science-2018:ENV.GP.4
science-2018:ENV.GP.4 Sustainability
Learning goal: Define sustainability and its three dimensions; explain and compute ecological and carbon footprints, including the "how many Earths" comparison; break a personal footprint into its components and construct a reduction plan; redesign a community for sustainability across waste, water, pollution, transportation, energy and biodiversity; and use the tragedy of the commons to identify strategies that prevent destruction of a shared resource.
What sustainability means. The most widely used definition comes from Our Common Future (1987), the report of the United Nations World Commission on Environment and Development (often called the Brundtland Commission), which describes sustainable development as "development that meets the needs of the present without compromising the ability of future generations to meet their own needs." In environmental science that idea is made concrete by three rules for resources. A renewable resource (timber, fish, fresh water, soil) is used sustainably only if the harvest rate does not exceed its regeneration rate. A non-renewable resource (fossil fuels, metal ores) is used sustainably only if it is used efficiently and a substitute is being developed as it is depleted. And a waste is discharged sustainably only if the environment can absorb and break it down as fast as it is produced. Sustainability is usually described as having three dimensions: environmental (the rules above), economic (people can afford to live) and social (benefits and burdens are shared fairly). A measure that satisfies one dimension while violating another, such as an energy-efficient home nobody can afford, is not sustainable in the full sense.
Ecological footprint. The ecological footprint is an accounting tool that converts everything a person, city or country consumes into the area of biologically productive land and water needed to produce those resources and absorb the wastes, especially the CO₂ from burning fossil fuels. It is expressed in global hectares, a hectare of world-average productivity. Its counterpart, biocapacity, is the productive area actually available. If footprint exceeds biocapacity, the difference is an ecological deficit. A single country can cover a deficit by importing biocapacity from other countries; for the world as a whole, which cannot import, a deficit (overshoot) is met by depleting stocks (drawing down fisheries, forests and soils) and by accumulating CO₂ in the atmosphere. The Global Footprint Network, which maintains the method, estimated in its early-2020s accounts (the basis of its annual Earth Overshoot Day) that humanity as a whole uses the equivalent of roughly 1.7 Earths, meaning it takes the planet about a year and eight months to regenerate what people use in a year. By the same method, if everyone lived like an average resident of the United States it would take on the order of five Earths, while some low-income countries use well under one. These are model outputs; they depend on the method's assumptions about yields and carbon uptake, and different calculators give different numbers for the same lifestyle. Their value is in comparison, not in the third decimal place.
Carbon footprint and its components. A carbon footprint is the mass of greenhouse gases, usually expressed as CO₂-equivalent, that a person's activities cause per year. It is the largest single part of most ecological footprints in developed countries. The components are usually grouped as shelter (heating, cooling and electricity for the home), food (with meat and dairy weighted heavily because of the land, feed and methane involved), transportation (fuel for cars and flights) and goods and services (the energy embedded in everything bought). The diagram's pie chart shows a hypothetical student's footprint divided as food 30%, transportation 25%, goods and services 25% and shelter 20%. A reduction plan starts with the largest slice and asks which changes are large, cheap and within the person's control. Cutting beef in half changes the food slice substantially; a shorter hot shower saves water-heating energy, so its carbon effect is real, though for most people it is smaller than changes in travel or diet and depends on how the water is heated. The order of effectiveness is usually: how you travel and how far, what you eat, how your home is heated and how much floor area is heated, and only then the many small habits.
Redesigning a community. A sustainable-town plan works through six systems and their links. Waste treatment: separate collection for recyclables and food scraps, composting, and a landfill designed to capture methane. Water resources: protect the source watershed, meter use, reuse treated wastewater for irrigation, and manage storm water with permeable surfaces and rain gardens instead of pipes. Pollution: zone industry away from housing and prevailing winds, and set limits with monitoring. Transportation: compact, mixed-use neighborhoods so daily trips are short, with safe walking and cycling routes and frequent transit on main corridors; transportation choices drive both air quality and the carbon slice. Energy: buildings insulated and oriented for sun, district heating where dense, and renewable generation sited where it does not destroy habitat. Biodiversity: a connected network of parks, stream corridors and native plantings so that green space is habitat, not just lawn. The systems interact: compact development reduces driving, which reduces pollution and the land taken from habitat. A good proposal states each recommendation, the evidence for it, its cost, who bears the cost, and what feedback from residents changed in the final version.
