Aerosols:  Solid or liquid particles suspended within the atmosphere (see "sulfate aerosols" and "black carbon aerosols"). 

Albedo:  Refers to the ratio of light from the sun that is reflected by the Earth's surface to the light received by it.  Unreflected light is converted to infrared radiation (i.e., heat), which causes atmospheric warming (see "radiative forcing").  Thus, surfaces with a high albedo (e.g., snow and ice) generally contribute to cooling, whereas surfaces with a low albedo (e.g., forests) generally contribute to warming.  Changes in land use that significantly alter the characteristics of land surfaces can therefore influence the climate through changes in albedo. 

Anthropogenic Emissions: Emissions of greenhouse gasses resulting from human activities. 

Black Carbon Aerosols:  Particles of carbon in the atmosphere produced by inefficient combustion of fossil fuels or biomass.  Black carbon aerosols absorb light from the sun, shading and cooling the Earth's surface, but contribute to significant warming of the atmosphere (see "radiative forcing"). 

Carbon Dioxide Equivalent (CO2e):  The emissions of a gas, by weight, multiplied by its "global warming potential."

Chlorofluorocarbons (CFCs): CFCs are synthetic industrial gases composed of chlorine, fluorine, and carbon.  They have been used as refrigerants, aerosol propellants, cleaning solvents and in the manufacture of plastic foam.  There are no natural sources of CFCs.  CFCs have an atmospheric lifetime of decades to centuries, and they have 100-year "global warming potentials" thousands of times that of CO2, depending on the gas.  In addition to being greenhouse gases, CFCs also contribute to ozone depletion in the stratosphere and are controlled under the Montreal Protcol.

Climate: The long-term average weather of a region including typical weather patterns, the frequency and intensity of storms, cold spells, and heat waves.  Climate is not the same as weather.

Climate change: Refers to changes in longer-term trends in the average climate, such as changes in average temperatures.  In IPCC usage, climate change refers to any change in climate over time, whether due to natural variability or as a result of human activity.  In UNFCCC usage, climate change refers to a change in climate that is attributable directly or indirectly to human activity that alters atmospheric composition.

Emissions Cap: A mandated restraint in a scheduled timeframe that puts a "ceiling" on the total amount of anthropogenic greenhouse gas emissions that can be released into the atmosphere.  This can be measured as gross emissions or as net emissions (emissions minus gases that are sequestered). 

Emissions Trading: A market mechanism that allows emitters (countries, companies or facilities) to buy emissions from or sell emissions to other emitters.  Emissions trading is expected to bring down the costs of meeting emission targets by allowing those who can achieve reductions less expensively to sell excess reductions (e.g. reductions in excess of those required under some regulation) to those for whom achieving reductions is more costly.

Global Warming: The progressive gradual rise of the Earth's average surface temperature thought to be caused in part by increased concentrations of GHGs in the atmosphere.

Global Warming Potential  (GWP):  A system of multipliers devised to enable warming effects of different gases to be compared.  The cumulative warming effect, over a specified time period, of an emission of a mass unit of CO2 is assigned the value of 1.  Effects of emissions of a mass unit of non-CO2 greenhouse gases are estimated as multiples.   For example, over the next 100 years, a gram of methane (CH4) in the atmosphere is currently estimated as having 23 times the warming effect as a gram of carbon dioxide; methane's 100-year GWP is thus 23.   Estimates of GWP vary depending on the time-scale considered (e.g., 20-, 50-, or 100-year GWP), because the effects of some GHGs are more persistent than others. 

Greenhouse Effect: The insulating effect of atmospheric greenhouse gases (e.g., water vapor, carbon dioxide, methane,etc.) that keeps the Earth's temperature about 60°F warmer than it would be otherwise.

Hydrofluorocarbons (HFCs): HFCs are synthetic industrial gases, primarily used in refrigeration and semi-conductor manufacturing as commercial substitutes for chlorofluorocarbons (CFCs). There are no natural sources of HFCs.  The atmospheric lifetime of HFCs is decades to centuries, and they have 100-year "global warming potentials" thousands of times that of CO2, depending on the gas.  HFCs are among the six greenhouse gases to be curbed under the Kyoto Protocol.

