Additionality

Additionality is the condition that an intervention causes outcomes beyond those expected in its absence. It is used to judge whether an action produced a real increment rather than merely coinciding with existing trends.

Counterfactual: A description of what would have happened under conditions different from those that actually occurred. Additionality compares observed outcomes with the counterfactual in which the intervention did not happen.
Carbon offset: A unit representing a quantified reduction or removal of greenhouse-gas emissions used to compensate for emissions elsewhere. Offset credibility depends on whether credited reductions exceed what would have happened anyway.
Leakage (climate policy): An increase in emissions outside a regulated area or project that offsets some emissions reductions within it. A project may be additional locally while leakage cancels part of its wider climate benefit.
Baseline manipulation: The strategic selection or alteration of a reference scenario to influence measured performance. An inflated no-intervention baseline can make ordinary outcomes appear additional.
Causal inference: Methods for estimating whether and how one event or intervention causes an outcome. It supplies methods for distinguishing intervention effects from background changes.
Clean Development Mechanism: A Kyoto Protocol program that issued credits for eligible emission-reduction projects in developing countries. Its project credits relied on demonstrating emissions reductions beyond a baseline scenario.
Permanence (carbon offsets): The durability of a carbon removal or emissions reduction over time. Additionality asks whether a benefit is caused; permanence asks whether it lasts.
Additionality in biodiversity conservation: The extent to which conservation actions produce biodiversity gains beyond likely outcomes without those actions. Biodiversity gains lack a single common metric, complicating counterfactual comparisons.
Baseline scenario: An estimate of conditions and outcomes expected without a proposed intervention. The baseline specifies the no-intervention outcome against which additionality is assessed.
Environmental subsidy: Public financial support intended to encourage activities that protect or improve the environment. Additionality distinguishes supported actions induced by a subsidy from actions firms would take anyway.

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