BackPage Primer from Environmental Building News
January 1, 2009
Weighting Environmental Impact Categories
Environmental life-cycle assessments (LCAs) score products or design options across a series of impact categories. Those categories typically include things like climate change, ozone depletion, resource depletion, habitat destruction, and human health. It’s rare that a product or option scores better than others in all of the categories, so making decisions based on those LCA results involves tradeoffs.
To make those tradeoffs, we have to decide how important each impact category is compared to the others. Sometimes we do this implicitly as we make choices. To communicate about our decisions, however, it helps to express our priorities through weighting factors. These factors can be used to combine multiple scores into one aggregate score, making comparisons easy. Getting to that simple comparison, however, is a process fraught with assumptions and simplifications.
When you have to assign weightings, you have several options. A common approach is to simply poll a group of people and ask them how important each category is. But when people answer that question, they usually have some arbitrary scale of impact in mind. It’s easy to say that avoiding climate change is more important than avoiding ozone depletion, for example, but are you willing to accept drastic ozone depletion to avoid a small amount of climate change? Probably not—but where do you draw that line?
Before you try to answer that question, it helps to establish a way of comparing the amount of impact you’re looking at in each category, which is tricky because they are not directly comparable. One way to put these apples and oranges onto the same scale is to use the average per-person impacts in each category. LCA practitioners sometimes use, for example, one person-year’s worth of climate change emissions to one person-year’s worth of ozone-depleting chemical emissions as the basis for comparisons.
Once you have a consistent unit of measure, you can weight the categories with a more specific and meaningful (if still hard-to-answer) question: which is worse, the climate change effect of one person-year’s worth of carbon dioxide equivalent emissions (24,000 tons), or the ozone depletion effect of one person-year’s worth of CFC-11 equivalent emissions (0.31 pounds)? And by how much?
In preparation for the release of version 4.0 of the Building for Environmental and Economic Sustainability (BEES) software tool, the LCA consulting firm Five Winds International worked with the National Institute for Standards and Technology (NIST) to poll a select group about environmental impacts and assign weightings to a set of environmental impact categories. Those weightings were subsequently adopted by the U.S. Green Building Council for use in distributing points among the credits in the LEED 2009 rating systems. USGBC intends to update these weightings for future updates to LEED.
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I think this is very interesting because on a smaller scale I do this every day in my own work. I do think that this system is fairly arbitrary. I would like to see a metric that has more variables and subsequently measures these components more accurately, especially elements such as geographical location.