Level 2 or level 3 calculations can be certified to PAS 2050 or ISO 14067, once it is available.
There are two established methods for calculating product carbon footprints. One method is to use product level input output models and combine this with an organisation input output model to calculate overheads.
The other method is process based and to measure all the elements that make up the carbon footprint. This has issues around where boundaries are defined, and can be very expensive to calculate.
PAS 2050 was created in October 2008 by BSI and is the first standard for calculating product carbon footprints it uses a process based method and has proved expensive to implement. There was a significant debate when PAS 2050 was created about using process or input output models or a hybrid. The PAS 2050 committee went to a pure process method.
ISO 14067 is based on PAS 2050, and due to be released in March 2011.