Left Foot

Watch Your Step:
Understanding the Impact of Your Personal Consumption on the Environment

Philip Camill
Department of Biology
Carleton College
Right Foot

Part I.  Future Imperfect?

Imagine you could see into the future and watch the next 100 years unfold.  Let's say that human populations, resource consumption, and waste continue to grow according to a business-as-usual scenario with few changes in policy or lifestyles to curb these trends.  Would you notice a point when Earth is no longer able to sustain human population growth, resource consumption, and pollution?  Have we already reached this point?  How could you tell?  What does it take to sustain life on Earth?  The issue of sustainability is a challenging one, and the long-term survival of life as we know it may depend on how we define sustainability as well as the policies and lifestyle changes necessary to achieve it (Brundtland 1987).

Understanding how the Earth system sustains human life is a fascinating problem.  Interestingly, space travel provided an early impetus for creating artificial life-support systems based on ecological principles similar to those of Earth's biosphere (Corey and Wheeler 1992, Galston 1992, Schwartzkopf 1992).  The goal was to develop a self-sustaining biosphere in miniature that could extend the duration of space flights or allow humans to colonize another planet like Mars.  In 1984, a company called Space Biospheres Ventures began the most ambitious example of this kind of research—a project called Biosphere 2—to replicate a self-sustaining biosphere like Earth's (a.k.a.  Biosphere 1).

Columbia University Biosphere 2 Center has become an important experiment for understanding the life-support functions of the biosphere.  Simply put, do we know enough about how the Earth's biosphere sustains human life to be able to replicate it or preserve it?

Click on images to enlarge
Biosphere 2 Ecosystems in Biosphere 2 Ocean ecosystem
Fig. 1. Biosphere 2 Lab Fig. 2. Ecosystems in Biosphere 2 Lab Fig. 3. Picture of the ocean ecosystem
All images of Biosphere 2 are the property of Columbia University Biosphere 2 Center and are used with permission.

Between 1984 and 1991, Columbia University Biosphere 2 Center was built in Oracle, Arizona, at a cost of over $200 million (Nelson et al.  1993, Cohen and Tilman 1996) (see Fig.  1).  The structure was not just a building where people could attempt to live sustainably—it was much bolder.  Biosphere 2 was a giant glass chamber sealed off from the atmosphere that contained living ecosystems including a tropical rainforest, ocean, savannah, desert, marsh, and agricultural landscape (see Figs.  2 and 3).  It enclosed 13,000m2 of land and a total volume of 204,000m3 (Cohen and Tilman 1996).

The original test of Biosphere 2 attempted to determine if these ecosystems could support the lives of eight people in perpetuity.  Their only source of food was what they could grow in the agricultural sector.  Their only source of oxygen was from plants and algae in the terrestrial and oceanic ecosystems.  In 1991, the eight researchers were sealed in Biosphere 2, and the world watched.

In this case study, you will examine the issue of sustainability of humans on Earth using the concept of the "ecological footprint" (Wackernagel and Rees 1996).  This is a powerful method for understanding sustainability and how to quantify it.  As we will see, Biosphere 2 provides a good example of the ecological footprint concept because it represents a specific amount of land area (footprint size) and land use types thought sufficient to sustain the lives of eight people.  We will see the outcome of the Biosphere 2 experiment at the end of the case.


Imagine you are a team of senior research scientists at the initial planning phase of Biosphere 2.  What kinds of specific conditions would be needed in Biosphere 2 to make it possible to sustain eight people for the rest of their lives?  You might consider the following questions and sources of information:

Part II.  A Delicate Balance

On October 12, 1999, Secretary-General of the United Nations, Kofi Annan, welcomed Adnan Nevic of Sarajevo, Kosovo, into the world, marking the birth of the six billionth living human.  The event rejuvenated long-standing debate about how many people the Earth can support before exhausting the supply of natural resources and a clean environment.  Most current estimates project a human carrying capacity, or the number of people that the Earth can support, at around 12 billion, occurring sometime within the next century (Cohen 1995).

