Life-Cycle Assessment (LCA) — Definition and GHG Accounting Context
Ask whether a paper cup is greener than a plastic one, or a timber frame than a steel one, and the honest answer is: it depends on where you look. Count only the factory and you get one verdict; count the forest, the transport, the years of use, and the landfill, and you can get the opposite. To answer such questions rigorously rather than rhetorically, you need a method that follows a product from the ground it came from to the grave it ends in.
That method is life-cycle assessment. Life-cycle assessment (LCA) is a standardised, ISO-governed method for quantifying the environmental impacts of a product or system across its whole life cycle — the backbone behind carbon footprints, EPDs, and embodied carbon.
Life-cycle assessment (LCA) is a method for quantifying the environmental impacts of a product, service, or system across its entire life cycle — raw materials, manufacturing, use, and end of life. Governed by ISO 14040 and 14044, it runs in four phases (goal & scope, inventory, impact assessment, interpretation) and assesses many impacts, of which the carbon footprint is one. It is the method behind embodied carbon, EPDs, and product CO₂e footprints.
Definition — Cradle to Grave, Quantified
Life-cycle assessment (LCA) is a standardised method for compiling and evaluating the environmental impacts of a product, service, or system across its entire life cycle — from the extraction of raw materials (“cradle”), through processing, manufacturing, distribution, and use, to disposal or recovery (“grave”). It is defined by the international standards ISO 14040 (principles and framework) and ISO 14044 (requirements and guidelines), which make it a repeatable, auditable procedure rather than an ad-hoc estimate. It is sometimes called life-cycle analysis, but ISO uses “assessment”.
A crucial and often-missed point: LCA is multi-impact. It assesses a range of environmental categories — climate change, acidification, eutrophication, water use, resource depletion, and more — not carbon alone. The carbon footprint is the single most-used LCA output, but it is one impact category (global warming potential) drawn from a wider assessment. Reducing one impact can worsen another, and only a full LCA reveals such trade-offs.
LCA matters because it is the scientific method underneath most product-level carbon numbers you will meet. Embodied carbon, Environmental Product Declarations, and product carbon footprints are all LCA applied and communicated in a particular way. Understanding LCA is therefore understanding where those numbers come from — and why they can legitimately differ.
Definition at a glance
| What it is | A standardised method to quantify a product’s environmental impacts across its whole life cycle |
|---|---|
| Governed by | ISO 14040 (framework) and ISO 14044 (requirements) |
| Scope of impacts | Multi-impact — climate, acidification, eutrophication, water, resources — not carbon alone |
| Four phases | Goal & scope · inventory (LCI) · impact assessment (LCIA) · interpretation |
| Two key choices | The functional unit and the system boundary — both shape the result |
| It underpins | Carbon footprints, EPDs, embodied carbon, product carbon footprints |
The Four Phases (ISO 14040/44)
ISO 14040/44 structures every LCA into four phases. They are iterative rather than strictly sequential — interpretation routinely sends the analyst back to refine the scope or gather better data.
| Phase | What happens |
|---|---|
| 1. Goal & scope definition | State why the LCA is done, the system studied, the functional unit, and the system boundary. |
| 2. Life-cycle inventory (LCI) | Compile every input and output — energy, materials, emissions, waste — across all life-cycle stages, usually with database and primary data. |
| 3. Life-cycle impact assessment (LCIA) | Translate the inventory flows into impact categories using characterisation factors — e.g. multiplying each gas by its GWP to get a climate-change result in kg CO₂e. |
| 4. Interpretation | Analyse the results, test their sensitivity and consistency, identify significant contributors, and draw conclusions within the stated scope. |
System Boundary: Where You Draw the Line
The single choice that most shapes an LCA result is the system boundary — which life-cycle stages are counted. The same material yields different numbers depending on where the boundary is set, so a fair comparison requires the same boundary on both sides.
| Boundary | Covers |
|---|---|
| Cradle-to-gate | Raw materials to the factory gate — production only (EN 15978 modules A1–A3) |
| Cradle-to-grave | Production through use to end-of-life disposal — the full life cycle |
| Cradle-to-cradle | Full life cycle where end-of-life feeds a new life cycle (closed-loop recycling) |
| Gate-to-gate | A single process step only — one factory’s contribution |
A concrete illustration with live data: the cradle-to-gate carbon result for galvanised steel is 2.48 kg CO₂e per kg. Extend the boundary to cradle-to-grave and end-of-life stages add a little more; extend it further and the recycling recovery — a separate figure of -1.11 kg CO₂e per kg (module D, a credit beyond the system boundary) — appears as its own line, never blended into the total. Move the boundary and the headline number moves with it, which is exactly why LCA insists the boundary be declared up front.
The Functional Unit
An LCA compares products by the function they deliver, not by their mass — and the functional unit is the precise, quantified statement of that function. “One litre of beverage packaged and delivered”, “one square metre of external wall with a 60-year service life”, “1,000 hours of light at a given brightness” are functional units. Everything in the assessment is scaled to it.
This is what makes comparisons fair. Comparing a kilogram of aluminium with a kilogram of steel is meaningless if a component needs three times as much steel to do the same job; comparing them per functional unit — per component that performs the required function for the required time — is meaningful. Get the functional unit wrong, or compare two studies that used different ones, and the conclusion is worthless no matter how precise the arithmetic.
