Consequential vs Attributional LCA

One of the most important methodological choices in LCA is whether to use an attributional or consequential approach. These aren't just technical variants—they answer fundamentally different questions and can produce substantially different results.

Two Different Questions

Attributional LCA (ALCA)

Question: "What share of global environmental impacts can be attributed to this product?"

ALCA describes the environmentally relevant physical flows to and from a product system. It's a snapshot of the current situation, allocating existing impacts to a product.

Analogy: Dividing a household's electricity bill among roommates based on their share of appliance use.

Consequential LCA (CLCA)

Question: "What environmental impacts will change as a consequence of a decision about this product?"

CLCA models how material and energy flows (and associated impacts) change in response to a decision. It considers market effects and substitution.

Analogy: Calculating how much the household's electricity bill will increase if one roommate adds a new appliance.

Key Differences

Understanding System Expansion vs Allocation

The treatment of multi-output processes best illustrates the difference.

Example: Soybean Processing

Soybean crushing produces both soybean oil and soybean meal. If we're studying soybean oil:

Attributional approach (allocation):

• Divide crushing impacts between oil and meal
• Common methods: by mass, by economic value, by energy content
• Result: Oil carries X% of crushing impacts based on allocation key

Consequential approach (system expansion):

• Ask: What happens in the market if we produce more soybean oil?
• More crushing → more meal produced as co-product
• More meal → less demand for alternative protein (e.g., fish meal)
• Credit soybean oil with avoided fish meal production
• Result: Oil impacts = crushing + downstream - avoided fish meal

The consequential result can even be negative if the avoided production is impact-intensive.

Visualizing the Difference

Attributional (allocation by mass):

Soybeans → Crushing → Oil (20% mass) → allocated 20% of impacts
                    → Meal (80% mass) → allocated 80% of impacts

Consequential (system expansion):

Soybeans → Crushing → Oil → carries full crushing impacts
                    → Meal → -credit for displaced fish meal
                    
Net oil impact = Crushing - Meal credit

Marginal vs Average Data

Average Data (ALCA)

Represents the current market mix:

• Average electricity grid (30% coal, 25% gas, 45% renewable...)
• Average production technology across suppliers
• Representative of what currently exists

Marginal Data (CLCA)

Represents what changes at the margin:

• Which power plant starts up when demand increases?
• Which supplier expands production when demand grows?
• What technology enters the market for new capacity?

Example: Electricity in a Renewable-Heavy Grid

The marginal supplier depends on time horizon and market conditions.

When to Use Each Approach

Use Attributional LCA For:

Environmental reporting and communication

• Carbon footprints for annual reports
• Environmental Product Declarations (EPDs)
• Product labeling and claims

Product comparison for purchasing

• Comparing suppliers' products
• Green procurement decisions
• Material selection

Regulatory compliance

• Most EPD programs require ALCA
• Product Environmental Footprint (PEF) uses ALCA
• Corporate reporting frameworks

Accountability and allocation

• Dividing responsibility among supply chain actors
• Setting baseline footprints
• Tracking improvement over time

Use Consequential LCA For:

Policy analysis

• Biofuel mandates and their effects
• Recycling policy impacts
• Material restrictions or bans

Strategic business decisions

• Should we change our production process?
• What happens if we enter/exit a market?
• Evaluating technology investments

Large-scale decisions

• Decisions that affect markets
• Decisions with indirect effects
• Long-term planning with market evolution

Research on system-level effects

• Rebound effects
• Market-mediated impacts
• Economy-wide implications

Common Pitfalls

Pitfall 1: Mixing Approaches

Using attributional background data in a consequential study (or vice versa) creates inconsistencies. ecoinvent offers separate system models for each approach—don't mix them.

Pitfall 2: CLCA for Small Decisions

If your decision won't affect markets, CLCA's complexity isn't warranted. A single consumer choosing between products doesn't change marginal suppliers.

Pitfall 3: ALCA for Policy Decisions

Using average data to model policy impacts misses market-mediated effects. A biofuel mandate changes agricultural markets—average data won't capture this.

Pitfall 4: Ignoring Time Horizons

Marginal technologies differ by time horizon:

• Very short term: Existing spare capacity
• Short term: Expandable capacity
• Long term: New investment decisions

Match your time horizon to the decision context.

Pitfall 5: Double Counting

In CLCA with system expansion, ensure avoided burdens aren't counted twice—once as a credit and again in another product's impact.

Hybrid and Integrated Approaches

Some studies combine elements:

Attributional + Sensitivity Testing

Use ALCA as baseline but test sensitivity to marginal scenarios:

• "What if all additional electricity came from gas?"
• "What if recycling displaces virgin production?"

Partial Equilibrium Models

Model markets explicitly for key flows (energy, commodities) while using attributional data for minor inputs.

Scenario-Based Consequential

When marginal technologies are uncertain, model multiple market scenarios and present results as ranges.

Database Support

ecoinvent System Models

ecoinvent provides three system models:

Always use a single system model throughout your study.

GaBi/Sphera

Primarily attributional data, with some consequential scenarios available.

USLCI and Other Free Databases

Generally attributional, representing average conditions.

Practical Guidance

Checklist for Choosing an Approach

1. What question are you answering?

   • Product share → ALCA
   • Decision consequences → CLCA

2. What's the decision scale?

   • Individual/small-scale → ALCA usually appropriate
   • Market-affecting → Consider CLCA

3. What do stakeholders expect?

   • EPDs, carbon footprints → ALCA required
   • Policy analysis → CLCA often expected

4. What data is available?

   • Marginal data requires additional analysis
   • If only average data, stick with ALCA

5. What's your expertise level?

   • CLCA requires understanding of markets
   • When in doubt, start with ALCA

Documentation Requirements

Regardless of approach, document:

• Which approach and why
• System model used (if database-based)
• How multi-output processes were handled
• Key assumptions about markets (for CLCA)
• Time horizon and scope

Key Takeaways

1. ALCA and CLCA answer different questions—choose based on your goal
2. ALCA uses average data and allocation; CLCA uses marginal data and system expansion
3. EPDs and most reporting require ALCA; policy analysis often needs CLCA
4. Never mix attributional and consequential system models
5. CLCA is more complex—only use when market effects matter to your decision
6. Document your choice and reasoning clearly

Practice Exercise

A company is deciding whether to: A) Switch from virgin plastic to recycled plastic B) Report the carbon footprint of their current product

For each decision:

1. Which LCA approach is more appropriate?
2. How would the treatment of recycled content differ between approaches?
3. What data would you need?

What's Next?

The next lesson introduces Social LCA—extending Life Cycle thinking to social and socioeconomic impacts alongside environmental ones.

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Further Reading

• Consequential LCA: Ekvall, T., & Weidema, B.P. (2004). System Boundaries and Input Data in Consequential Life Cycle Inventory Analysis. International Journal of Life Cycle Assessment.
• Comparison: Plevin, R.J., et al. (2014). Greenhouse Gas Emissions from Biofuels' Indirect Land Use Change Are Uncertain but May Be Much Greater than Previously Estimated. Environmental Science & Technology.
• Guidance: JRC (2010). International Reference Life Cycle Data System (ILCD) Handbook - General Guide for Life Cycle Assessment.