Scenario analysis: how end-of-life assumptions decide the result

Calculating what happens to a construction material at end-of-life means assuming something about the waste management system fifty years from now. That assumption is inevitably wrong — the question is how wrong, and how much the result depends on it.

A scenario is not a forecast

Scenario analysis does not claim to predict the future. Its purpose is to show how the result changes across different plausible futures. A conclusion that points the same way in all three scenarios is robust; one that reverses between them tells you something far more valuable than a single number.

Building end-of-life scenarios

• Conservative — today's recycling rates, no infrastructure improvement
• Baseline — assuming current policy targets are met
• Optimistic — high separation, closed-loop recycling
• Give the recycling, energy recovery and landfill shares separately in each scenario
• Model degradation behaviour by material — methane from timber in landfill is critical

Under EN 15804 the scenario used for the C modules must be defined explicitly in the report. Without a statement such as 'ninety per cent recycling assumed', the figures in the C and D modules cannot be interpreted.

The future electricity mix

For long-lived products the second major scenario variable is the carbon intensity of use-stage electricity. Holding today's grid factor constant for fifty years systematically overstates operational carbon. In a scenario where the grid decarbonises, the relative weight of embodied carbon rises and material decisions become more critical.

How to report scenarios

The bad practice is running three scenarios and putting the most favourable one in the report. The right practice is showing all of them and marking one as the baseline with a justification. A reader must be able to see how much the conclusion depends on which assumptions.