One way of preserving this integrity would be to require abstractors/polluters who use this “safety valve” to make the necessary abstraction/emission reductions in later years. This reassessment can be very important when “qualitatively new” instruments are added to the existing mix, such as when a quantity based instrument (e.g. a quota-based trading system) is combined with price-based instruments (e.g. taxes and subsidies). It is also important to regularly review the effectiveness and efficiency of the instrument mix that is in place - to ensure that the programme performance anticipated ex ante has indeed been realised.