Greenhouse gases such as methane, nitrous oxide and carbon dioxide differ in how long they last in the atmosphere and how effective they are in absorbing heat radiation.
To allow emissions of those gases to be compared, emissions are often weighted according to the standards adopted by countries under the UNFCCC, so that emissions of all gases can be expressed in a single unit called “CO2-equivalent” emissions. For the purpose of national inventories, it is critical that national accounts are consistent with the reporting conventions adopted internationally.
The metric used for reporting under the UNFCCC is called the “Global Warming Potential” (GWP), which is based on the average warming effect of greenhouse gases over 100 years after their emission.
The UNFCCC has adopted the Global Warming Potential (GWP) as the universal metric for reporting GHG emissions, and for measuring the success of mitigation. The GWP allows us to compare emissions from different sectors, set overall targets, and inform emissions trading.
The GWP uses carbon dioxide (the most important greenhouse gas) as a reference, against which the warming effect of other gases can be compared. Emissions of each gas are calculated by multiplying its weight by its GWP, which is then reported as a 'carbon dioxide equivalent' emission (sometimes abbreviated as ‘CO2-eq’).
The GWP is based on the amount of carbon dioxide that would have produced the same cumulative warming effect (technically known as ‘radiative forcing’) over a given period as the gas being emitted. For instance, over the course of a century, emitting one tonne of methane is estimated, based on measurements and climate model calculations, to have an approximately 25 times greater warming effect than one tonne of carbon dioxide, while nitrous oxide has an effect 298 times greater. So methane's 100-year GWP is 25, while nitrous oxide's is 298; and the emission of one tonne of methane is reported as 25 tonnes of CO2-eq. 
Greenhouse gas metrics are being updated routinely based on new scientific knowledge and as greenhouse gas concentrations and the world’s climate continue to change.
Read more 

NZAGRC Factsheet: Economic and Policy Implications of Alternative GHG Metrics

 
Note
Our 2016 factsheets use the GWP weighting values that are being used to report greenhouse gas emissions to the UNFCCC fromthe year 2015 onwards.
Compared to values used in earlier emission inventories, this update has increased the weighting given to methane emissions (by 19%) and slightly decreased the weighting of nitrous oxide emissions (by 4%) relative to emissions of carbon dioxide.
These changes do not alter the amount of gases being emitted, but they do affect the reported CO2-equivalent emissions from agriculture in the national inventory.