Glossary Terms: All | A-D | E-I | J-Q | R-Z |

Adaptation: Actions by individuals or systems to avoid, withstand, or take advantage of current and projected climate changes and impacts. Adaptation decreases a system’s vulnerability, or increases its resilience to impacts.

Adaptive Capacity: A system’s inherent ability to adapt to climate change impacts.

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

Afforestation: Planting of new forests on lands that have not been recently forested.

Air Retarder: A material or structural element that inhibits airflow into and out of a building’s envelope or shell. It can be a house wrap, which reduces air infiltration and exfiltration yet allows water to easily diffuse through it. An air barrier is an assembly of materials and construction details that severely reduces air infiltration. (BSC/WAPTAC)

Air Sealing: Sealing penetrations in the walls, floor, and ceiling where outside air enters the home. It’s the most cost-effective way to improve the energy efficiency of a home. (PATH)

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.

Allowance: An authorization to emit a fixed amount of a pollutant.

Allocation: Under an emissions trading scheme, permits to emit can initially either be given away for free, usually under a ‘grandfathering’ approach based on past emissions in a base year or an ‘updating’ approach based on the more recent emissions. The alternative is to auction permits in an initial market offering.

Ancillary Benefits: Complementary benefits of a climate policy including improvements in local air quality and reduced reliance of imported fossil fuels.

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

Appliance Age: If you did not purchase your appliance new or do not remember its date of purchase, you may contact a local dealer to find out how to determine the year model of your appliance. You may also be able to find the age of your appliance online by searching the web. Try here:

Assigned Amount: In the Kyoto Protocol, the permitted emissions, in CO2 equivalents, during a commitment period. It is calculated using the Quantified Emission Limitation and Reduction Commitment (QELRC), together with rules specifying how and what emissions are to be counted.

ATV: All Terrain Vehicle. A small, open, motorized vehicle with three or four wheels designed for recreational use.

Ballast: A device used to control the voltage in a fluorescent lamp. (EERE)

Barrel: A unit of volume equal to 42 U.S. gallons. One barrel weighs 306 pounds, or 5.80 million Btu of crude oil. Barrel is abbreviated as bbl.

Base Year: Targets for reducing GHG emissions are often defined in relation to a base year. In the Kyoto Protocol, 1990 is the base year for most countries for the major GHGs; 1995 can be used as the base year for some of the minor GHGs.

Baseload: For residential customers, the remaining energy consumption after the energy used for cooling and/or heating has been subtracted. It includes energy used for water heating, refrigeration, clothes dryer, lighting, electronics, etc. Baseload use is more or less constant each month, year round.

Baselines: The baseline estimates of population, GDP, energy use and hence resultant greenhouse gas emissions without climate policies, determine how big a reduction is required, and also what the impacts of climate change without policy will be.

Basket of Gases: This refers to the group six of greenhouse gases regulated under the Kyoto Protocol. They are listed in Annex A of the Kyoto Protocol and include: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6).

Battery: An energy storage device made up of one or more electrolyte cells.

Biodiesel: An alternative fuel that can be made from any fat or vegetable oil. It can be used in any diesel engine with few or no modifications. Although biodiesel does not contain petroleum, it can be blended with diesel at any level or used in its pure form.

Biodiversity: The variety of organisms found within a specified geographic region.

Biofuel: Biomass that is converted directly into liquid fuels, usually in the form of ethanol or biodiesel. Biofuels: Liquid fuels and blending components produced from biomass (plant) feedstocks, used primarily for transportation.

Biogas: A mixture of methane and carbon dioxide produced by the bacterial decomposition of organic wastes and used as a fuel.

Biomass: Any organic (plant or animal) material that is available on a renewable basis, including agricultural crops and agricultural wastes and residues, wood and wood wastes and residues, animal wastes, municipal wastes, and aquatic plants.

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“).

Blower Door: A device used by energy auditors and raters to pressurize a building in order to locate places of air leakage and energy loss. (WAPTAC)

British Thermal Unit (Btu): A unit of heat energy. One Btu is the amount of energy needed to raise the temperature of one pound of water one degree Fahrenheit. Natural gas is sometimes billed in units of million Btus, which is also referred to as a decatherm (or dekatherm). One Btu is equal to 252 calories.

Bubble: An option in the Kyoto Protocol that allows a group of countries to meet their targets jointly by aggregating their total emissions. The member states of the European Union are utilizing this option.

Building Heat-Loss Factor: The measure of the heating requirements of a building expressed in BTU per degree-day. (EERE)

Capital Stock: Existing investments in energy plant and equipment that may or may not be modified once installed.

Carbon Capture and Storage (CCS): Carbon capture and storage (also referred to as carbon capture and sequestration) is a technique for trapping carbon dioxide as it is emitted from large point sources, compressing it, and transporting it to a suitable storage site where it is injected (sequestered) into the ground.

Carbon Dioxide (CO2): CO2 is a colorless, odorless, non-poisonous gas that is a normal part of the ambient air. Of the six greenhouse gases normally targeted, CO2 contributes the most to human-induced global warming. Human activities such as fossil fuel combustion and deforestation have increased atmospheric concentrations of CO2 by approximately 30 percent since the industrial revolution. CO2 is the standard used to determine the "global warming potentials" (GWPs) of other gases. CO2 has been assigned a 100-year GWP of 1 (i.e., the warming effects over a 100-year time frame relative to other gases).

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

Carbon Footprint: The total set of greenhouse gas emissions caused directly and indirectly by an [individual, event, organization, product] expressed as CO2e.

Carbon Monoxide: A colorless, odorless, poisonous gas produced by incomplete fossil fuel combustion.

