Yellow, red, and brown energy: leveraging water footprinting concepts for decarbonizing energy systems

As the energy system changes, metrics used to describe energy use for modelling, socioenvironmental assessment, and other applications should be continually evaluated to ensure ongoing relevance and applicability. Decarbonization highlights the need for fit-for-purpose assessment tools as energy systems undergo an expected transition from mostly fossil to mostly nonfossil resources. Energy use has historically been a high-quality proxy for socioenvironmental impacts of interest, but this characteristic depends on the relatively stable historical relationship between energy use (typically measured as exchanges of marketed energy resources and carriers like natural gas and electricity) and these impacts—a relationship that is increasingly weak. Already, energy use metrics used in tools like energy footprinting and life cycle assessment have developed maladaptations to include nonfossil resources, including many flow resources. For example, nonmarketed energy use is typically ignored; metrics like heat rate are applied to nonthermal resources in ways with limited physical meaning; and definitional exceptions are made without clear justification. Part of the challenge is that energy is a conserved quantity with highly variable quality, but energy footprint metrics have historically implicitly assumed that all energy, and energy use, is the same. The assessment community can improve the clarity and value of energy use quantification under decarbonization by drawing on the experience of footprinting with another highly heterogeneous conserved resource: water. This discussion introduces the concept of a yellow, red, and brown energy footprint framework as an expansion of traditional energy footprinting and analogue of the green, blue, and grey water footprinting framework.