Intergovernmental Panel on Climate Change (IPCC): The IPCC was established in 1988 by the World Meteorological Organization and the UN Environment Programme.  The IPCC is responsible for providing the scientific and technical foundation for the United Nations Framework Convention on Climate Change (UNFCCC), primarily through the publication of periodic assessment reports (see "Second Assessment Report" and "Third Assessment Report").

Kyoto Protocol: An international agreement adopted in December 1997 in Kyoto, Japan.  The Protocol sets binding emission targets for developed countries that would reduce their emissions on average 5.2 percent below 1990 levels.

Methane (CH4): CH4 is among the six greenhouse gases to be curbed under the Kyoto Protocol. Atmospheric CH4 is produced by natural processes, but there are also substantial emissions from human activities such as landfills, livestock and livestock wastes, natural gas and petroleum systems, coalmines, rice fields, and wastewater treatment.  CH4 has a relatively short atmospheric lifetime of approximately 10 years, but its 100-year GWP is currently estimated to be approximately 23 times that of CO2.

Nitrous Oxide (N2O): N2O is among the six greenhouse gases to be curbed under the Kyoto Protocol.  N2O is produced by natural processes, but there are also substantial emissions from human activities such as agriculture and fossil fuel combustion.  The atmospheric lifetime of N2O is approximately 100 years, and its 100-year GWP is currently estimated to be 296 times that of CO2.

Radiative Forcing: The term radiative forcing refers to changes in the energy balance of the earth-atmosphere system in response to a change in factors such as greenhouse gas emissions, land-use change, or solar radiation.  The climate system inherently attempts to balance incoming (e.g., light) and outgoing (e.g., heat) radiation.  Positive radiative forcings increase the temperature of the lower atmosphere, which in turn increases temperatures at the Earth's surface.  Negative radiative forcings cool the lower atmosphere.  Radiative forcing is most commonly measured in units of watts per square meter (W/m2).

Renewable Energy: Energy obtained from sources such as geothermal, wind, photovoltaic, solar, and biomass.

Sinks: Any process, activity or mechanism that results in the net removal of greenhouse gases, aerosols, or precursors of greenhouse gases from the atmosphere.

Substitution: The economic process of trading off inputs and consumption due to changes in  prices arising from a constraint on greenhouse gas emissions.  How the extremely flexible U.S. economy adapts to available substitutes and/or finds new methods of production under a greenhouse gas constraint will be critical in minimizing overall costs of reducing emissions.

Sulfate Aerosols:  Sulfur-based particles derived from emissions of sulfur dioxide (SO2) from the burning of fossil fuels (particularly coal).  Sulfate aerosols reflect incoming light from the sun, shading and cooling the Earth's surface (see "radiative forcing") and thus offset some of the warming historically caused by greenhouse gases. 

Sulfur Hexafluoride (SF6): SF6 is among the six types of greenhouse gases to be curbed under the Kyoto Protocol.  SF6 is a synthetic industrial gas largely used in heavy industry to insulate high-voltage equipment and to assist in the manufacturing of cable-cooling systems.  There are no natural sources of SF6.  SF6 has an atmospheric lifetime of 3,200 years.  Its 100-year GWP is currently estimated to be 22,200 times that of CO2.

United Nations Framework Convention on Climate Change (UNFCCC): A treaty signed at the 1992 Earth Summit in Rio de Janeiro that calls for the "stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system."  The treaty includes a non-binding call for developed countries to return their emissions to 1990 levels by the year 2000.  The treaty took effect in March 1994 upon ratification by more than 50 countries.  The United States was the first industrialized nation to ratify the Convention.

Water Vapor (H2O):  Water vapor is the primary gas responsible for the greenhouse effect.  It is believed that increases in temperature caused by anthropogenic emissions of greenhouse gases will increase the amount of water vapor in the atmosphere, resulting in additional warming (see "positive feedback").

Courtesy Pew Center Global Climate Change