The difficulty of determining the uppermost limit of human population is that the human carrying capacity changes through time and will likely rise to some extent with increasing population.  Former President George H.W.  Bush argued that "Every human being represents hands to work and not just another mouth to feed" (Bush 1991).  This suggests that more people can acquire more resources to sustain larger populations, such as building irrigation canals to grow more food or building more housing.  More people may lead to greater ingenuity and technical innovation, such as medical advances and genetically engineered crops.  However, others have pointed out that this view does not specify the cultural, environmental, and economic resources available to make additional hands productive and, therefore, does not specify by how much the additional hands can increase human carrying capacity (Cohen 1995).  For example, there is no advantage to more people in drought-stricken countries where irrigation canals are useless.  There is no advantage to more people if raw materials, such as lumber, are not available to build homes.  The advantage of additional scientists may be limited if research funding declines.

Potential limits to human population size depend critically on the relative balance between the rate of consumption of natural resources and the production of waste versus the rate at which natural resources and services are provided by the planet.  Joel Cohen of Rockefeller University argues that "To believe that no ceiling to population size or carrying capacity is imminent entails believing that nothing in the near future will stop people from increasing Earth's ability to satisfy their wants by more than, or at least as much as, they consume" (Cohen 1995).  He emphasizes this point poignantly:  "How many people the earth supports depends on how many people will wear cotton and how many polyester; on how many will eat meat and how many bean sprouts; on how many will want parks and how many will want parking lots.  These choices will change in time and so will the number of people the Earth can support" (Cohen 1995).

Not only does human sustainability on Earth depend on population size, it also depends on rates of material consumption and waste production by the population.  Although human population growth rates are often highest in developing nations, income and material consumption are often more than an order of magnitude greater in wealthy industrialized nations, such as the United States, Japan, Canada, France, Germany, and England (see Table 1).

Table 1.  Population growth rates and doubling times of selected countries for year 2001.
Data Sources:  Population Reference Bureau and World Bank.

Annual Population Increase (%)
Population Doubling Time (yr)*
Per-capita purchasing power (1999 $)
Palestinian Territory
Middle East
3.7 %
Middle East
3.5 %
Solomon Islands
3.4 %
Middle East
3.3 %
3.3 %
3.2 %
3.1 %
Demo.  Rep.  Congo
3.1 %
3.1 %
3 %
3 %
Saudi Arabia
Middle East
2.9 %
Central America
2.9 %
2.8 %
Central America
2.8 %
South America
2.7 %
0.9 %
United States
North America
0.6 %
Western Europe
0.4 %
North America
0.3 %
0.2 %
United Kingdom
Western Europe
0.1 %
Western Europe
0 %
Western Europe
0 %
Eastern Europe
0 %
Western Europe
0 %
Western Europe
-0.1 %
Czech Republic
Eastern Europe
-0.2 %
Eastern Europe
-0.6 %
Eastern Europe
-0.7 %
Eastern Europe
-0.7 %
*  Population doubling time is calculated as t = ln(2)/ln(R), where ln = natural logarithm and R is popuation growth rate, which is equivalent to 1 + the fractional annual population increase.

To the extent that material consumption drives energy use, resource extraction, pollution, climate change, and landfill buildup, this means that industrialized nations have a larger per-capita impact on the environment (Brown et al.  1993, Herendeen 1998).  The United States, by virtue of its massive per-capita consumption of natural resources and energy and its generation of CO2 and waste, could be considered the most overpopulated country in the world in terms of environmental impact (Cohen 1995, Gardner and Sampat 1999).

Material Consumption and Pollution

Earth is strained by rapidly rising per-capita consumption, especially in industrial nations.  Modern societies are consuming more natural resources and generating more waste than at any time in human history.  For example, the average American uses 101 kilograms of materials in a given day in both direct consumption of goods and indirect consumption of materials required to make those goods (Gardner and Sampat 1999).

Material consumption is having significant impacts on the global environment.  The global cycles of carbon and nitrogen are now dominated by human inputs, often eclipsing natural rates of movement of material by the biosphere and geosphere.  From 1860 to 1991, the human population quadrupled, whereas energy consumption rose 93 fold (Cohen 1995).  Most of this energy consumption is provided by fossil fuels combustion, which releases greenhouse gases like carbon dioxide into the atmosphere.  In 1998, the top five industrial polluters alone—USA, China, Russia, Japan, and India (see http://cdiac.esd.ornl.gov/trends/emis/graphics/top20_1998.gif)—contributed more than half of global carbon emissions (Marland et al.  2001).  The anthropogenic rise in carbon dioxide is occurring at a rate hundreds of times faster that any change seen during the Pleistocene Epoch (the most recent Ice Age) spanning the past 1.8 million years.  By the end of the 21st century, atmospheric carbon dioxide levels will reach the highest levels seen in the last 30 million years (Pearson and Palmer 2000).

