The Tier 2 approach is a more detailed analysis that builds on the Tier 1 approach by multiplying the results of the Tier 1 analysis by the relative uncertainty of each category. This method is likely to reduce the number of key categories under consideration.

Using the Tier 2 approach, key categories represent 90 percent of the quantified uncertainty contribution, as opposed to those that sum to the pre-determined cumulative emissions or trend threshold (i.e., 95 percent).

A simple spreadsheet can account for the uncertainty contribution by applying the category percentage uncertainty estimates and the Tier 1 level and trend assessments (see Chapter 7 of the IPCC Good Practice Guidance for a complete discussion of how this threshold was established).