Left Foot

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

Philip Camill
Department of Biology
Carleton College
Right Foot


This case was developed for my freshman-to-sophomore-level course entitled Global Change Biology.  The course fulfills a science distribution requirement for nonbiology majors, but is taken for elective credit by science majors.  A typical class may be composed of students from economics, biology, philosophy, sociology/anthropology, environmental studies, English, political science, studio art, and religion.  I do not presuppose a strong background in biology or science.  To serve this broad audience, I introduce scientific methods, but I also take advantage of student diversity to examine the multidisciplinary nature of environmental problems.

This case would also be appropriate for the following sophomore-to-senior level courses: 

This case is useful for everyone, and the ecological footprint spreadsheet should be disseminated widely.  I strongly recommend that the instructor also use the case along with the students in the course.  Many students enjoy this case so much that they pass it along to family and friends.


Upon completion of this case study, students will be able to examine:


The students will have read the following course readings before engaging this case.

The case is an extended, out-of-class project that can be integrated with course topics on sustainability.  It is suitable as a two-week or one-month project.

On the first day of the class in which I use this case, 48 students are divided into small groups of three students each.  Over the 10-week quarter, I deliver my course material using three teaching styles:  (1) lecture (20 class periods), (2) case studies (seven cases requiring seven class periods), and (3) in-class strategy sessions, where groups present in-depth statements on practical methods for mitigating global change (three class periods). 

Three broad sections structure this course:

I present this case after we have covered issues related to land use and urban sprawl, private property rights rights, social and economic valuation of the environment, and sustainability.  I use two case studies prior to this case:  one on the impacts of sprawl in southern Florida on the Everglades and another on local decisions governing deforestation of the Amazon

Before introducing the case, I introduce the students to common uses of the phrase "sustainability" and challenge them to think about what sustainability really means.  For emphasis, I use examples of environmental mission statements from selected national liberal arts colleges, which all use the terminology of sustainability to describe their goals for campus operation.  I then ask them if any of these schools are sustainable and show that they are not by using the "glass dome" analogy of Wackernagel and Rees (1996).  Basically, the inhabitants of no city (or college) would be able to survive if a glass dome were placed on top of the city, sealing it off from inputs or exports of energy, water, food, and air from outside and exports of solid and gaseous waste.  The question becomes how big does the dome have to be to enable all of the inhabitants to survive by including enough land to grow food and forests to absorb CO2 from fossil fuel emissions?  This idea of the "ecological footprint" is a definition of how much land it takes to sustain a human population of any size.  Most cities are not sustainable because the ecological footprint of cities is vastly larger than the geographic area in which they lie.  For example, the ecological footprint of Vancouver, British Columbia, is 19 times that of the city's geographic area.

Starting with this general discussion, I move into an analysis of how we can quantify sustainability in terms of human impact on the environment.  We study an example of carbon emissions from Herendeen (1998, pp.  34-35) to show that the product of population (P), affluence (A), and technological impact (T) can lead to direct environmental impact (I).  That is, I = P * A* T.  Increasing P, A, or T will lead to greater environmental impact, I.

I then ask if these three direct factors are the only ones contributing to how we impact the environment in our daily lives.  I use two examples to show that there are also indirect effects of our consumption (Herendeen 1998):  (1) How much energy does it take to drive and maintain your car?  (2) How much energy does it take to feed you?

I ask them to take about 10 minutes to list all energy-requiring items necessary to carry out the activities (driving and maintaining the car and feeding themselves), and they do a good job in showing that operating a car requires substantially more energy than just gasoline.  For example, personal indirect energy use from owning a car includes energy needed to (1) drill, extract, refine, and ship petroleum, (2) operate auto retail dealers and maintenance shops, (3) construct highway infrastructure, and (4) run auto insurance industries, among others.  Herendeen shows that this indirect energy use can be up to 63% higher than direct fuel use alone.  Our impact on the environment is, therefore, much broader than we imagine because of indirect effects. 