Carbon Sinks: Processes that remove more carbon dioxide from the atmosphere than they release. Both the terrestrial biosphere and oceans can act as carbon sinks.

Carbon Taxes: A surcharge on the carbon content of oil, coal, and gas that discourages the use of fossil fuels and aims to reduce carbon dioxide emissions.

Certified Emissions Reduction (CER): Reductions of greenhouse gases achieved by a Clean Development Mechanism (CDM) project. A CER can be sold or counted toward Annex I countries’ emissions commitments. Reductions must be additional to any that would otherwise occur.

Chemical Energy: Energy stored in a substance and released during a chemical reaction such as burning wood, coal, or oil.

Chimney (or Stack) Effect: The tendency of heated air or gas to rise in a duct or other vertical passage, such as in a chimney, small enclosure, or building, due to its lower density compared to the surrounding air or gas. (EERE)

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 Protocol.

Clean Development Mechanism (CDM): One of the three market mechanisms established by the Kyoto Protocol. The CDM is designed to promote sustainable development in developing countries and assist Annex I Parties in meeting their greenhouse gas emissions reduction commitments. It enables industrialized countries to invest in emission reduction projects in developing countries and to receive credits for reductions achieved.

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 long-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 UNFCC usage, climate change refers to a change in climate that is attributable directly or indirectly to human activity that alters atmospheric composition.

Climate Sensitivity: The average global air surface temperature change resulting from a doubling of pre-industrial atmospheric CO2 concentrations. The IPCC estimates climate sensitivity at 1.5-4.5oC (2.7-8.1oF).

Climate Variability: Refers to changes in patterns, such as precipitation patterns, in the weather and climate.

Coal-Fired Power Plant: A power plant that uses coal as the fuel to generate electricity.

Cogeneration: The production of electrical energy and another form of useful energy (such as heat or steam) through the sequential use of energy.

Coke (coal): A solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents are driven off by baking in an oven at temperatures as high as 2,000 degrees Fahrenheit so that the fixed carbon and residual ash are fused together. Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Coke from coal is grey, hard, and porous and has a heating value of 24.8 million Btu per ton.

Colfiring: The process of burning natural gas in conjunction with another fuel to reduce air pollutants.

Combined Cycle: Characteristic of a power producing engine or plant that employs more than one thermodynamic cycle. Heat engines are only able to use a portion of the energy their fuel generates; the remaining heat from combustion is generally wasted. Combining two or more "cycles" results in improved overall efficiency.

Commitment Period: The period under the Kyoto Protocol during which Annex I Parties' GHG emissions, averaged over the period, must be within their emission targets. The first commitment period runs from January 1, 2008 to December 31, 2012.

Compact Fluorescent Light Bulb (CFL): A smaller version of standard fluorescent lamps that can directly replace standard incandescent lights. These lights consist of a gas-filled tube and magnetic or electronic ballast. The newer CFLs are vastly improved. They fit in most light fixtures and have warmer color tones and longer life. They will reduce the energy used by 75% over an incandescent bulb and last 6-10 years. (EERE/PATH)

Conference of the Parties (COP): The supreme decision-making body comprised of the parties that have ratified the UN Framework Convention on Climate Change. It meets on an annual basis. More than 190 government delegations are expected to attend the 14th COP in Poland in December 2008.

Convective Airflow: Air movement where less dense (warmer) air is displaced by more dense (cooler) air. Often expressed by the phrase “hot air rises.” Convective airflow can be useful if controlled, as in gravity hot-air heating systems, but is more often a contributor to heat loss. (WAPTAC)

Conventional Computer: A computer that has not received the ENERGY STAR rating.

Conventional Thermostat: A thermostat that does not have an ENERGY STAR rating and may have a mercury switch.

CRT: Cathode Ray Tube. A sealed tube in which electrons are emitted from a cathode to a phosphorescent screen to produce an image. Until several years ago, CRT displays were the most common type of displays used for televisions and desktop computer monitors.

Daily Peak: The greatest amount of electricity used during a certain period in a day, such as one hour, half hour, or quarter hour.

Decoupling: Separating a utility’s sales from its profits during rate adjustments, so the utility can still earn a profit when revenues go down as a result of energy efficiency programs. (ACI)

Discounting: The process that reduces future costs and benefits to reflect the time value of money and the common preference of consumption now rather than later.

Early Crediting: A provision that allows crediting of emission reductions achieved prior to the start of a legally imposed emission control period. These credits can then be used to assist in achieving compliance once a legally imposed system begins.

Ecosystem: A community of organisms and its physical environment.

Eco-Tourism: Responsible travel to natural areas that conserves the environment and improves the well-being of local people.

EER/Energy Efficiency Ratio: The measure of the instantaneous energy efficiency of room air conditioners; the cooling capacity in Btu/hr divided by the watts of power consumed at a specific outdoor temperature (usually 95 degrees Fahrenheit). While the SEER considers yearlong efficiency (kWh), EER is a measure of the maximum use at a given time (kW). (EERE/PATH)

Emissions: The release of substances (e.g., greenhouse gases) into the atmosphere.

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 Reduction Unit (ERU): Emissions reductions generated by projects in Annex B countries that can be used by another Annex B country to help meet its commitments under the Kyoto Protocol. Reductions must be additional to those that would otherwise occur.

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.

Energy: The ability to do work or the ability to move an object. Electrical energy is usually measured in kilowatt hours (kWh), while heat energy is usually measured in British thermal units (Btu).

