Frameworks are useful in that they suggest how specific conditions might be connected with one another, and how long, indirect trains of connections might lie behind any specific environmental impact. Models have been developed that state the precise value or character of any human and environmental condition, and the direction and strength of the connections between those conditions. The most elaborate models now in use are theGlobal Circulation Models used to chart the links between greenhouse gas emissions and climate change. Models work precisely because they simplify reality. In modelling, simplification allows us to quantify a limited number of conditions and their dynamic interconnections. It would not be possible to build a model that included all the conditions of any action. The utility of any model depends on its accuracy. This applies both to current conditions, and to the relationships between conditions.