A simple way of explaining how Overseer models GHG emissions is available in the Methane Technical Manual Chapter, Figure 1:
The emissions from the component parts of the farm system are calculated, in kg of the specific gas.
At the point where the arrows then come together, and before the results are split out into emissions per ha or emissions per kg of product, the AR4 conversion factors are used to convert the kg/tonnes of constituent gas into kg/tCO2e, using GWP100.
Currently, all farm reporting is done in either kg/tonnes of gas or using GWP100. Should that situation change, Overseer Limited will update our reporting.
The AR4 values for GWP100 are below, and come from the Working Group of IPCC, published in 2007:
Co2 = 1
CH4 = 25
N2O = 298
Overseer Limited are currently working with the National Inventory team to understand the implications of moving Overseer to the AR5 or AR6 values, which have been updated since the release of AR4 in 2007 and its ratification in 2010: