environmental impacts from the same substance or activity may vary across space eg ecosystem medium topology human population and time eg season weather it is also the case that environmental issues interact so that for example watersecurity policies may need to include policies for the adaptation to and mitigation of climate change as well as the provision and protection of water resources for example an economic instrument that affects prices will not be effective if consumers are not aware of prices or the means by which they can respond to price increases that said instrument mixes are not always either necessary or efficient oecd a