(STAR) STAR SCIENCE By Francis L. De Los Reyes III, PH.D. -(First of two parts)

What’s your environmental footprint? How many Earths are needed to support the global population with your lifestyle? Well, there is a website that calculates exactly that (, by the Global Footprint Network). You input things like the size of your house, your diet, car miles traveled per week, and the website uses a model, obviously with several assumptions about energy consumption, carbon emissions and land use, and voilà, you get the number of planet Earths needed to support YOU.

These kinds of tools are useful for showing how your lifestyle choices affect the environment. Human activity impacts the environment in many ways: how we live, the goods we use, and services we consume all have environmental costs. The results are eye-opening. Today, we - the global population - need 1.3 planets to support our collective lifestyle, which means it now takes the Earth one year and four months to regenerate what we use in a year ( The 2005 data show that the average Filipino uses 0.9 global hectares (for comparison, the average American uses 9.4 global hectares). If you compare the ecological footprint of the Philippines to its biocapacity, it turns out we need 1.6 times the area of the Philippines to support the population (the US needs 1.9 times its area). It doesn’t take a Ph.D. to realize that global ecological “overshoot” is a bad thing. How can we approach this problem in a scientific way? We need a method for quantifying sustainability over time and at different geographical scales.

In the 1970s, academic debates over the underlying cause of environmental degradation resulted in the famous IPAT identity. The formula states that environmental impact, I, is equal to the product of P, A, and T, where P is the population, A is the per capita affluence (expressed as $ GDP/capita), and T is the impact of technology (expressed as impact/$ GDP). A is a rough measure of individual wealth and T measures the intensity of the impact (i.e., how much environmental damage occurs per dollar produced in the economy). Intuitively, the identity makes sense: the more people, and the more “affluent” the population, the more total resources are needed and the more wastes need to be absorbed. The factor T accounts for the effect of technology on environmental impact. If we agree that P, A, T are the dominant factors contributing to environmental degradation, then the IPAT identity gives us a quantitative framework to assess the total contribution of a specific region, country or even the world to environmental impacts. In addition, the identity allows us to analyze the relative contribution of each factor over time to environmental impact.

In general, the population term (P) increases CO2 emissions (the more people in a country, the higher the total CO2 emissions). The affluence term (A) also increases CO2 emissions; the “richer” the population, the more consumption of energy, materials, and services. We see this in the comparison of data for the US and the Philippines.

(To be concluded)

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Francis L. de los Reyes III is an associate professor of Environmental Engineering at North Carolina State University. He conducts research and teaches classes in environmental biotechnology, biological waste treatment, and molecular microbial ecology. He is on the editorial board of Water Research, and was a 2008 Balik-Scientist of the DOST. He is a member of the Philippine-American Academy of Science and Engineering. E-mail at

Joseph F. DeCarolis is an assistant professor of Environmental Engineering at North Carolina State University. He conducts research and teaches classes on energy and climate issues at the intersection of engineering, economics, and public policy. E-mail at

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