Biomass burning occurs in many types of land uses causing emissions of CO₂, CH₄, N₂O, CO, and NO_x. The IPCC provides Tier 1 methods for estimating emissions from two general types of biomass burning: burning within managed forests (i.e. forest land remaining forest land) and burning in the course of land use conversion (i.e. Land converted to Forest land; Land converted to Cropland; Land converted to Grassland).

IPCC methods group fires into two categories: prescribed (or controlled) fires and wildfires. Prescribed fires include: (i) land clearing fires in the course of forest conversion, (ii) slash-and-burn agriculture, (iii) post-logging burning of harvest residues (slash); and (iv) low-intensity prescribed fire for fuel load management. Wildfires are typically more variable (i.e. in temperature and thoroughness of biomass combustion) than prescribed fires making estimation of emissions from these events more difficult.

In accordance with IPCC⁽¹⁾, when managed land is burned, emissions resulting from both prescribed fires and wildfires should be reported so that carbon losses and non-CO₂ GHG emissions on managed lands are taken into consideration.⁽²⁾

At Tier 1 IPCC assumes that emissions from fire are the sum of the area burnt multiplied by the fuel available for combustion per unit area taking into consideration the fraction of available fuel combusted and the mass of each GHG emitted per unit of fuel combusted⁽³⁾. Emissions of each gas are estimated individually, and then are summed to give the total GHG emissions due to fire. The location of relevant IPCC Tier 1 factors are summarised in Table 18 below.

Tier 2 or 3 is required where fire is a key category of GHG emissions. Countries applying Tier 2 are likely to have national data at disaggregated level on the mass of fuel available according to forest types and management systems. Tier 3 estimation requires spatial estimates of the mass of fuel available according to different forest types, regions and management systems. Tier 3 estimation methods may also be able to distinguish fires burning at different intensities, resulting in different amounts of fuel consumption. Fully integrated, mass-balance Tier 3 systems estimate emissions based on the ecosystem type, the biomass on the site at the time of the fire and the type (e.g., wildfire, prescribed burning) and intensity of the fire. These systems also estimate the subsequent recovery from fire (uptake of CO₂) and ongoing release from trees killed by the fire. These systems are particularly effective when maps of fire extent are available.

Tier 2 or 3 methods should avoid double counting of CO₂ emissions from fire in particular where carbon loss is derived from repeated field measurements (i.e. NFI) since estimates of change from ground measurements could include the carbon loss from fires.