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The quantity of greenhouse gas (GHG) emissions that have been reduced or removed as the result of an offset or transfer project. In quantitative terms, additionality is the difference between the baseline emissions (emissions that have not been subjected to any intentional reductions) and the volume of emissions released after the implementation of an offset project.


The planting of new forests on lands that historically have not contained forests.

Alternative Energy

Energy derived from nontraditional sources (e.g., compressed natural gas, solar, hydroelectric, wind).


Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.


The multilayer gaseous envelope surrounding the Earth. The dry atmosphere consists almost entirely of nitrogen (78.1% volume mixing ratio) and oxygen (20.9% volume mixing ratio), together with a number of trace gases, such as argon (0.93% volume mixing ratio), helium, radiatively active greenhouse gases such as carbon dioxide (0.035% volume mixing ratio), and ozone. In addition, the atmosphere contains water vapor, the amount of which is highly variable, but typically 1% volume mixing ratio; clouds; and aerosols.

Atmospheric Lifetime

The average time that a molecule resides in the atmosphere before it is removed by chemical reaction or deposition. This can also be thought of as the time that it takes after a human-caused emission of a gas for the concentrations of that gas in the atmosphere to return to natural levels. Greenhouse gas lifetimes can range from a few years to a few thousand years.