Energy Conservation: To reduce or manage energy consumption in a cost-effective and energy-efficient manner. (PUC)

Energy Efficiency: Refers to activities that are aimed at reducing the energy used by substituting technically more advanced equipment, typically without affecting the services provided. Examples include high-efficiency appliances, efficient lighting programs, high-efficiency heating, ventilating and air conditioning (HVAC) systems or control modifications, efficient building design, advanced electric motor drive, and heat recovery systems.

Energy-Efficient Mortgages: A type of home mortgage that takes into account the energy savings of a home with cost-effective energy saving improvements that reduce energy costs, thereby providing the homeowner additional money for the mortgage payment. A borrower can qualify for a larger loan amount than otherwise would be possible. (EERE)

Energy Resources: The available supply and price of fossil and alternative resources will play a huge role in estimating how much a greenhouse gas constraint will cost. In the U.S. context, natural gas supply (and thus price) is particularly important, as it is expected to be a transition fuel to a lower carbon economy.

ENERGY STAR: A joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy that establishes voluntary, minimum energy efficiency standards for various products, and allows those products that meet the standards to bear the ENERGY STAR label.

Energy Star Qualified New Homes Program: A program of the U.S. Environmental Protection Agency (EPA) and Dept. of Energy (DOE) to distinguish homes that are more energy efficient (30% > than the norm) as required by code. An Energy Star qualified home means that the home builder has taken extraordinary steps to design and build a structure that will be among the most efficient new homes. The homes are inspected by home energy raters, who verify that the home meets the technical specifications and performance standards established for Energy Star qualification. (

Energy Star Qualified Products: Appliances and other products that meet strict energy efficiency guidelines set by the EPA and DOE. Products in more than 50 categories are eligible for the Energy Star label. They use less energy, save money, and help protect the environment and are identified with the Energy Star label.

ENERGY STAR Rated Programmable Thermostat: A thermostat generally having two different programming schedules (one for weekdays and a second for weekend programming), at least four possible programming events for each programming period (i.e., wake, leave, return, and sleep settings), a setback/setup period of at least eight hours in length, with schedules included in the default program.

Enhanced Greenhouse Effect: The increase in the natural greenhouse effect resulting from increases in atmospheric concentrations of GHGs due to emissions from human activities.

Entry Into Force: The point at which international climate change agreements become binding. The United Nations Framework Convention on Climate Change (UNFCCC) has entered into force. In order for the Kyoto Protocol to do so as well, 55 Parties to the Convention must ratify (approve, accept, or accede to) the Protocol, including Annex I Parties accounting for 55 percent of that group's carbon dioxide emissions in 1990. As of June 2003, 110 countries had ratified the Protocol, representing 43.9 percent of Annex I emissions.

Ethanol: A colorless liquid that burns to produce water and carbon dioxide. The vapor forms an explosive mixture with air and may be used as a fuel in internal combustion engines.

European Community: As a regional economic integration organization, the European Community can be and is a Party to the UNFCCC; however, it does not have a separate vote from its members (Austria, Belgium, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Luxemburg, the Netherlands, Portugal, Spain, Sweden, and the United Kingdom).

Evapotranspiration: The process by which water re-enters the atmosphere through evaporation from the ground and transpiration by plants.

Flat-Plate Solar Connector: A device designed to capture the sun’s energy and produce low-temperature heat energy. They are commonly used as collectors in solar heating systems.

Forest Carbon Sequestration: The uptake and storage of carbon by forests.

Fossil Fuels: Fuels (coal, oil, natural gas, etc.) that result from the compression of ancient plant and animal life formed over millions of years.

Gas-Condensing Hot Water Heater: Gas-condensing water heaters are is similar to conventional gas, tank hot water heaters with the exception that they are designed to condense the water vapor in the hot flue gas, thereby transferring the heat of condensation to the hot water, improving the efficiency of the heater, but resulting in a slightly acidic water drainage.

Gas to Liquids (GTL): A process that combines the carbon and hydrogen elements in natural gas molecules to make synthetic liquid petroleum products, such as diesel fuel.

General Circulation Model (GCM): A computer model of the basic dynamics and physics of the components of the global climate system (including the atmosphere and oceans) and their interactions which can be used to simulate climate variability and change.

Generating Capacity: The amount of electrical power a power plant can produce.

Generator: A device that turns mechanical energy into electrical energy. The mechanical energy is sometimes provided by an engine or turbine.

Geothermal Energy: The heat or energy that is produced by natural processes inside the earth. It can be taken from hot springs, reservoirs of hot water deep below the ground, or by breaking open the rock itself.

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.

Gray Water: Domestic wastewater composed of wash water from kitchen, bathroom, and laundry sinks, tubs, and washers.

Green Building: The practice of increasing the efficiency of buildings and their use of energy, water, and materials and reducing building impacts on human health and the environment through optimal siting, design, construction, operation, maintenance, and waste removal – the complete building life cycle. Energy Efficiency is the key element for green building.

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.

Green Electricity or Green Power: Electricity produced from sustainable sources (wind, solar) by methods that have minimal negative impact on the environment, including energy conservation. (GGGC)

Greenhouse Gas (GHG): Any gas that contributes to the "greenhouse effect."