A sustainable lifestyle. The same six systems scale down to a household: an energy-efficient living space (insulation, efficient appliances, a heated area no larger than needed), mindful use of resources (repair, reuse, buying durable goods), alternative transportation (walking, cycling, transit, shared trips), dietary sources (more plants, less waste, local and seasonal where practical) and outdoor space that supports pollinators and absorbs rain. None of these requires deprivation; most save money, which is the economic dimension of sustainability.
The tragedy of the commons and how to avoid it. Three ideas need separating. A common-pool resource is one that many users draw on and from which it is hard to exclude anyone: a fishery, a pasture, an aquifer, the atmosphere. If no one controls its use, it is open access. If a defined community owns or manages it, with rules about who may use it and how much, it is common property, and such governance can work well, as discussed below. Hardin's 1968 argument, which fits open access best, was that each user profits fully from taking one more unit while the cost of depletion is shared by everyone, so rational individuals over-harvest until the resource collapses. A classroom simulation, in which teams "fish" tokens from a shared bowl that replenishes by a fixed fraction each round, usually produces collapse within a few rounds when there are no rules and stable yields when rules are agreed. Strategies that prevent destruction, drawn from both the simulation and real cases, are: limits (a total allowable catch set below the regeneration rate), allocation (quotas, permits or property rights so each user has a stake in the future), monitoring (someone counts), enforcement (penalties that exceed the gain from cheating) and communication (users who talk and trust each other cooperate). Elinor Ostrom, who won the 2009 Nobel Memorial Prize in Economics, documented many communities that manage commons successfully for centuries with such rules, showing that the tragedy is a tendency, not a law.
Misconception. "Sustainability means using no resources." It means using renewables no faster than they regenerate and non-renewables while developing substitutes. A second misconception: "industrial emissions have nothing to do with consumers." Industry produces goods that people buy, and footprint accounting traces those emissions back to consumption. How much weight to give individual choices compared with policy and infrastructure is a question of values and strategy that the accounting cannot settle; what it does show is which consumption categories (travel, housing, food) carry the most emissions.
Worked footprint breakdown: A hypothetical student's carbon footprint is 8.0 metric tons CO₂-equivalent per year, divided as in the diagram: food 30% (2.4 t), transportation 25% (2.0 t), goods and services 25% (2.0 t), shelter 20% (1.6 t). Plan: (1) halve beef and lamb consumption, estimated to cut the food slice by about a quarter: saves 0.6 t; (2) replace a daily 10 km solo car commute with bus or bicycle on three days out of five: saves about 60% of commuting fuel, which was half the transportation slice, so 0.6 t; (3) lower the thermostat 2 °C in winter and seal draughts, estimated to save 10% of shelter: 0.16 t. New footprint ≈ 8.0 − 0.6 − 0.6 − 0.16 = 6.64 t, a 17% reduction. The largest saving came from the two largest slices, as expected; the thermostat change is worth doing but is a small share.
Worked "how many Earths": Suppose a footprint calculator reports a lifestyle at 8.5 global hectares per person and the world's biocapacity at about 1.6 global hectares per person. Earths required if everyone lived that way = 8.5 ÷ 1.6 ≈ 5.3. For a lifestyle at 1.2 global hectares, 1.2 ÷ 1.6 = 0.75 Earths. The comparison is what matters: the first lifestyle would need about seven times the share of the planet the second does. The 1.6 figure is itself a modeled estimate that changes with the method and year.
Worked commons simulation: A bowl holds 20 tokens (fish). Each round, four teams may each take 0 to 5 tokens; after taking, the remaining stock doubles up to a maximum of 20. Round 1 with no rules: each team takes 5; 0 remain; the fishery is gone in one round. Round 1 with a rule "total take ≤ 10, 2 or 3 each": teams take 10, 10 remain, the stock doubles back to 20, and the same take is available every round for ever. The sustainable yield here is 10 per round, half the stock, because the regeneration rate is 100% of what remains, up to the cap. A team that cheats by taking 5 gains 2 or 3 extra tokens once and costs everyone, including itself, the future rounds unless monitoring catches it.