Humanity also strongly modifies the productivity of the biosphere through land use change and the addition of nutrients.  Stanford ecologist, Peter Vitousek, determined that humans co-opt approximately 40% of total plant productivity on Earth (Vitousek 1986).  Moreover, the exponential growth of nitrogen fertilizers following WWII now rivals nitrogen fixation by the biosphere in terms of the amount of new nitrogen introduced into the environment each year, leading to chronic problems of acid rain, eutrophication, and the destruction of stratospheric ozone (Vitousek 1994).

Lastly, wood comprises one fraction of the materials consumed by humans, and current logging rates in tropical countries to supply this demand is leading to record rates of rainforest destruction (Laurance 1997, Laurance 2000).  Given that the tropics may support 50 to 80% of the world's biodiversity, destruction of this habitat may be driving eight to 11 species extinct per day (Wilson 1992).

Clearly, consumption in the industrialized world is rapidly converting natural capital to material goods, resulting in declines in primary forests, biodiversity, and clean air and water, and increases in ozone, atmospheric pollution, and solid-waste landfills.  At the global level, steady changes in these indicators of environmental health all suggest that rates of material consumption are not sustainable in the long term

Can we Measure Sustainability?

Instead of the difficult task of estimating the human carrying capacity (number of people per total land area of Earth), Mathis Wackernagel and colleagues at Redefining Progress developed the concept of "ecological footprint" to quantify humanity's long-term impact on the global environment (Wackernagel and Rees 1996).  The ecological footprint represents the inverse of carrying capacity because it quantifies the amount of land area that is required to sustain the lifestyle of a population of any size—an individual, household, community, city, country, or world.

For example, consider the ecological footprint of one human.  Given Earth's 8.9 billion hectares (where a hectare is an area equal to a 100-m x 100-m square) of productive land and its 6 billion human inhabitants, the average ecological footprint should be about 1.5 hectares per person if we assume that land use should be distributed equitably among all of the planet's citizens.  This per-capita footprint provides an unambiguous benchmark with which to assess the long-term sustainability of population growth and material consumption.  Individual footprints below 1.5 hectares are sustainable whereas footprints above 1.5 hectares are not.  Footprint calculations by Wackernagel and Rees (1996) suggest that the individuals in industrialized countries often have footprints as large as four to 10 hectares.

Wackernagel and Rees (1996) posed a thought experiment analagous to Biosphere 2 to illustrate the concept of sustainability and the ecological footprint. Imagine you could cover your town with a big glass dome that sealed off the area of the town and its immediate atmosphere from the rest of Earth.  The question of sustainability becomes this:  How large does the dome have to be and what sorts of land use types need to be included to ensure the survival of all inhabitants?  The more your town is comprised of industrial capital, such as asphalt, buildings, and vehicles, the more difficult it becomes to sustain life because of the lack of natural capital, such as land to grow food and plants to provide life-supporting oxygen and remove life-threatening carbon dioxide.  It is clear from this example that every urban center is unsustainable, as indicated by massive fluxes of energy, water, food, and clean air into cities and exports of solid and gaseous waste out of cities.  Sealing off an urban center under a glass dome would be fatal for its inhabitants.  The ecological footprint of cities is vastly larger than the geographic area in which they lie (Wackernagel and Rees 1996).

In fact, Wackernagel and Rees estimated that if all global citizens aspired to the American lifestyle and attendant levels of consumption, we would require two additional Earths to provide enough land area to supply natural resources and to absorb industrial waste!  In a startling new study, their work suggests that the ecological footprint of humanity began to exceed the capacity of the Earth to supply resources in the early 1980s (see Fig.  1 in Wackernagel et al.  2002).

Understanding the interactions of population growth and material consumption presents a challenging study of the impacts of humans on the global environment and the long-term sustainability of humanity: 

Part III.  Benchmark for Improvement

To what extent do our lifestyles impact the environment and contribute to the number of people the Earth can support?  How can the concept of the ecological footprint be used to measure the impacts of our personal consumption on the environment?  Is your lifestyle sustainable?  In this case study, you will have the opportunity to calculate your ecological footprint based on your own consumption of resources.  You will be able to judge whether your lifestyle is sustainable relative to the global benchmark of 1.5 hectares per person.

