The reasons are all taken from the Audit Scotland report Reducing Scottish Greenhouse Gas Emissions published today.
- Uncertainties about the measurement of emissions: "There are numerous uncertainties with reported levels of emissions." Annexe 1 para 4.
- Emissions for many sources are estimated - sometimes using basic assumptions - for the devolved administration in Scotland. Overall, the data for the devolved administrations are less certain than for the UK as a whole. Annexe 1 para 5
- Current assessment methods do not cater for shipping. Annexe 1 para
- Uncertainties associated with greenhouse gases other than carbon dioxide can be significant, particularly concerning emissions from agriculture and related land use. For example, the uncertainty level is plus or minus 12 per cent for Scottish emissions of carbon dioxide in 2009, but plus or minus 290 per cent for nitrous oxide.
- Reported emission levels are estimated not by directly measuring them but by calculating them from the quantities of fossil fuels used and from other relevant processes relating to industry and agriculture - but there are gaps in the data on use of fuel use, and the emissions behaviour of soils is still poorly understood.
- It takes almost two years to obtain data on Scottish emissions. It is difficult to assess the effectiveness of policies on emissions due to the length of time it takes to obtain data on actual levels of emissions. Report para 54.
- It is challenging to assess emissions associated with imported goods and services. "This is a highly challenging field of work" Report para 56.
- Different greenhouse gases vary in their effectiveness at warming the atmosphere. Annexe para 3.