The greenhouse gas emission estimates are based on a wide range of data sources and include statistical differences, assumptions, proxy datasets and some expert judgement. In addition, natural variability in processes that are being modelled introduces a degree of uncertainty. For the base year, the 95% confidence interval is ±9%, and for 2015 it is ±7%. For the percentage reduction between the base year and 2015, the 95% confidence interval ranges from 9% to 29%, with a central estimate of 17.8%.
Each year the entire back series of data to 1990 is revised with updated data and/or methodological changes.