Green Power Purchasing/ Aggregation Policies: Municipalities, state governments, businesses, and other nonresidential customers can play a critical role in supporting renewable energy technologies by buying electricity from renewable resources. At a local level, green power purchasing can mean buying green power for municipal facilities, streetlights, water pumping stations, and the like. A few states allow local governments to aggregate the electricity loads of the entire community to purchase green power and even to join with other communities to form an even larger green power purchasing block. This is often referred to as “Community Choice.” (DSIRE)

Green Tags (or Renewable Energy Certificates): Green tags are transferable rights for renewable energy. A renewable energy provider is issued one green tag for each 1,000 KWh of energy it produces. The energy is sold into the electrical grid, and the green tag or REC can be sold on the open market for additional profit. The credits are purchased by firms or individuals who either want or are required to generate a portion of their energy from renewable sources. Green Tags (or Renewable Energy Certificates): Green tags are transferable rights for renewable energy. A renewable energy provider is issued one green tag for each 1,000 KWh of energy it produces. The energy is sold into the electrical grid, and the green tag or REC can be sold on the open market for additional profit. The credits are purchased by firms or individuals who either want or are required to generate a portion of their energy from renewable sources. (Wikipedia)

Grid: A network for the transmission of electricity throughout the state, region, or nation. The term is also used to refer to the layout of an electric distribution system.

Ground-Level Ozone: In the troposphere, the air closest to the Earth's surface, ground-level or "bad" ozone is a pollutant that is a significant health risk, especially for children with asthma. It also damages crops, trees and other vegetation. It is a main ingredient of urban smog.

Group of 77 and China, or G77/China: An international organization established in 1964 by 77 developing countries; membership has now increased to 133 countries. The group acts as a major negotiating bloc on some issues including climate change.

Healthy Homes: Homes that are designed, built, or remodeled to create and maintain a healthy indoor environment. Healthy home building is an integral part of whole house and green building practices, with a focus on making the homes dry, clean, well ventilated, free of combustion by-products and toxic chemicals, pest-free, and comfortable.

Heating Oil (Oil): Also commonly known as oil heat or No. 2 heating oil, is a liquid petroleum product used in boilers or furnaces. Heating oil is usually delivered by truck to residential and commercial buildings.

Heat Pump Hot Water Heater: Heat pump water heater technology uses a vapor compression refrigeration system to transfer heat from the surrounding air to water stored in a tank. These compressors in these heaters are typically powered by electricity.

Heavy metals: Metallic elements with high atomic weights; (e.g. mercury, chromium, cadmium, arsenic, and lead); can damage living things at low concentrations and tend to accumulate in the food chain.

HGWP (High Global Warming Potential): Some industrially produced gases such as sulfur hexafluoride (SF6), perfluorocarbons (PFCs), and hydrofluorocarbons (HFCs) have extremely high GWPs. Emissions of these gases have a much greater effect on global warming than an equal emission (by weight) of the naturally occurring gases. Most of these gases have GWPs of 1,300 - 23,900 times that of CO2. These GWPs can be compared to the GWPs of CO2, CH4, and N2O which are presently estimated to be 1, 23 and 296, respectively.

High Efficiency Tank: A natural gas, tank hot water heater with an Energy Factor of at least 0.62. (The Energy Factor is a measure of the heat energy in the hot water relative to the energy input to the water heater; a higher number indicates greater efficiency.)

High-Performing Tank: A natural gas, tank hot water heater with an Energy Factor of at least 0.67. (The Energy Factor is a measure of the heat energy in the hot water relative to the energy input to the water heater; a higher number indicates greater efficiency.)

Home Energy Audit: An assessment of a home or apartment to determine the energy efficiency of the home and its equipment. An audit should include an analysis of the utility bills, interview with the residents, inspection of the heating and cooling equipment, appliances, building structure, and insulation from basement to attic. It may involve the use of diagnostic equipment, such as a blower door, to identify leaks. The audit results in a list of recommended measures to improve the energy efficiency of the home.

Home Energy Rating Systems (HERS): A nationally recognized energy rating program that gives builders, mortgage lenders, secondary lending markets, homeowners, sellers, and buyers a precise evaluation of the energy features in homes. Builders can use this system to gauge the energy quality in their home and also to have a rating on this home on which prospective buyers and lenders can compare to other similarly built homes. (EERE)

Hot Air: A situation in which emissions (of a country, sector, company or facility) are well below a target due to the target being above emissions that materialized under the normal course of events (i.e. without deliberate emission reduction efforts). Hot air can result from over-optimistic projections of growth. Emissions are often projected to grow roughly in proportion to GDP, and GDP is often projected to grow at historic rates. If a recession occurs and fuel use declines, emissions may be well below targets since targets are generally set in relation to emission projections. If emission trading is allowed, an emitter could sell the difference between actual emissions and emission targets. Such emissions are considered hot air because they do not represent reductions from what would have occurred in the normal course of events.

Horsepower: A unit for measuring the rate of work (or power) equivalent to 33,000 foot-pounds per minute, or 746 watts.

House as a System (Whole House System): The approach to home design, building, remodeling, and weatherization that recognizes how all the features in a home are connected and that changing one component can greatly affect another part of the house. It is based on the principles of building science and relies on diagnostics to verify results. Using this approach will improve not only the overall energy efficiency of the home but also its comfort, indoor air quality, safety, durability, and affordability.

Hydroelectric Power Plant: A power plant that uses moving water to power a turbine generator to produce electricity.

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.

Hydrogen: A colorless, odorless, highly flammable gaseous element. It is the lightest of all gases and the most abundant element in the universe, occurring chiefly in combination with oxygen in water and also in acids, bases, alcohols, petroleum, and other hydrocarbons.

Hydrokinetic and Wave Energy Conversion Devices: Generate electricity from waves or directly from the flow of water in ocean currents, tides, or inland waterways.

Impact: An effect of climate change on the structure or function of a system.

Incandescent Bulb: A light bulb that produces light by passing electrical current through a thin filament, heating the filament and causing it to emit light. Conventional light bulbs with screw bases are incandescent bulbs.