I then spend about five minutes introducing the ecological footprint idea as a way to measure sustainability using data from Wackernagel and Rees (1996).  Using a laptop in the classroom, I open the Excel spreadsheet and walk the students through the footprint spreadsheet, showing them how to enter data and the kinds of consumption categories that they will need to keep track of over a two-week or one-month period.

The students then spend two weeks to a month outside of class accounting for all of their consumption in six categories:  food, housing, transportation, goods, services, and waste.  I provide access to a table that provides a guide for the specific items and quantities students should monitor over the time span.  Data are entered into the blue cells in the spreadsheet.  These are monthly rates of consumption, so if students recorded data for a full month, then they can directly enter their data given the appropriate units.  However, if they tallied their consumption for the shorter two-week period, then they must multiply their data by two before entering it into the spreadsheet.  I prefer to have students tally these categories for two weeks because it takes considerable effort to follow them for a whole month, and students have found this too difficult to handle in a busy schedule.  Once students have entered monthly consumption for these items, the footprint spreadsheet converts the amount of goods and services consumed and waste produced into an area of land needed to support a person's lifestyle.

There are three levels of difficulty and engagement that make this case suitable for high school levels up to senior-level undergraduate courses.

  1. Simple—This scenario is appropriate for high school students or college freshmen-level courses.  In this scenario, students just plug numbers into the spreadsheet and then examine their ecological footprint size.  This is what most online footprint calculators do.
  2. Intermediate—This level is best suited for the majority of introductory and mid-level college courses.  Here, students should understand how each of the consumption categories contributes to their footprints.  They can trace the numbers in the spreadsheet to learn how the six consumption categories require the use of fossil energy land, arable land, pasture, forest, built-up land, and the sea.  This information can be found on the introduction to calculations page.  This is the level that I expect in my 100-level Global Change Biology course.
  3. Advanced—At this level, students should understand the mathematical and theoretical basis of calculations for converting each good or service into land area.  This information is detailed and will probably require that the instructor walk through a sample calculation with the students.  Detailed information on how the spreadsheet calculations work is provided in specific notes about the spreadsheet calculations:

For advanced students who want to determine how indirect effects are incorporated into the calculations, the instructor will need to describe the differences between "bottom-up" and "top-down" footprint approaches and how national aggregate data can be used to determine appropriate correction factors.  This kind of information is provided in the following additional notes about the spreadsheet calculations:

There are generally few surprises for the instructor as a result of using this spreadsheet.  Students with the largest footprints tend to consume more materials and energy, travel more, eat diets rich in meat and other animal products, and recycle little.  Students may be surprised at how much land area it takes to support the six consumption categories.

I often have students convert the footprint land area into the equivalent number of football fields so that they can visualize this area easily (1 hectare = 100m x 100m = 10,000 m2 = two football fields side-by-side).  Footprints should range between 1.5 to 10 hectares, with an average usually around 4 to 6 hectares.  Footprints larger than 10 to 15 hectares are quite high, so if students have footprints larger than this, instructors might have them double check their calculations for errors.

Follow-up Assignment

As described in the case study, the case assignment includes three parts that are turned in to the instructor. 

This is intended to use the introductory story about Biosphere 2 to get students brainstorming about a strict definition of sustainability.  Most people think they know what sustainability means, but it's often a vague or weak notion.  I will use this prompting question the next time I teach my course, but it can be optional.  The question can be posed in class and students can work as a team in or out of class to come up with answers.  The online reading by Daily et al.  (1997) provides a nice foundation for the students and instructor about the kinds of ecological services required to support life on Earth.

This is the heart of the case study, and it is the part of the project where students work independently.  Students will need at least two weeks to record their consumption of the six factors listed.  I usually assign this case study during week 5 of a 10-week term, and I allow them to work on it up to the last week of classes.  This four- to five-week window allows some flexibility as to when students start to record their information.  Having the students complete the case near the end of the term also allows me to lead into the next discussion as one of the topics I use to wrap up the course.  I have students print out copies of their Excel spreadsheet to hand in.  One important caveat about having this assignment come in at the end of the term is that this is a substantial case, so care must be taken by the instructor not to pile on too many assignments or exams close to the due date of this case study.


