To understand the magnitude of the pressures of resources and ecosystems it is useful to turn to the conceptual equation that is widely attributed to Paul R. Ehrlich (1968). Ehrlich was an American biologist and educator in the year 1968. The equation relates impact (I), to population (P), affluence (A), and technology (T). This equation suggests that impacts, which could be energy use, material use, or emissions, and the product of the population, the affluence of the population divided by the number of people in the nation or region, and the impacts associated with the technologies used in the delivery of the affluence (impact per unit of gross domestic product). For example, if the IPAT equation were used to describe energy use in the U.S, then one would represent energy per year, P would represent the population of the United States, A would represent the GDP per capital, and T would represent the energy use per dollar of GDP. IPAT has been chosen by many scholars in both the social and natural sciences as a starting point for investigating interactions of population, economic growth, and technological change. .
Environmental issues have gained increasing height in the latter of the 20th century and the beginning of the 21st century. Growing populations and affluence around the world have put growing pressure on natural resources and environmental systems to support the needs and wants of global populations, from now into the future, is part of a developing awareness of sustainability. Sustainability is the ability to meet humanities current needs without compromising the ability of future generations to meet their needs. Sustainability is achieved when the environment can tune indefinitely without going into a decline from the stresses that human society imposes on natural systems such as fertile soil, water and air. This applies to many levels including individual, communal, regional, national, and global levels.