Plate Nº 78 · recorded November 11, 2026
Earth & ClimateReported finding
Four Methods Collapse Into Two: Study Simplifies Environmental Impact Analysis
PSI researchers proved four hybrid life cycle assessment methods reduce to two, clearing a path for open-source tools that measure product footprints without arbitrary boundaries.
By Elena Vasquez4 min read734 words
In brief
- A PSI study shows four established hybrid life cycle assessment methods can be reduced to two.
- The findings were published in two papers in the Journal of Industrial Ecology (2026).
- Hybrid methods date back to the 1970s; their differences had never been clearly defined.
- The EU's Carbon Border Adjustment Mechanism depends on objective footprint calculations.
- The researchers aim to build user-friendly software on the open-source tool Brightway.
Four established methods for measuring a product's full environmental footprint can be mathematically reduced to just two, according to research from the Paul Scherrer Institute (PSI) published in the Journal of Industrial Ecology.
The finding matters beyond academic tidiness. Life cycle assessment, or LCA, is the standard tool for quantifying how green a car, a wheel of cheese or a wind turbine really is, and it feeds directly into political decisions, including the European Union's new Carbon Border Adjustment Mechanism. Simplifying the underlying math is a first step toward open-source software that could let anyone run such analyses without deep mathematical training.
"Life cycle assessment is ultimately a mathematical method: it is only as good as the data it is based on," said Michael Weinold, who carried out the research at PSI's Laboratory for Energy Systems Analysis and now works at ETH Zurich.
Why is completeness so hard?
An LCA is supposed to trace a product's entire life: raw materials, manufacturing, energy supply, transportation and more. Take gasoline. Combustion matters, but so does where the fuel came from. "For crude oil, this chain usually ends where the oil is extracted," Weinold said. "Theoretically, however, research, administration and real estate projects of the oil companies could also be included."
Conventional assessments simply cannot capture every process. Someone has to decide where to draw the boundary, and that choice is subjective — even though the procedure itself is standardized by ISO. "This makes the life cycle assessment, partly, a subjective process," Weinold said.
The subjectivity has real costs. The EU's Carbon Border Adjustment Mechanism charges imported goods according to their carbon footprint, which will make importing Chinese steel more expensive. "The calculations must be as objective as possible — because ultimately, we all pay the costs as consumers," Weinold said.
How do hybrid methods fix it?
The fix is to combine two kinds of data. Detailed process data — milk volumes, electricity consumption in a dairy — gets merged with so-called input-output tables, which map how economic sectors connect worldwide. If you know how much the U.S. aviation industry spends on steel, and you know the CO₂ emissions of steel production, you can attribute a share of those emissions to aviation.
These combinations are called hybrid life cycle assessments (HLCAs), and because the economic tables are complete by definition, analysts no longer need to draw an arbitrary line. "There's no need to draw an arbitrary line anymore," Weinold said.
Since the 1970s, researchers have developed four scientifically recognized main methods plus several variants, with no clear definition of how they differ. Weinold and his co-author Guillaume Majeau-Bettez, a professor at the CIRAIG Life Cycle Assessment Research Center in Canada, analyzed them across two studies and showed all four reduce mathematically to two.
"That was no simple proof," Weinold recalls. The authors of one of the four methods were initially skeptical and requested additional calculations for special cases. In the end, the differently formulated methods proved mathematically equivalent.
Which method when?
The two survivors differ in complexity. The simpler "tiered HLCA" supplements classic process data with averages from economic tables — cheese-specific figures where available, dairy-sector averages where not. It suits products with small production volumes in the wider economy, such as Swiss watches.
The "integrated HLCA" lets data flow both ways: from the economic table into the process chain and back again, which requires reclassifying economic sectors. It's needed when a process matters to the whole economy. Weinold's example: a Swiss pilot project with 10 wind turbines barely dents the national energy system, so the tiered approach works. But a scenario where wind covers 20% of Swiss electricity involves everyone as consumers. "Then the integrated method is needed," he said.
The next step is practical: software for hybrid assessments usable without deep math skills, built on Brightway, PSI's open-source tool that has become one of the most widely used in the field.
The bottom line: messy, subjective footprint accounting just got a cleaner mathematical foundation — and simpler tools may follow.
The papers: Michael P. Weinold et al., Mathematical unification of methods for hybrid life cycle assessment, Journal of Industrial Ecology (2026), DOI: 10.1007/s44498-026-00187-w; and Critical analysis of the path-exchange method for hybrid life cycle assessment, Journal of Industrial Ecology (2026), DOI: 10.1007/s44498-026-00023-1.
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More from Elena Vasquez
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Correspondent covering money, funding and market shifts around general science news at SciBeat.
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