Thinking about attributional v. consequential models

Tom Greenwood from Wholegrain Digital shared this analogy recently in an email thread. It's too good to lose, and so I'm putting it here for my future self to refer back to.

Attributional models are used for most carbon accounting purposes, such as vehicles, buildings etc. These tend to take wide system boundaries and attribute the full system energy to units of "usage". The simplest example is the emissions per passenger on a plane, is the total emissions of the plane divided by the average number of passengers. This is useful for carbon accounting, reporting and offsetting purposes, as well as raising awareness of the scale of general impacts.

Consequential models are used more for investigating more detailed behaviours within systems so that the system can be optimised. To take the plane example again, looking at the incremental emissions of a single passenger boarding a plane would reveal whether passenger weight is a key factor in emissions. This might lead to discussions around whether or not childrens plane tickets have lower emissions, or whether luggage allowances are an important variable. Basically, it facilitates more nuanced discussions to allow engineers to focus on the things that move the needle the most, but the actual emissions reported are generally far lower than attributional emissions and not really suited for broader reporting purposes.