aerosol A suspension of liquid or solid matter in air. Some are produced mechanically, such as dust, sea salt, or soot, and some are produced chemically through the conversion of gases into solids or liquids, such as the conversion of sulfur dioxide gas to sulfuric acid.

albedo A measure of how much insolation is reflected back into space. It is usually given as the fraction or percent of the incoming solar radiation energy that is reflected back to space.

Anasazi A people who lived at the Four Corners region of the southwestern United States between 1AD and 1300AD. They had a well-developed corn agriculture and a road system for trading between relatively large cities constructed of adobe and stone. Famous archeological sites such as Chaco Canyon and Mesa Verde provide evidence of the Anasazi people. Anasazi is a Navajo word meaning "ancient enemies".

Arrhenius, Svante (1859-1927) A Swedish chemist who in predicted that humans would warm the global climate by increasing the carbon dioxide content of the atmosphere.

Beringia A name given by scientists to the region around the Bering Strait between Alaska and Siberia during the last ice age when it was greatly enlarged because of the lowered sea level at that time.

carbon dioxide A gas consisting of one carbon atom and two oxygen atoms. Plants use it during photosynthesis to make carbohydrates. It is released back to the atmosphere during the processes of combustion or decay. It is an important greenhouse gas.

climate The average or expected weather conditions in a particular region and season. It is quantified by measures such as the monthly mean temperature or precipitation averaged over a number of years.

climate model Computer programs designed to represent, in terms of equations, atmospheric and chemical processes that occur in nature.

cloud condensation nuclei Aerosol particles on which molecules of water vapor may condense to form cloud droplets or ice particles.

Dust Bowl A period of severe drought in Colorado, New Mexico, Oklahoma, Kansas, and parts of Texas in 1934-1937. It coincided with the economic disruption of the Great Depression and caused considerable hardship for people living in this area.

dynamical system A system consisting of interacting parts that may exhibit complex variability in time.

El Niño/Southern Oscillation (ENSO) Two phenomena closely linked in the tropics. El Niño refers to variations in temperature and circulation in the tropical Pacific Ocean, while Southern Oscillation refers to atmospheric pressure and wind variations that are closely coupled to changes in the tropical oceans. Together they produce important climate fluctuations on intervals of two to five years that have a significant impact on the seasonal weather in the tropics and globally. During a warm El Niño event warm water normally in the western Pacific Ocean spreads east toward South America. Effects range from droughts in Australia to flooding in some parts of the U.S.

energy balance Heat entering and leaving a system are equal. For Earth, the solar energy entering the climate system is equal to the infrared radiation emitted to space.

feedback A process in which one event affects another, which then affects the original event. In climate, water vapor feedback causes the climate to be more sensitive to small influences, because warming causes atmospheric water vapor to increase, which through the greenhouse effect causes the climate to warm further.

GCM An acronym derived from "general circulation model", which is a computer model that calculates the detailed large-scale motions of the atmosphere or the ocean explicitly from the equations of physics.

greenhouse effect A condition in which the atmosphere warms the surface by being relatively transparent to solar radiation and absorbing and emitting terrestrial radiation very effectively. Solar radiation is transmitted through the atmosphere and heats the surface, but the thermal infrared radiation emitted by the surface cannot easily pass through the atmosphere, so the surface must heat up to achieve an energy balance.

ground sloth Large ancestor of the two- and three-toed tree-loving sloth bear. Ground sloths were oxen-sized herbivores with large claws, who preferred the ground to the trees. They became extinct approximately 10,000 years ago.

halocarbons A family of industrial gases containing halons, such as chlorine, bromine, and carbon. They are manufactured for use in refrigeration units, as cleaning solvents, and in the production of insulating foams. Common examples are CFC-11 and CFC-12, which are only destroyed when they travel up into the stratosphere and encounter ultraviolet radiation, so they have long lifetimes in the atmosphere.

ice-albedo feedback A process by which ice-albedo can change the sensitivity of the climate response and thus amplify climate changes produced by other means. Surface ice cover increases when the climate cools, the higher albedo of ice reflects more solar radiation and this causes the climate to cool further.

