Carbon dioxide equivalents (CO2 eq) provide a universal standard of measurement against which the impacts of releasing (or avoiding the release of) different greenhouse gases can be evaluated. Every greenhouse gas has a Global Warming Potential (GWP), a measurement of the impact that particular gas has on 'radiative forcing'; that is, the additional heat/energy which is retained in the Earth's ecosystem through the addition of this gas to the atmosphere.

The GWP of a given gas describes its effect on climate change relative to a similar amount of carbon dioxide and is divided into a three-part "time horizon" of twenty, one hundred, and five hundred years. As the base unit, carbon dioxide numeric is 1.0 across each time horizon. This allows the greenhouse gases regulated under the Kyoto Protocol to be converted to the common unit of CO2 eq.

Global Warming potentials for the greenhouse gases regulated under the Kyoto Protocol under a 100 year timeframe are as follows
(Source - US EPA):

Carbon dioxide has a GWP of 1
Methane has a GWP of 21
Nitrous oxide has a GWP of 310
Halocarbons (HFC) has a GWP of 140 to 11,700
Sulphur Hexafluoride has a GWP of 23,90

This means that in 100 years, one tonne of methane will have an effect on global warming that is 21 times greater than one tonne of carbon dioxide, and so forth. No single number can accurately represent the GWP of a given gas, as certain gases remain in the atmosphere much longer than others. The uncertainty ranges for the fluorocarbon derivative indicates the continued uncertainty regarding their long term decomposition in the atmosphere as these are fairly newly 'designed' gases. Short-lived gases are less harmful in the long-term than they are in the short-term, which means that the carbon dioxide equivalency of a given gas can vary dramatically over time.