Attributional vs Consequential LCA
LCA can answer two different questions, and the modelling choice changes the result. It is important to know which one a study did.
| Attributional | Consequential | |
|---|---|---|
| Question | What impacts are attributable to this product as it exists? | What impacts would a decision or change actually cause? |
| Data | Average data — a snapshot of the existing system | Marginal data — the effect at the margin, including market responses |
| Typical use | Carbon footprints, EPDs, corporate reporting | Policy and decision analysis (e.g. biofuel mandates) |
Most carbon footprints and EPDs are attributional — they describe a product as it is, using average data. Consequential LCA asks the harder “what if we did more of this?” question and brings in market-mediated effects such as indirect land-use change. Neither is wrong; they answer different questions, and mixing them up leads to false comparisons.
What LCA Produces and Where It’s Used
LCA is rarely the end product — it is the engine that feeds more familiar outputs. The same underlying assessment, communicated under different rules, becomes:
- Carbon footprint / product carbon footprint — the climate-change impact category alone, per ISO 14067.
- Environmental Product Declaration (EPD) — a standardised, third-party-verified LCA report, per ISO 14025 and, for construction, EN 15804.
- Embodied carbon — the material and construction stages of a building’s LCA, assembled at building level and assessed against EN 15978.
Underneath them all sit life-cycle inventory databases — such as ecoinvent and ÖKOBAUDAT — that supply the background data for the inventory phase. The whole-building LCA calculator is one applied example, assembling a building’s life-cycle carbon from material quantities and factors. For companies, LCA-derived product footprints are the basis of most Scope 3 reporting.
Common Confusions
- Treating LCA as a carbon-only tool. LCA is multi-impact; a carbon footprint is one impact category drawn from it. Optimising for carbon alone can worsen water, land, or resource impacts an LCA would reveal.
- Comparing across different boundaries. A cradle-to-gate figure and a cradle-to-grave figure are not comparable. Always match the system boundary — and the functional unit — on both sides.
- Ignoring the functional unit. Comparing per kilogram instead of per function is a classic error: the material that weighs less per job can lose on a per-kilogram basis and win per functional unit.
- Confusing attributional and consequential results. One describes a product as it is; the other estimates the effect of a decision. They answer different questions and should not be compared directly.
- Assuming one “true” LCA number. Results depend on data quality, boundary, functional unit, and method choices, so two defensible LCAs of the same product can differ. Transparency about assumptions matters more than a single figure.
Frequently Asked Questions
Life-cycle assessment is a standardised method for quantifying the environmental impacts of a product, service, or system across its entire life cycle — from raw-material extraction, through manufacturing, distribution, and use, to disposal or recovery. It is governed by the international standards ISO 14040 and ISO 14044, which make it a repeatable, auditable procedure. LCA assesses many environmental impact categories — climate change, acidification, eutrophication, water use, resource depletion — not carbon alone, though the carbon footprint is its most-used single output. It is the scientific method behind embodied carbon, Environmental Product Declarations, and product carbon footprints.
ISO 14040/44 defines four phases. Goal and scope definition sets out why the study is done, the system studied, the functional unit, and the system boundary. Life-cycle inventory (LCI) compiles every input and output — energy, materials, emissions, waste — across all life-cycle stages. Life-cycle impact assessment (LCIA) translates those inventory flows into impact categories using characterisation factors, for example multiplying each greenhouse gas by its global warming potential to give a climate result in kg CO₂e. Interpretation analyses the results, tests their sensitivity and consistency, and draws conclusions. The phases are iterative, not strictly linear — interpretation often sends the analyst back to refine the scope or the data.
No — a carbon footprint is one output of an LCA, not the whole thing. LCA is multi-impact: it assesses climate change alongside acidification, eutrophication, water use, resource depletion, and other categories. A carbon footprint is just the climate-change impact category — the global-warming-potential result — extracted and reported on its own, typically under ISO 14067. Because it is the most policy-relevant and easily communicated impact, carbon often stands in for the whole assessment, but focusing on it alone can hide trade-offs: a change that cuts carbon might increase water use or resource depletion, which only the full LCA would show.
The functional unit is the precise, quantified statement of the function a product delivers, to which all the impacts in an LCA are scaled — for example “one square metre of external wall with a 60-year service life” or “one litre of beverage packaged and delivered”. It exists to make comparisons fair. Comparing a kilogram of one material with a kilogram of another is meaningless if they perform different amounts of work; comparing them per functional unit — per unit of the job actually done, for the time required — is meaningful. Two LCAs can only be compared if they use the same functional unit and the same system boundary.
They answer different questions. Attributional LCA asks what environmental impacts are attributable to a product as it currently exists, using average data — a snapshot of the system. Most carbon footprints, EPDs, and corporate reports are attributional. Consequential LCA asks what impacts a decision or change would actually cause, using marginal data and including market-mediated effects such as indirect land-use change. It is used mainly for policy and decision analysis. Neither is more correct — they answer different questions — but their results are not directly comparable, and a study should state which approach it used.