The consultancy Ecometrica have produced a very clear and useful paper - ref as follows: Brander M, Tipper R, Hutchison C, Davis G (2008). Technical Paper: Consequential and Attributional Approaches to LCA: a Guide to Policy Makers with Specific Reference to Greenhouse Gas LCA of Biofuels, Ecometrica The paper sets out the difference between consequential and attributational LCA approaches and assesses the extent to which the EU's Renewable Energy Directive (RED), the Renewable Fuel Standard in the US, and the UK's Renewable Transport Fuel Obligation (RTFO) uses one, the other or a combination of the two approaches. The paper points out that the two approaches aim to answer different questions, and failure to distinguish them can result in the wrong method being applied, a mixture of the two approaches within a single assessment, or misinterpretation of results. Here is the paper's definition of the difference between the two - note that the issue has strong relevance to all forms of agricultural production, and not just biofuels. Attributional LCA (ALCA) provides information about the impacts of the processes used to produce (and consume and dispose of) a product, but does not consider indirect effects arising from changes in the output of a product. ALCA generally provides information on the average unit of product and is useful for consumption-based carbon accounting. Examples of ALCA methodologies include the PAS 2050 Specification for the assessment of the life cycle greenhouse gas emissions of goods and services, and to a large extent ISO 14044 Environmental Management - Life Cycle Assessment - Requirements and Guidelines. ALCA informs comparisons between the direct impacts of products , and is used to identify opportunities for reducing direct impacts in different parts of the life cycle. Consequential LCA (CLCA) provides information about the consequences of changes in the level of output (and consumption and disposal) of a product, including effects both inside and outside the life cycle of the product. CLCA models the causal relationships originating from the decision to change the output of the product, and therefore seeks to inform policy makers on the broader impacts of policies which are intended to change levels of production. Whereas ALCAs are generally based on stoichiometric relationships between inputs and outputs, and the results may be produced with known levels of accuracy and precision, CLCAs are highly dependent upon economic models representing relationships between demand for inputs, prices elasticities, supply, and markets effects of co-products. Such models rarely provide known levels of accuracy or precision and should therefore be interpreted with caution.
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