Emissions reductions are "additional" if they occurred because of the presence of incentives associated with the existence of GHG markets, voluntary or mandatory. A variety of stakeholders have proposed many different additionality "test," but at its root, demonstrating the additionality of a carbon offset means showing that the emissions reductions being unsed as offsets are not "business as usual."

Carbon Footprint

The estimated emissions of carbon dioxide (CO2) and other greenhouse gases (GHGs) associated with a particular activity, use of your car, your family's overall lifestyle, or use of a particular product or service.  The scope of carbon footprint analyses can vary, and may or may not include all GHGs or reflect a "life cycle" approach to quantifying "upstream" and "downstream" GHG emissions.  When it includes all GHGs, the footprint is commonly expressed in "CO2 equivalent" (CO2e) units.  The personal carbon footprint of a typical individual in the United States is approximately 10 tons of CO2e per year, reflecting emissions from the activities listed above that are under a person's direct control.

Carbon Neutral

The idea of helping consumers, organizations, and businesses neutralize their personal or corporate GHG emissions (i.e. their "carbon footprint") by offsetting all or some of the emissions associated with their lives and activities.

Carbon Offset
The act of reducing or avoiding GHG emissions in one place in order to "offset" GHG emissions occurring somewhere else.  Unlike most conventional pollutants, GHGs mix well in the atmosphere and can travel around the planet quickly.  As a result, it doesn't really matter from the standpoint of global warming mitigation where a reduction takes place.  Carbon offsets are intended to take advantage of the radically different costs and practicalities of achieving GHG emission reductions by sector and geography.

Certified Emissions Reductions (CER)

A tradable certificate reflecting the reduction or avoidance of one ton of CO2e. CERs are the currency used by the Clean Development Mechanism (CDM) under the Kyoto Protocol for GHG trading between developing countries (countries without emissions reduction targets) and industrialized countries (those with emissions reduction targets).

Chicago Climate Exchange (CCX)

The CCX operates a voluntary GHG cap-and-trade program in the US and has branched out into Europe and other countries. The US program is a pilot program to generate learning and test how a domestic GHG cap-and-trade system might function. CCX members contractually committ to GHG emissions reductions of a certain magnitude per year from their original baseline. Reductions beyond that level can be sold to other CCX members who need additional reductions. A small fraction of the CCX market consists of project-based reductions.

Clean Development Mechanism (CDM)

An emissions trading mechanism under the Kyoto Protocol. It is intended to help Annex B Parties (industrialized countries) reduce the costs of meeting their emissions targets under the Protocol by achieving emissions reductions in other countries at lower costs than they could domestically. CDM emission reduction projects allow developing countries to advance sustainable development objectives by creating and selling CERs (see Certified Emissions Reductions).

Gold Standard

An offset standard based on the Kyoto Protocol's Clean Development Mechanism and developed by several international nonprofit organizations. Gold Standard certification is limited to small-scale renewable energy and energy efficiency projects that have received approval through the CDM process. A voluntary market version of the Gold Standard has recently been released that does not require CDM project approval; it, too, limits certification to renewable energy and energy efficiency projects. The voluntary standard seeks to apply a standard of review similar to the CDM, but notes that additionality testing procedures are relaxed.

Greenhouse Gas (GHG)

The primary gases (both naturally existing and man-made) that contribute to global warming by trapping more energy in the earth's atmosphere than would occur in their absence.  Greenhouse gases covered by the Kyoto Protocol are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), sulfur hexafluoride (SF6), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs).  Chlorofluorocarbons are also powerful GHGs, but are regulated separately as a means of addressing stratospheric ozone depletion.  Water vapor is also a GHG that responds automatically to changes in temperature and other conditions, but it cannot be directly influenced by human activities.  It is therefore not generally considered a greenhouse gas for global warming mitigation purposes.

Kyoto Protocol

An internationally binding agreement that falls under the more general United Nations Framework Convention on Climate Change (UNFCCC).  The Protocol sets GHG targets for countries that sign and ratify the agreement.  The United States and Australia are among the few countries that ratified the UNFCCC but did not ratify the Protocol and thus are not subject to its GHG reduction targets.  A corollary to this is that emissions reduction projects in the United States cannot be used for compliance with the Kyoto Protocol.

Renewable Energy Certificate (REC)

A certificate that represents the environmental attributes of 1 MWh of electricity from a renewable energy source. RECs can be used to satisfy regulatory mandates or to supply voluntary green energy markets.

Voluntary Carbon Standard

A new standard proposed by the Climate Group and the International Emissions Trading Association for carbon offsets bought and sold in the voluntary market. As of December 2006, it is still under development and has been submitted to the public for comment.

White Tag

A certificate equivalent to 1 MWh of energy savings. White tags have the goal of commoditizing energy efficiency, much as renewable energy certificates have commoditized renewable energy generation.