cause the entire Earth to move toward a colder period. If the northern half gets more than its usual amount of summer irradiation, the entire Earth will move toward a warmer period.
ozone: Ozone (O3) is the molecular form of oxygen having three atoms instead of two as in its normal form (O2). Ozone is continuously formed by the interaction of ultraviolet radiation from the Sun with O2. The concentration of ozone is greatest in the middle of the stratosphere, but is also found at all other altitudes down to Earth's surface. The so-called "stratospheric ozone layer" serves the vital function of absorbing and thereby removing most of the ultraviolet light coming from the Sun.
stratosphere: This is the portion of the atmosphere immediately above the troposphere. The air in this region is not vertically well mixed and, in fact, is stagnant due to its inverted temperature gradient (the temperature increases with increased altitude in the stratosphere). The stratosphere extends from the top of the troposphere up to an altitude of about thirty miles.
temperature scales: We will usually use the Fahrenheit (oF) scale here because Americans are most familiar with it. Occasionally, the more scientifically preferred Celcius (oC) scale will also be used. If a temperature change or difference is being referred to, that temperature change in units of oC can be converted to units of oF simply by multiplying by 1.8. If a specific temperature of some object is being indicated in units of oC, that temperature can be converted to units of oF by multiplying by 1.8 and then adding 32. Thus the freezing point of water (0.0 oC) becomes 32.0 oF and the boiling point of water (100 oC) becomes 212 oF. Note that the change in temperature when water is heated from its freezing point to its boiling point is +100 oC or +180 oF.
troposphere: This is the lower portion of Earth's atmosphere in which we live. It is well mixed from top to bottom and its height varies from about five miles in the polar regions to about twelve miles near the equator. The temperature at the top of the troposphere is very cold (approximately -70 oF at mid-latitudes), thereby inhibiting the passage of water vapor and other compounds of moderate volatility into the stratosphere.
ultraviolet (UV) radiation: This is the short wavelength (0.1 to 0.4 microns) portion of the sun's incoming radiation. Unlike visible or infrared radiation, it can cause the rupture of the chemical bonds within most molecules.
visible (Vis) radiation: Most of the electromagnetic radiation emitted by the sun is of this type, having wavelengths between 0.4 and 0.8 microns. As its name suggests, our eyes detect visible light.
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