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1.2. The Magnitude of the Sustainability Challenge

To grasp the magnitude of the pressures on resources and ecosystems, it is useful to invoke a conceptual equation that is generally attributed to Ehrlich and Holdren (1971). The equation relates impact (I) to population (P), affluence (A), and technology (T):

I = P * A* T

This conceptual relationship, referred to as the IPAT equation, suggests that impacts, which could be energy use, materials use, or emissions, are the product of the population (number of people), the affluence of the population (generally expressed as gross domestic product, GDP, of a nation or region, 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 GDP). For example, if the IPAT equation were used to describe energy use in the United States, I would represent energy use per year, P would represent the population of the United States, A would represent the annual GDP per capita, and T would represent the energy use per dollar of GDP.

While the IPAT equation should not be viewed as a mathematical identity, it can be used to assess the magnitude of the challenges that our societies face in materials use, energy use, and environmental impacts. By estimating growth in population and affluence, we can get an indication of the amount by which energy use, materials use, and emissions might increase over the next several decades, if our technologies remain static. Estimates from the United Nations (United Nations, 2007) suggest that world population will increase at the rate of 1% to 2% per year until peaking at somewhere near 10 billion, over the next century. Affluence, as measured in economic output (e.g., GDP), is growing in some regions of the world by 8% to 10% per year. On average, worldwide, affluence is growing by roughly 2% to 4% per year, depending on economic conditions. If these trends continue for several decades, compounded growth would lead world economic output (P*A) to increase by 50% in 10 years, by 300% in 25 years, and by more than a factor of 10 in 50 years.

Invoking the IPAT equation, the implications of population and economic growth are that if technology remains static, energy use, materials use, and environmental impacts will grow 10-fold over the next 50 years. Reducing the impacts of technology (T in the IPAT equation) by an order of magnitude will be necessary if the world is to support 10 billion people, all aspiring to better living standards. Reducing energy use, materials use, and emissions will be a central challenge for engineers of the 21st century, and engineers will need to develop and master technical tools that will integrate the objectives of energy efficiency, materials efficiency, and reduced environmental emissions into design decisions.

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