Industrial Revolution The change in economic and social organization resulting from the replacement of hand tools with powered machinery and large-scale industrial production, beginning in England in 1760 and later in other countries.

infrared radiation Radiation with wavelengths between 4 and 200 microns. It is emitted by all objects with temperatures like those near the surface of Earth. It is invisible to the human eye, but the heat from it can be felt by human skin. Earth cools by emitting infrared radiation to space.

insolation Incoming energy from the sun. The first two parts of the word come from "in=in" and "sol=sun".

large scale Having spatial scales of a few thousand kilometers or greater. Midlatitude high and low pressure systems are large scale.

latent heat The heat released or absorbed per unit mass by a system in a reversible change of phase, such as the heat released when water condenses.

Little Ice Age A period of expansion of mountain glaciers from about 1350 to 1800 in the Alps, Norway, Iceland, Alaska, and probably elsewhere.

mastodon Extinct relative of the modern elephant. A favored food source of earliest Americans.

mesoscale Having spatial scales from one to several hundred kilometers. Thunderstorms are mesoscale phenomena.

methane A gas consisting of one carbon atom and four hydrogen atoms, also known as natural gas. It is produced during decay in the absence of oxygen, such as takes place under water, within the earth, or in the guts of animals. It is an important energy source and an important greenhouse gas whose concentration in the atmosphere has doubled since preindustrial times.

Milankovitch, Milutin (1879-1958) A Serbian mathematician who offered a theory for what causes the advances and retreats of ice sheets.

nitrous oxide A gas consisting of two nitrogen atoms and one oxygen atom. It is an important greenhouse gas.

orbital parameters The characteristics of the Earth's elliptical orbit about the sun and rotation about its axis that control the distribution of insolation through latitude and season. The eccentricity measures the extent to which Earth's orbit deviates from a perfect circle. Obliquity or tilt measures the angle between the Earth's axis of rotation and the plane of its orbit around the sun. The longitude of perihelion measures the season of the year in which the Earth passes closest to the sun.

ocean sediment The solid material that falls to the bottom of the ocean and builds up over time. It consists of shells of small sea creatures, organic material from sea life, and dust or silt transported from continents. It forms a record of the ecology of the ocean at the time the sediment was laid down. This record can be retrieved from cores drilled in ocean sediments.

photosynthesis The process by which plants produce sugars from carbon dioxide, water, and sunlight.

small scale Having spatial dimensions on the order of a few tens of meters or less. Air turbulence near the surface of earth is a small-scale phenomenon.

solar radiation Radiation emitted by the sun. It consists of ultraviolet radiation with wavelengths between 0.1 and 0.4 microns, visible radiation with wavelengths between 0.4 and 0.7 microns and near-infrared radiation with wavelengths between 0.7 and 4 microns. Of the total energy emitted by the sun, about half is visible and half near infrared, with ultraviolet being a small, but important constituent. Much of the ultraviolet is absorbed in the stratosphere and above, which prevents it from harming life at the surface of Earth.

stratosphere The part of the atmosphere 10 miles above earth's surface.

sulfur dioxide A gas consisting of one sulfur atom and two oxygen atoms. It is produced by life, by volcanoes, and during the combustion of fuels containing sulfur. In the atmosphere it is converted into sulfuric acid aerosol, which may form hazes and serve as cloud condensation nuclei.

thermals Rising parcels of warm air caused by the heating of Earth's surface by sunshine.

transpiration The release of moisture through the pores in the surface of leaves or other parts of plants.

water vapor feedback A process by which water vapor can change the sensitivity of the climate response and thus amplifies climate changes produced by other means. Water vapor is the most important greenhouse gas, and its concentration in the atmosphere increases as the surface temperature increases, giving a positive feedback.

weather The short-term events of the atmosphere, in days, hours, or minutes.