Sustainability Indicators (SIs) emerged for the purposes of Sustainable Development as a product of ecological studies quantifying the value of certain parts of an ecosystem in an attempt to provide policymakers with a system for comparing the potential value or harm of one policy over another, i.e., understanding the trade-offs. SIs also provide decision makers with a means for measuring progress over time (e.g., improvement or decline in the system or part of the system.) A simple example is that pollution levels in water resources could indicate whether certain efforts to regulate those pollutants were effective. However, the measure of pollution levels alone does not indicate whether an action is sustainable. It is only in the context of a broader set of indicators (of parts of the system) that the measure of pollution levels would begin to inform us about the sustainability of an action or set of actions.
Urban systems studies are an area that has seen significant attempts at creating comprehensive sets of Urban SIs, as you will encounter in Chapter 3. Many researchers in sustainability and/or urban studies consider the challenge of making our cities sustainable as the main hurdle to achieving a sustainable society. This is for various reasons, including the projected growth of human populations between now and at least 2100 will be in urban centers, where more than 50% of the world's population currently lives, roughly two-thirds of which are in developing countries. Yet, for all of the projected growth, urban living (per capita) provides the best opportunity for a low-impact lifestyle (e.g., per capita, residents of NYC have the lowest greenhouse gas footprints in the U.S.) This is because of high-density living with efficient and readily available public transportation.