When it comes to climate change, there really is only one measure that matters, the total quantity of Greenhouse Gasses (GHGS) in the atmosphere. That is why policy-makers tend to favour absolute measures of emissions. The UK Climate Change Bill targets set out a legal obligation to reduce absolute emissions of “carbon units” by 80% by 2050 compared to 1990 levels.  This equates to around 3% per annum reduction every year for the next 40 years