Incentive-Based Regulation: A regulation that uses the economic behavior of firms and households to attain desired environmental goals. Incentive-based programs involve taxes on emissions or tradable emission permits. The primary strength of incentive-based regulation is the flexibility it provides the polluter to find the least costly way to reduce emissions.

Infrared Thermography: The science of using infrared imaging to detect radiant energy or heat loss in a building. The infrared camera or scanner electronically senses heat radiated by objects and converts that thermal energy into images visible to the human eye. Some scanners can automatically record these images on video. Used in conjunction with a blower door during home audits, the scanner can provide valuable data about air leakage sites and thermal bypasses. (WAPTAC)

Integrated Gasification Combined Cycle (IGCC): An alternative process for achieving efficiency improvements in conventional pulverized coal-fired power plants. IGCC plants use a gasifier to convert coal (or other carbon-based materials) to syngas, which drives a combined cycle turbine to produce electricity. (World Coal Institute)

Inter-City Bus: A bus used to travel from one city to another, generally tens to hundreds of miles away, with limited stops.

Inter-City Train: A train used to travel from one city to another, generally tens to hundreds of miles away, with limited stops.

Intergenerational Equity: The fairness of the distribution of the costs and benefits of a policy when costs and benefits are borne by different generations. In the case of a climate change policy the impacts of inaction in the present will be felt in future generations.

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 (UNFCC); primarily through the publication of periodic assessment reports (see "Second Assessment Report" and "Third Assessment Report").

Joint Implementation (JI): One of the three market mechanisms established by the Kyoto Protocol. Joint Implementation occurs when an Annex B country invests in an emissions reduction or sink enhancement project in another Annex B country to earn emission reduction units (ERUs).

Kilowatt (kW): A unit of power, usually used for electric power or for energy consumption (use). A kilowatt equals 1,000 watts.

Kilowatt-hour (kWh): A unit of electrical energy. One kilowatt hour is the amount of energy consumed by 1000 watts of electrical power in one hour. A 100 watt bulb operated for 100 hours will consume 10 kilowatt hours of electricity. Kilowatt-hour is the most common billing unit for residential electricity consumption.

Kyoto Mechanisms: The Kyoto Protocol creates three market-based mechanisms that have the potential to help countries reduce the cost of meeting their emissions reduction targets. These mechanisms are Joint Implementation (Article 6), the Clean Development Mechanisms (Article 17).

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.

Land Use, Land-Use Change and Forestry (LULUCF): Land uses and land-use changes can act either as sinks or as emission sources. It is estimated that approximately one-fifth of global emissions result from LULUCF activities. The Kyoto Protocol allows Parties to receive emissions credit for certain LULUCF activities that reduce net emissions.

Landfill Gas (LFG): The result of anaerobic bacteria that thrive in a landfills oxygen-free environment, resulting in the decomposition of the organic materials and the production of primarily carbon dioxide and methane.

Liquid Crystal Display (LCD): A display technology that uses liquid crystals to create images. LCD monitors are the most popular display technologies for desktop and laptop computers.

LEED-Leadership in Energy and Environmental Design: LEED-Leadership in Energy and Environmental Design: A rating system by the US Green Building Council (USGBC) that rates how “green” a building is in five categories: site design, energy-efficiency, resource-efficiency, water-efficiency, and health and indoor air quality. Previously only for commercial buildings, LEED is now expanding into homes (see below). (PATH)

LEED for Homes: A voluntary rating system of USGBC that promotes the design and construction of high-performance “green” homes. A green home uses less energy, water, and natural resources; creates less waste; and is healthier and more comfortable for the occupants. USGBC is now developing a LEED standard for existing homes. (USGBC)

Life Cycle Assessment (LCA): LCA is a technique to assess the environmental aspects and potential impacts associated with a product, process, or service, by: compiling an inventory of relevant energy and material inputs and environmental releases; evaluating the potential environmental impacts associated with identified inputs and releases; interpreting the results to help you make a more informed decision.

Life Cycle (Product): All stages of a product's development, from extraction of fuel for power to production, marketing, use, and disposal.

Liquefied Petroleum Gas (LPG): A group of hydrocarbon-based gases derived from crude oil refining or natural gas fractionation. They include ethane, ethylene, propane, propylene, normal butane, butylenes, isobutane, and isobutylene. For convenience of transportation, these gases are liquefied through pressurization.

Load: The power and energy requirements of users on the electric power system in a certain area or the amount of power delivered to a certain point.

Low-E Windows: Windows that are coated with a metallic glass (low emissivity) film to resist the flow of radiant heat. (EERE)

Low-Flow Faucet: A kitchen faucet meeting federal standards of 2.5 gallons per minute (gpm) maximum flow at 80 pounds per square inch water pressure or a bathroom faucet meeting the federals standard of 2-2.2 gallons per minute at 60 pounds per square inch of water pressure. Faucets with lower flow rates are available—2-2.2 gpm for kitchen faucets and as low as 0.5 gpm for bathroom faucets.

Low-Flow Showerhead: A showerhead meeting the federal standard of 2.5 gallons per minute (gpm) maximum flow at 80 pounds per square inch water pressure. Very high efficiency showerheads that operate at 1.5 gpm or less are currently available.

Manual J: The standard method for calculating residential cooling loads developed by the Air-Conditioning and Refrigeration Institute (ARI) and the Air-Conditioning Contractors of America (ACCA) based largely on the American Society of Heating, Refrigeration, and Air-Conditioning Engineer’s (ASHRAE) “Handbook of Fundamentals.” (WAPTAC)

Market Benefits: Benefits of a climate policy that can be measured in terms of avoided market impacts such as changes in resource productivity (e.g., lower agricultural yields, scarcer water resources) and damages to human-built environment (e.g., coastal flooding due to sea-level rise).