  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?
    This is a new question that I have not asked before.  I hope that it will foster a consideration of issues like material consumption, land use types necessary for life support and waste absorption, etc.  It helps students make a solid link between Biosphere 2 and the concept of the ecological footprint.
  2. List the factors, from largest to smallest, that contributed to your footprint.  What surprises you about this list?
    These results vary from student to student, but generally transportation is the largest contributor, especially for students that drive or commute a lot.  Meat-based diets almost always require more land than do vegetarian diets.
  3. In terms of the amount of land required to maintain your lifestyle, where might you consider your lifestyle to be sustainable?  Not sustainable?
    Answers vary from student to student depending on personal consumption.  "Sustainable" is used loosely here.  It is hard to know if having a transportation footprint of 3 hectares is especially nonsustainable relative to a food footprint of 2.5 hectares.  However, the global average per-capita allotment of 1.5 hectares of biologically productive land serves as a fairly unambiguous benchmark against which students can evaluate their footprint categories.  Generally, for this question, I direct students to discuss parts of their footprint that are unusually large relative to other factors.  For example, students in the past have commented that they traveled a lot to interviews over the month and that they could not sustain that level of travel if they hoped to reduce or minimize their footprint. 
  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?
    For the first question, students are usually pretty specific about lifestyle changes they could make that would lower their footprint size.  In the past, many have commented that they know they eat too much meat and that they will try to eat several vegetarian meals per week.  Others suggest that they will start walking or biking across campus instead of driving.  Others have affirmed their committment to recycling.  I hope that the second and third questions get students thinking about the kinds of steps it would take to adapt their current lifestyles to a situation as dramatic as Biosphere 2.  This is a really concrete way to get students thinking about the strict meaning of sustainability.
    There is a wide range of student sentiment that results from these questions.  Several students are perfectly honest that they know that living a more sustainable lifestyle requires eating less meat, lowering utilities, and driving less, but they describe themselves as reluctant to do so because they are accustomed to a comfortable way of life.  Most students seem to want to make an honest effort to lower their footprint size and are not apathetic to this issue.  Even if students don't seek to change their lifestyle dramatically, this might be the first time that they come face-to-face with a real number that describes the impact of their lifestyle, and that is valuable.
  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?
    Although a normative question, most students make thoughtful statements about the general equity of their consumption with respect to people in other parts of the world.  Some feel guilty.  Some express gratitude that they have as much as they do.  This question may require special prodding by the instructor to help students work through overly simplistic answers such as "We should recycle more." Push the students to critically evaluate whether this would have any major effect.  The waste category of the footprint, after all, is usually relatively small.  Other ways to encourage deeper analysis include having students present specific proposals that have worked elsewhere (e.g., Europe) or having the students role play the part of poorer nations.  Remind students that their generation needs to shoulder part of the responsibility for thinking of solutions for moving society towards sustainability if they feel that is important.

An open-class discussion of the footprint calculation and questions needs to be an integral part of the course to achieve closure on this project.  I usually spend about 30 minutes in class discussing the footprints and questions.  This is a powerful and rewarding assignment, and this final discussion is often emotional and spirited.  Many are shocked by how large their footprint is.  Most agree that it is very enlightening seeing specific details about their consumption, and many indicate that they are now much more concientious about how much they consume.  For fun, I usually award a footprint award certificate to the person with the smallest, most sustainable footprint, which usually is around 1.3 hectares for vegan students living on campus.  It is a great way to end an introductory course in environmental studies because it ties in well with many topics examined over the term.  Students can be required to answer these questions in teams or individually.  I have tried both ways, and both ways work.  Students usually turn in questions when they hand in the spreadsheet.

Overall, I have run this case as a purely independent assignment and as a partial-group assignment.  Depending on the level of group involvement on this case, instructors may want to include a peer evaluation to determine the level of student participation in the first and third parts above.

Common Pitfalls Encountered by Students and Instructors

There are a few areas where students often have difficulties.

