Each Party shall report a consistent annual time series starting from 1990; those developing country Parties that need flexibility in the light of their capacities with respect to this provision have the flexibility to instead report data covering, at a minimum, the reference year/period for its NDC under Article 4 of the Paris Agreement and, in addition, a consistent annual time series from at least 2020 onwards. The CRT tables to be submitted shall be all of them, for all years (1990 to present -2), but reporting using “FX” notation key for those years finally not considered part of the inventory period. All emissions estimates in a time series should be estimated consistently, which means that as far as possible, the time series should be calculated using the same method and data sources in all years. Using different methods and data in a time series could introduce bias because the estimated emission trend will reflect not only real changes in emissions or removals but also the pattern of methodological refinements. If there are gaps in the activity data time series which prevent GHG emissions being calculated, try to use one of the IPCC “gap filling” approaches to complete the time series; see Volume 1 Chapter 5: “Time Series Consistency” of the IPCC 2006 Guidelines.

In many cases, historical data are no longer available if they were not collected or archived in a timely manner, making it nearly impossible to reconstruct complete time series for emissions—particularly for earlier years like 1990.

For sectors such as Energy and Waste, it may be feasible to estimate missing data using international statistics and population data, applying interpolation techniques where necessary. These approaches can provide reasonably fair estimates, especially when national data are limited.

However, the AFOLU sector presents a much greater challenge. In many countries where historical statistical data are limited, AFOLU tends to account for a larger share of emissions and removals compared to the Energy sector. The lack of consistent historical data makes it extremely difficult to construct a complete time series, even with flexibility provisions. Inventory compilers may find themselves with very limited options for improving historical coverage in this sector.

Therefore, while flexibility can be applied in principle, the practical limitations—especially for AFOLU—may leave countries with few viable options for improving historical coverage.