Mauna Loa Record: The record of measurement of atmospheric CO2 concentrations taken at Mauna Loa Observatory, Mauna Loa, Hawaii, since March 1958. This record shows the continuing increase in average annual atmospheric CO2 concentrations.

Megawatt (MW): A unit of electrical power equal to 1,000 kilowatts or 1,000,000 watts.

Methane (CH4): A colorless, nonpoisonous, flammable gas created by anaerobic decomposition of organic compounds. A major component of natural gas used in the home. 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.

Micro dams: A micro hydropower plant has a capacity of up to 100 kilowatts. A small or micro-hydroelectric power system can produce enough electricity for a home, farm, ranch, or village. Unlike large-scale hydro power, it does not attempt to interfere significantly with river flows.

Microwave Sounding Units (MSU): Sensors carried aboard Earth orbiting satellites that have been used since 1979 to monitor tropospheric temperatures.

Mitigation: Actions to reduce greenhouse gas emissions.

Montreal Protocol: (on Substances that Deplete the Ozone Layer) An international agreement that entered into force in January 1989 to phase out the use of ozone-depleting compounds such as methyl chloroform, carbon tetrachloride, and CFCs. CFCs are potent greenhouse gases which are not regulated by the Kyoto Protocol since they are covered by the Montreal Protocol.

Municipal Solid Waste (MSW): Common garbage or trash generated by industries, businesses, institutions, and homes.

National Action Plans: Plans submitted to the Conference of the Parties (COP) by all Parties outlining the steps that they have adopted to limit their anthropogenic GHG emissions. Countries must submit these plans as a condition of participating in the UN Framework Convention on Climate Change and, subsequently, must communicate their progress to the COP regularly.

Natural Gas: An odorless, colorless, tasteless, nontoxic, clean-burning fossil fuel. It is usually found in fossil fuel deposits and used as a fuel.

Negative Feedback: A process that results in a reduction in the response of a system to an external influence. For example, increased plant productivity in response to global warming would be a negative feedback on warming, because the additional growth would act as a sink CO2, reducing the atmospheric CO2 concentration.

Net Metering: For those consumers who have their own electricity-generating units, net metering allows for the flow of electricity both to and from the customer through a single, bi-directional meter. With net metering, during times when customer’s generation exceeds his or her use, electricity from the customer to the utility offsets electricity consumed at another time. (DSIRE)

Nitrogen Oxide (NOx): The result of photochemical reactions of nitric oxide in ambient air; major component of photochemical smog. Product of combustion from transportation and stationary sources and a major contributor to the formation of ozone in the troposphere and to acid deposition.

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.

Non-Market Benefits: Benefits of a climate policy that can be measured in terms of avoided non-market impacts such as human-health impacts (e.g., increased incidence of tropical diseases) and damages to ecosystems (e.g., loss of biodiversity).

Non-Party: A state that has not ratified the UNFCCC. Non-parties may attend talks as observers.

Nonrenewable Energy: Nonrenewable energy sources come out of the ground as liquids, gases and solids and cannot be replenished in a short period of time. Nonrenewable sources of energy can be categorized into fossil fuels (e.g. oil, natural gas, and coal) and nuclear (e.g. uranium). We get most of our supplied energy from nonrenewable sources. (EIA)

Off-Grid Home: A home (or building) that is not connected to the power grid or to municipal sewer or water lines. They must produce their own energy and find and dispose of their own water. (PATH)

Off-Peak/On-Peak: Blocks of time when energy demand and price is low (off-peak) or high (on-peak)

Passive Solar (Building) Design: A building design that uses structural elements of a building to heat and cool a building without the use of mechanical equipment. It requires careful consideration of the local climate and solar energy resource, building orientation, and landscape features. Heat is distributed primarily by natural convection and radiation, though fans can also be used to circulate room air or ensure proper ventilation. (EERE)

Particulate Matter: Also known as particle pollution or PM, is a complex mixture of extremely small particles and liquid droplets.

Peak Load Reduction: Reducing electric load during the times when electric demand is high, such as on a hot summer day. Load reduction can be accomplished by temporary demand response, load shifting, or through permanent demand reduction from energy-efficiency. (NYSERDA)

Perfluorocarbons (PFCs): PFCs are among the six types of greenhouse gases to be curbed under the Kyoto Protocol. PFCs are synthetic industrial gases generated as a by-product of aluminum smelting and uranium enrichment. They also are used as substitutes for CFCs in the manufacture of semiconductors. There are no natural sources of PFCs. PFCs have atmospheric lifetimes of thousands to tens of thousands of years and 100-year GWPs thousands of times that of CO2, depending on the gas.

Personal Income Tax Incentives: An incentive to encourage purchase of renewable energy or energy-efficient equipment. Some states offer personal income tax credits up to a certain percentage or predetermined dollar amount for the cost or installation or renewable energy equipment. (DSIRE)

Phantom Load: Any appliance that consumes power even when it is turned off. Examples of phantom loads include equipment chargers, appliances with electronic clocks or timers, appliances with remote controls, and appliance with wall cubes (a small box that plugs into an AC outlet to power appliance). Phantom loads can be a significant part (6%) of a household’s electric use. (EERE)

Polluter Pays Principle (PPP): The principle that countries should in some way compensate others for the effects of pollution that they (or their citizens) generate or have generated.

Positive Feedback: A process that results in an amplification of the response of a system to an external influence. For example, increased atmospheric water vapor in response to global warming would be a positive feedback on warming, because water vapor is a GHG.

ppm or ppb: Abbreviations for “parts per million” and “parts per billion,” respectively - the units in which concentrations of greenhouse gases are commonly presented. For example, since the pre-industrial era, atmospheric concentrations of carbon dioxide have increased from 270 ppm to 370 ppm.