Part IV.  Calculating Your Footprint

To complete this case, you will need to record your consumption of energy and materials in each of the following six categories:  food, housing, transportation, goods, services, and waste.

The following table provides a guide for the specific items and quantities you should monitor over the time span of two weeks or a month.  Once you have tallied your monthly consumption of these items, use the footprint spreadsheet to directly convert the amount of goods and services you consume and waste you produce into an area of land needed to support them.  If you assess your consumption of these categories over two weeks, it is important that you double these values to convert them to monthly values for the spreadsheet.

Downloading the Miscosoft Excel spreadsheet

  • For PC users, right click your mouse on the footprint.xls link, choose the "Save link as" option, and select a directory you want to save the spreadsheet to.
  • For Mac users, click your mouse over this link, choose the "Save link as" option, and select a directory you want to save the spreadsheet "ecological footprint.xls" to.

Using the Spreadsheet

You will use a spreadsheet developed by Mathis Wackernagel and colleagues at Redefining Progress (Wackernagel et al.  2000).  The following description provides an overview of how the spreadsheet works: 

  • Open the footprint.xls spreadsheet using Microsoft Excel.
  • Data are entered in the blue cells.  Notes are indicated by the red tabs.  You can acces the information in the note by moving your mouse over the cell with the red tab. 
  • In cell H4, you enter whether you want to use metric or US standard units.  In cell D8, you enter the number of people for which you are calculating a footprint. If you are only accounting for your personal lifestyle, please enter 1. 
  • Next, you will see the six consumption categories:  food, housing, transportation, goods, services, and waste.  Examine the kinds of information that you need to enter in the blue cells.  These data are what you will need to keep track of over the month that you are assessing your footprint. 
  • Note that the units of each of the categories are presented in column C.
  • The spreadsheet calculates the amount of land required to sustain each of these consumption categories.  Notice in columns G-L that there are six Earth surface categories that can be used to produce materials consumed or to absorb the waste of producing and using them:  fossil energy land, arable land, pasture land, forest land, built-up land, and sea. Fossil energy land is the amount of land surface covered with forests that is required to absorb CO2 emitted from the combustion of fossil fuels.  Living sustainably requires that the amount of CO2 gas in the atmosphere does not rise as a result of fossil fuel combustion. Arable and pasture land and the sea are required for food production.  Forests are required for timber and paper resources. Built-up land is required for housing, transportation, and the production/processing of materials and energy.
  • When all of your data are entered, the spreadsheet converts the amount of materials and energy consumed and wasted into land areas for each of the six land use types.
  • The spreadsheet then totals the land areas required to support the six consumption categories and presents the ecological footprint in cell G148.

How does the ecological footprint work?  Understanding the spreadsheet calculations

Using the spreadsheet to calculate an ecological footprint does not require a detailed understanding of how the spreadsheet converts material and energy consumption and waste into land area.  However, examining the relationship between land area and material and energy consumption provides a rich understanding of how humanity impacts the environment—impacts that we may not commonly think about.

The following links describe these calculations in depth, showing the innovative approaches that Wackernagel and colleagues developed to assess individual sustainability. 

Specific Notes about the Spreadsheet Calculations

The following section describes how specific land use types are important for producing the materials and absorbing waste in the spreadsheet. As explained below, some calculations are modified from the general description presented in the "Introduction to calculations" page.  By clicking on the red triangles in the spreadsheet, you can show comments describing each calculation.

Other notes about the Spreadsheet Calculations

Tips for Determining Your Consumption of Resources

Depending on your living arrangements, it may be easy or challenging to account for your consumption of commodities like water, natural gas, and electricity.  Some students live in houses where the quantities of these factors appear on monthly utility bills or meters outside the home that can be easily read.  Other students live in large dorms where it is more difficult to determine one's personal consumption.  Learning to to measure your consumption of water, electricity, and gas is a valuable first step in learning about your consumption patterns.  Here are some tips for making this easy to do in case you don't have meters or monthly bills to use:

If you spend considerable time in particular buildings outside your dorm, such as a library, you may wish to estimate how much electricity and water you use in other buildings.  The goal is not rocket-science accuracy; just do the best you can, and have fun estimating these.  How many hours do you spend in these buildings?  How many lights are in the ceiling of these rooms?  How many other people are in the rooms at the same time (use the total # of people to calculate your contribution).  For example, let's say you spend 5 hours/day in the library.  Let's say there are 30 100-Watt bulbs in the room you work in there and that an average of 10 other students work in that room at any given time.  Energy consumption per month from that room would be 30 bulbs * 100 watts/bulb * 5 hours/day * 30 days/month = 450 kilowatt hours/month for all 10 people, or 45 kilowatt hours for you alone.  Pretty straightforward.  You can do this kind of estimate for electricity in all the rooms you inhabit on campus. 
Note how this is a great learning experience.  The fact that it might be really tough for you to nail down your exact consumption of fossil fuels required for heating underscores the ease with which people can be naive about their impacts on the environment. 