A.  Energy use and heating are some of the most challenging categories for students to track consumption, so I point them to helpful suggestions listed in the case study under the section "Tips for determining your consumption of resources" and I often send them an email with information on how to do this.

All students should be able to estimate their consumption of all of the resources listed in the spreadsheet using the tips provided.  Although not required of the instructor, I have found it useful to do a little bit of the legwork to determine average annual consumption of electricity, water, floor space, and natural gas for major dorms on campus.  I have found the staff in Carleton's maintenance and facilities unit to be extremely helpful.

Here is an example email that I have sent to my class the day we begin the footprint case; most of this information is repeated from the "Tips for determining your consumption of resources" section in the case study.  It is not necessary for instructors to send out an email like this so long as students are aware of the "Tips" section.

I am attaching an excel spreadsheet file with gas, water, and electricity for the buildings on campus for which these records are kept.  I am also putting this file on the course folder.  The columns that are starred are the values that you will work with. 

A couple of points:

  1. Most of your dorms/houses will be on here.  Calculating YOUR share of energy use requires that you divide the total energy used by the number of students in the dorm.  If your dorm is not listed, please do the best you can estimating your water, electricity and heat use.  For water, it's as simple as figuring out how much you use to drink, shower, etc.  For electricity, it's how much you use various appliances and lights.  Most appliances and lights are rated in watts, so you can figure out how many watts you use per hour.  For example, if I use a 100 watt bulb for 20 hours, that amounts to 2000 watt hours or 2 kilowatt hours (since electricity is about 5 cents per kilowatt hour, I would have used about 10 cents of energy).  For water, you can place a 2-liter bottle under faucets or shower and quantify the amount of water flowing over a given time.  For example, let's say I placed a bottle under the shower tap for 1 minute and collected 2 liters.  If I take 10-minute showers, I know I use 20L of water.
  2. The units of housing are square feet; the units of water are gallons; the units of gas are CCF (hundreds of cubic feet); and the units of electricity are kilowatt hours (kwh).
  3. The values in the data spreadsheet here are ANNUAL.  To enter them in the blue cells on the footprint spreadsheet, you will need to make them MONTHLY values by dividing by 12.
  4. Note that the square footage of the dorms are given.  You can divide this value by the number of people in the dorm to estimate your contribution to the housing footprint.
  5. Estimate how much electricity and water you use in other buildings, such as the library.  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 number of people to calculate your contribution).  For example, let's say I spend 5 hours/day in the library.  Let's say there are 30 100 watt bulbs in the room I work in there and that an average of 10 other students work in that room at any given time.  My own energy consuption 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 me alone.  Pretty straightforward.  You can do this kind of estimate for electricity in all the rooms you inhabit on campus.

[NOTE TO INSTRUCTORS:  Wackernagel and colleagues would recommend eliminating the accounting in step 5, because it may double count electricity that is already captured in the indirect effects corrections.  I prefer to have students do step 5 anyway because most students live more in the library than they do in their dorm, and it gives them practice with explicitly accounting for personal energy consumption.]

  1. For food, do the best job you can in estimating weight.  If you live off campus and don't use a meal plan, the weight of the food will be on the packages, or you could use a small scale.  If you eat on a meal plan, you can estimate the weight of each food item.

B.  Students should not enter the cost of their tuition in the education component of the services category (doing so will lead to an astronomical footprint size).  This category is intended for educational services, like paying for someone to make photocopies.  I point this out in the "Tips for determining your consumption of resources" section of the case study.

C.  This third issue is rare, but very important to consider.  Some students have eating disorders and they have been advised by their physicians and psychologists to not account for their food in any way.  Make the general point known to students that if they are unable to keep track of any of the categories to come talk to you.  For the one student that had an eating disorder, I recommended that this person skip the food category but keep track of consumption in other categories. 


Go back to the case

Acknowledgements:  This case was developed with support from The Pew Charitable Trusts.

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

SEND Comments HOME READ Comments click left to send comments, middle to go home, right to read comments