Programmable Thermostat: A thermostat for space heating and cooling that allows the temperature to be set automatically to a value that results in less heating or cooling in periods when they are not needed.

Quantified Emission Limitation and Reduction QELRC: Also known as QELRO (Quantified Emission Limitation and Reduction Objective): The quantified commitments for GHG emissions listed in Annex B of the Kyoto Protocol. QELRCs are specified in percentages relative to 1990 emissions.

R-Value: A measure of a material’s resistance to heat flow in units of Fahrenheit degrees x hours x square feet per Btu. The higher the R-value of a material, the greater its insulating capability.

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 gases, 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).

Radiosondes: Sensors carried aboard weather balloons that have been in continuous use since 1979 for the monitoring of tropospheric temperatures.

Ratification: After signing the UNFCCC or the Kyoto Protocol, a country must ratify it, often with the approval of its parliament or other legislature. In the case of the Kyoto Protocol, a Party must deposit its instrument of ratification with the UN Secretary General in New York.

Reforestation: Replanting of forests on lands that have recently been harvested.

Regional Groups: The five regional groups meet privately to discuss issues and nominate bureau members and other officials. They are Africa, Asia, Central and Eastern Europe (CEE), Latin America and the Caribbean (GRULAC), and the Western Europe and Others Group (WEOG).

Renewable Energy: Renewable energy sources can be easily made or “renewed”. We can never use up renewable fuels. Types of renewable energy sources include solar, wind, geothermal, and biomass.

Repurposing: To convert or use an object in a way that was not originally intended.

Resilience: The ability of a system to withstand negative impacts without losing its basic functions.

Revenue Recycling: If permits are auctioned, this gives considerable sums of money to be recycled back into the economy, either through a lump sum payment of offsetting other taxes. If the existing taxes that are correspondingly reduced were very inefficient, this allows this allows the possibility of both environmental and economic benefits from the trading system, commonly called the 'double dividend.'

Second Assessment Report (SAR): The Second Assessment Report, prepared by the Intergovernmental Panel on Climate Change, reviewed the existing scientific literature on climate change. Finalized in 1995, it is comprised of three volumes: Science; Impacts, Adaptations and Mitigation; and Economic and Social Dimensions of Climate Change.

Secretariat of the UN Framework Convention: The United Nations staff assigned the responsibility of conducting the affairs of the UNFCCC. In 1996 the Secretariat moved from Geneva, Switzerland, to Bonn, Germany.

Sequestration: Opportunities to remove atmospheric CO2, either through biological processes (e.g. plants and trees), or geological processes through storage of CO2 in underground reservoirs.

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

Solar Cell: An electric cell that changes radiant energy from the sun into electrical energy by the photovoltaic process.

Solar Energy: The radiant energy of the sun, which can be converted into other forms of energy, such as heat or electricity.

Solar Hot Water Heater: A water heater technology that uses the sun’s thermal energy to heat water.

Source: Any process or activity that results in the net release of greenhouse gases, aerosols, or precursors of greenhouse gases into the atmosphere.

Spot Market: This is a term used in the wholesale purchase of electricity by suppliers within the regional transmission organization.

SRES Scenarios: A suite of emissions scenarios developed by the Intergovernmental Panel on Climate Change in its Special Report on Emissions Scenarios (SRES). These scenarios were developed to explore a range of potential future greenhouse gas emissions pathways over the 21st century and their subsequent implications for global climate change.

Standard Tank Hot Water Heater: A hot water heater that heats hot water and stores it in a tank until it is needed.

Stratosphere: The region of the Earth's atmosphere 10-50 km above the surface of the planet.

Subsidiary Body for Implementation (SBI): A permanent body established by the UNFCCC that makes recommendations to the COP on policy and implementation issues. It is open to participation by all Parties and is composed of government representatives.

Subsidiary Body for Scientific & Tech. Advice: (SBSTA) A permanent body established by the UNFCCC that serves as a link between expert information sources such as the IPCC and the COP.

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 Dioxide (SO2): A pungent, colorless, gasformed primarily by the combustion of fossil fuels; becomes a pollutant when present in large amounts.

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.

Supplementarity: The Protocol does not allow Annex I parties to meet their emission targets entirely through use of emissions trading and the other Kyoto Mechanisms; use of the mechanisms must be supplemental to domestic actions to limit or reduce their emissions.

Sustainable Energy Systems: Sustainable (or renewable) energy systems are those that provide energy services to people without depleting resources, endangering the planet or compromising the ability of future generations to use the same energy services.

System: A population or ecosystem; or a grouping of natural resources, species, infrastructure, or other assets.

System Load Factor: The ratio of average load to peak load during a specific period of time, expressed as a percent. The load factor indicates to what degree energy has been consumed compared to maximum demand or the utilization of units relative to total system capability. An electric system's load factor shows the variability in all customers' demands.

Tankless Hot Water Heater: A hot water heater that heats water only when there is a demand for hot water and does not store hot water in a tank.

Targets and Timetables: Targets refer to the emission levels or emission rates set as goals for countries, sectors, companies, or facilities. When these goals are to be reached by specified years, the years at which goals are to be met are referred to as the timetables. In the Kyoto Protocol, a target is the percent reduction from the 1990 emissions baseline that the country has agreed to. On average, developed countries agreed to reduce emissions by 5.2% below 1990 emissions during the period 2008-2012, the first commitment period.