You will notice that some items like glass are listed both in goods and waste.  You may wonder if this leads to double counting.  For example, if you buy a soda in a glass bottle and then throw away the bottle, do you have to enter the weight in both the goods and waste categories?  The answer is no—just enter it in one or the other, preferably the waste category since bottles are not generally considered durable goods.  If you recycle all of your glass bottles, enter the weight in the spreadsheet and then enter 100% in cell H129.

Part V.  Assignment

The assignment for this case includes three parts that will be turned in to your instructor:


  1. What parallels can you draw between sustaining the life of a person in Biosphere 2 and the sustainability of a person's lifestyle in the real world?
  2. List the factors, from largest to smallest, that contributed to your footprint.  What surprises you about this list?
  3. In terms of the amount of land required to maintain your lifestyle, where might you consider your lifestyle to be sustainable?  Not sustainable?
  4. What specific actions could you take to reduce your footprint?  If you were to take actions to reduce your footprint, in what ways would your lifestyle be fundamentally different?  How realistic/achievable are these reductions?
  5. How do you feel about the fact that the average footprint of a citizen in the United States is 4 to 10 hectares compared to the global average of 1.5 hectares?  Why is this the case?  Should anything be done about it?  If so, what?  What are the global consequences of being apathetic about this question?

Part VI.  Epilogue:  Life in a Dome

Columbia University Biosphere 2 Center was a living experiment of Wackernagel and Rees' big-glass-dome analogy.  Eight people were sealed in the dome in 1991, but Biosphere 2 failed to sustain these eight scientists for even two years.  By 1993, the oxygen concentration in the air inside Biosphere 2 fell to 14%—roughly equivalent to that at the peak of a 17,500-ft mountain (Nelson et al.  1993, Cohen and Tilman 1996).  Atmospheric CO2 rose to about 1700 parts per million (ppm), similar to a level last seen approximately 50 million years ago shortly after the extinction of the dinosaurs (Pearson and Palmer 2000).  Nitrous oxide (N2O), a trace gas emitted from the microbial decomposition of soil nitrogen, rose to 79 ppm—a level that can reduce vitamin B12 synthesis to levels that damage the brain (Cohen and Tilman 1996).  Species extinctions were startling.  Nineteen of the 25 vertebrate species in Biosphere 2 went extinct (Cohen and Tilman 1996).  All pollinating species went extinct, in part because ants took over the insect world, forcing the researchers to pollinate plants themselves.  In 1993, fresh air was pumped into Biosphere 2, the researchers were able to leave the system, and the test was over.

Why did Biosphere 2 fail?  Much of the answer lies in the fact that researchers did not correctly judge the quantity of different ecosystems that were needed to sustain human life, and they did not anticipate how the entire system would adjust after it was sealed off.  Specifically, too much soil was added in the tropical rainforest biome.  The microbes in the soil, under the warm conditions in Biosphere 2, decomposed the soil carbon, releasing CO2 to the atmosphere and consuming O2.  This, in part, drove down the quantity of breathable air and created a greenhouse effect with the high CO2.  Another factor that contributed to the decline in oxygen was the cement materials use to construct the foundation.

Other surprises included the collapse of the animal kingdom and the elimination of insect species valuable for ecosystem services like pollination.  Ecosystem services are invaluble to humanity and, if degraded, can prove impossible or too costly to replace (Costanza et al.  1997, Daily et al.  1997).

Biosphere 2 and the ecological footprint are valuable lessons about sustainability.  Even in our best attempt to sustain the lives of just eight people, we could not develop an artificial biosphere that was sufficient.  Consider what would be required for a glass dome over your house, town, city, or country to perform any better. 

VII.  References

Date Posted:  08/22/02 nas
This file is also available in Adobe Portable Document Format (PDF).  PDF Version