Impact assessment

Each substance, released in or captured from the biosphere, is characterized by various impacts on the environment. For instance, one substance may increase the greenhouse gas effect of the atmosphere, or reduce the amount of drinkable water. In general, a substance is associated with various indicators and characterization factors. An indicator qualifies an impact category. A characterization factor quantifies the impact of a substance with respect to a given indicator.

Let us now illustrate how to express these concepts in our language.

Substance

We declare another type of top-level block using the substance keyword. The definition of a substance requires specific information. Let us start with a minimal example:

substance wood_smoke {
    name = "Woodfire Smoke"
    type = Emission
    compartment = "air"
    // optional: sub_compartment = "low altitude"
    reference_unit = m3
}

The required information is:

• The name of the substance, a human-readable text put between quotes. This has meaning for the humans who will read the model: you can put anything in it, the computer won't care.

• The type of the substance: as explained in the previous section, this can be an Emission, a Resource or a Land_use. You cannot use a substance of type Emission in a block Resource or Land_use, and vice versa.

• The compartment and, optionally, sub_compartment qualify the location the substance comes from or goes to. The characterization factors of a substance depend on this location: 1 kg of CO2 released in the atmosphere does not have the same global warming potential as 1 kg of CO2 captured in the ground.

• Finally, the reference_unit specifies the unit by which a quantity of this substance is measured. Again, this is because, the characterization factors of a substance implicitly refer to a nominal quantity of this substance: 1 kg of CO2 does not have, numerically, the same global warming potential as 1 liter of CO2. Moreover, our tool notifies you if you use an incompatible unit when referring to that substance.

Impacts

The above information is not very useful as is: when evaluating our system, we get the same result as before (don't believe the author, check it!). We need to characterize that substance. We do so by declaring a block impacts:

substance wood_smoke {
    name = "Woodfire smoke"
    type = Emission
    compartment = "air"
    reference_unit = m3

    // We use the special unit "u" for an arbitrary unit of something
    impacts {
        1 u climate_change     // contains co_2, which has a greenhouse effect
        0.5 u human_health     // contains co, which is dangerous
        0.5 u ecosystem_health // co is dangerous to everyone
    }
}

For the sake of simplicity, we have used the unit u, which represents a dimensionless quantity. Of course, real-world indicators have specific units (kgCO2eq, CTUh, etc.). We will see in later sections how to define and use such units.

Putting all the pieces together

By now, you should have the following code.

substance wood_smoke {
    name = "Woodfire smoke"
    type = Emission
    compartment = "air"
    reference_unit = m3

    // We use the special unit "u" for an arbitrary unit of something
    impacts {
        1 u climate_change     // contains co_2, which has a greenhouse effect
        0.5 u human_health     // contains co, which is dangerous
        0.5 u ecosystem_health // co is dangerous to everyone
    }
}

process bake {
    products {
        1 kg bread
    }
    inputs {
        1 kg flour
    }
    emissions {
        1 m3 wood_smoke (compartment="air")
    }
    resources {
        1 kg wood
    }
}

process mill {
    products {
        1 kg flour
    }
    inputs {
        2 kg wheat
    }
}

And if we assess the process bake again, using the green "Assess" button, we obtain:

What happened? The assessment engine notices the presence of a block substance characterizing wood_smoke and automatically connects the pieces together. Informally,

• Producing 1 kg of bread using the process bake requires 2 kg of wheat, 1 kg of wood from the biosphere, and releases 1 cubic meter of wood smoke
• But 1 cubic meter of wood smoke is characterized by 1 unit impact on climate change, 0.5 unit impact on human and ecosystem health.
• Therefore, producing 1 kg of bread induces a 1 unit impact on climate change, and 0.5 unit impact on human and ecosystem health.

You may have guessed the pattern now. For any target process, the assessment engine quantifies, as deeply as possible, the required products, substances or impact indicators. From now on, composing processes and defining substances as you go, you should know enough to model your first real-world model!