Technological Change: How much technological change will be additionally induced by climate policies is a crucial, but not well quantified, factor in assessing the costs of long-term mitigation of greenhouse gas emissions.

Therm: A unit of heat energy equivalent to 100,000 Btus. This is the most common unit for billing residential gas consumption in the U.S. One decatherm (or dekatherm) equals 10 therms or 1 million Btus.

Thermal Bypass: Similar to a convection loop, this structural heat loss is characterized by heated air traveling up exterior or interior stud cavities and leaking out the top of that cavity to the attic through joints and cracks in the framing, wiring, and plumbing holes, etc. These types of heat loss sources are sometimes the most difficult to locate. (WAPTAC)

Thermal Expansion: Expansion of a substance as a result of the addition of heat. In the context of climate change, thermal expansion of the world's oceans in response to global warming is considered the predominant driver of current and future sea-level rise.

Thermohaline Circulation (THC): A three-dimensional pattern of ocean circulation driven by wind, heat and salinity that is an important component of the ocean-atmosphere climate system. In the Atlantic, winds transport warm tropical surface water northward where it cools, becomes more dense, and sinks into the deep ocean, at which point it reverses direction and migrates back to the tropics, where it eventually warms and returns to the surface. This cycle or "conveyor belt" is a major mechanism for the global transport of heat, and thus has an important influence on the climate. Global warming is projected to increase sea-surface temperatures, which may slow the THC by reducing the sinking of cold water in the North Atlantic. In addition, ocean salinity also influences water density, and thus decreases in sea-surface salinity from the melting of ice caps and glaciers may also slow the THC.

Third Assessment Report (TAR): The most recent Assessment Report prepared by the Intergovernmental Panel on Climate Change, which reviewed the existing scientific literature on climate change, including new information acquired since the completion of the Second Assessment report (SAR). Finalized in 2001, it is comprised of three volumes: Science; Impacts and Adaptation; and Mitigation.

Trace Gas: A term used to refer to gases found in the Earth’s atmosphere other than nitrogen, oxygen, argon and water vapor. When this terminology is used, carbon dioxide, methane, and nitrous oxide are classified as trace gases. Although trace gases taken together make up less than one percent of the atmosphere, carbon dioxide, methane and nitrous oxide are important in the climate system. Water vapor also plays an important role in the climate system; its concentrations in the lower atmosphere vary considerably from essentially zero in cold dry air masses to perhaps 4 percent by volume in humid tropical air masses.

Troposphere: The region of the Earth's atmosphere 0-10 km above the planet's surface.

Turbine: A device with blades, which is turned by a force, e.g., that of wind, water, or high-pressure steam. The mechanical energy of the spinning turbine is converted into electricity by a generator.

U-Value (Coefficient of Heat Transmission): The reciprocal of R-Value. The lower the number, the greater the heat transfer resistance (insulating) characteristics of the material. (EERE)

Umbrella Group: Negotiating group within the UNFCCC process comprising the United States, Canada, Japan, Australia, New Zealand, Norway, Iceland, Russia, and Ukraine.

UN 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.

Uncertainty: Uncertainty is a prominent feature of the benefits and costs of climate change. Decision makers need to compare risk of premature or unnecessary actions with risk of failing to take actions that subsequently prove to be warranted. This is complicated by potential irreversibilities in climate impacts and long term investments.

Uranium: A heavy, naturally occurring radioactive element.

Uranium Fuel Cycle: The series of steps involved in supplying fuel for nuclear power reactors. It includes mining, refining, the making of fuel elements, their use in a reactor, chemical processing to recover spent (used) fuel, re-enrichment of the fuel material, and remaking into new fuel elements.

Urban Heat Island (UHI): Refers to the tendency for urban areas to have warmer air temperatures than the surrounding rural landscape, due to the low albedo of streets, sidewalks, parking lots, and buildings. These surfaces absorb solar radiation during the day and release it at night, resulting in higher night temperatures.

Vector-borne Disease: Disease that results from an infection transmitted to humans and other animals by blood-feeding anthropods, such as mosquitoes, ticks, and fleas. Examples of vector-borne diseases include Dengue fever, viral encephalitis, Lyme disease, and malaria.

Volatile Organic Compounds (VOCs): Any organic compound that participates in atmospheric photochemical reactions except those designated by EPA as having negligible photochemical reactivity.

Vulnerability: The potential for a system to be harmed by climate change, considering the impacts of climate change on the system as well as its capacity to adapt.

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").

Watt: A metric unit of power, usually used in electric measurements, which work is done or energy is used.

Weather: Describes the short-term (i.e., hourly and daily) state of the atmosphere. Weather is not the same as climate.

White Tags: White tags are similar to Green Tags except they represent one MWh of electricity savings due to the use energy conservation methods and equipment. White Tags are determined through precise calculations of energy savings derived from conservation measures, such as the use of more efficient lighting, heating, and cooling.

Yellowcake: A natural uranium concentrate that takes its name from its color and texture. Yellowcake typically contains 70% to 90% U3O8 (uranium oxide) by weight. It is used as feedstock for uranium fuel enrichment and fuel pellet fabrication.

Xeriphytic: Plants that are adapted to withstand dry conditions.

Xeriscaping: Xeriscaping refers to landscaping in ways that do not require supplemental irrigation. This practice employs the use of xeriphytic – or drought-resistant – plant species and is promoted in areas that do not have easily accessible supplies of fresh water.

Zero Energy House: A house that produces as much energy as it uses, resulting in a yearly energy bill of $0. Building America has a goal that all new homes will have zero net energy use by 2020. The key is to minimize the energy required to operate the home and then add a power sources like photovoltaics. (PATH)