IPCC tier levels
A three-step hierarchy of methodological rigour, from global defaults to country-specific measurement, used to classify emission estimates.
The IPCC guidelines describe three tiers. Tier 1 applies globally available default factors with coarse activity data. Tier 2 substitutes country or region-specific factors and better data. Tier 3 uses direct measurement, detailed modelling and site-specific systems.
Higher tiers reduce uncertainty but demand data, capacity and cost. The appropriate tier depends on how significant a source is within a national inventory: a minor category can reasonably stay at Tier 1, while a dominant one warrants the effort of Tier 3.
For project-level carbon accounting the tier concept is borrowed rather than binding. It still frames a useful question — is this estimate built on defaults borrowed from elsewhere, or on measurement from this landscape? — and verifiers often use that framing when judging whether an uncertainty claim is credible.
Frequently asked questions
Is a higher tier always better?
Not automatically. Effort should follow significance; over-engineering a trivial source wastes resources without improving the inventory.
Can tiers be mixed within one estimate?
Yes, and it is normal. Different pools and gases are commonly estimated at different tiers.