Worked town system link: A town proposes moving a planned housing development from farmland at the edge to a vacant industrial site near the center. Effects across systems: transportation (shorter trips, transit within reach), pollution (less driving, but the site's soil must be tested and cleaned), biodiversity (farmland and its hedgerows kept), water (existing pipes reused; storm water must be handled with permeable paving), energy (denser buildings share walls and lose less heat), waste (existing collection routes). The trade-off is cleanup cost versus the cost of new roads and pipes at the edge.
Representation task: Draw a pie chart of your own estimated footprint components (shelter, food, transportation, goods and services) using an online ecological or carbon footprint calculator your teacher approves, or the hypothetical figures in the diagram if you prefer not to use personal data. Keep the two measures apart: an ecological footprint is an area (global hectares, which can be expressed as a number of Earths), while a carbon footprint is a mass of gases (metric tons CO₂-equivalent) and is only one component of it; if you use a carbon calculator or the diagram, label your chart as the carbon part only. Beside it, list the three changes you judge largest, cheapest and most within your control, and estimate the percentage each would save. State which numbers are calculator outputs and which are your estimates.
Commons simulation: Run the token fishery described in the worked example for five rounds with no rules, record the stock each round, then five rounds with rules the class agrees on. Record: the rule, who monitored, whether anyone cheated, and the yield per round. Write which of the five strategies (limits, allocation, monitoring, enforcement, communication) your class used and which real commons (a fishery, an aquifer, the atmosphere) each strategy has been applied to.
Independent performance: Redesign a real or hypothetical town of about 10,000 people to be more sustainable. Success criteria: (1) one specific recommendation for each of waste treatment, water resources, pollution, transportation, energy resources and biodiversity; (2) at least two links between systems explained (for example, how transportation choices affect air quality and land use); (3) an estimate of one cost and who bears it; (4) evidence of feedback: present a draft to two classmates, record their objections, and show what changed in the final proposal; (5) a closing paragraph on which of the three dimensions of sustainability (environmental, economic, social) your plan serves least well and why.
An adult may read the prompts aloud. Answer independently with words, objects, drawings or a demonstration before opening the answer guide. Award 1 point for a correct result or claim and 1 point for a matching explanation or demonstration; where a question asks only for a result (a corrected sentence, a spelling, a count), a fully correct result earns both points, and the answer guide shows what a complete response contains. Record prompting separately. The total (two points per question) is local practice feedback, not an Ohio proficiency level.
Define sustainability and state the three resource rules that make the definition operational.
Sustainability is meeting present needs without compromising future generations' ability to meet theirs. Operationally: harvest renewable resources no faster than they regenerate; use non-renewable resources efficiently while developing substitutes; and discharge wastes no faster than the environment can absorb and break them down.
What is an ecological footprint, in what unit is it measured, and what does it mean if a country's footprint exceeds its biocapacity?
It is the area of biologically productive land and water needed to supply what a person or population consumes and to absorb its wastes, especially CO₂, measured in global hectares (hectares of world-average productivity). If a country's footprint exceeds its biocapacity it has an ecological deficit, which it covers by importing biocapacity from other countries, by depleting its own stocks such as forests, fisheries and soils, or by adding CO₂ to the atmosphere; for the whole world, which cannot import, a deficit means depletion and cannot continue indefinitely.
A calculator reports a lifestyle at 6.4 global hectares per person against a world biocapacity of 1.6 global hectares per person. How many Earths would be needed if everyone lived that way, and why should the answer be treated as approximate?
6.4 ÷ 1.6 = 4 Earths. The figure is approximate because both numbers are model estimates that depend on assumptions about crop yields, forest carbon uptake and the conversion of different land types to a common unit, and different calculators give different values; the comparison between lifestyles is more reliable than the absolute number.
Using the diagram's 8.0-metric-ton footprint, compute the metric tons in the food and shelter slices and explain which offers the larger absolute saving from a 25% reduction.
Food = 30% of 8.0 = 2.4 t; shelter = 20% of 8.0 = 1.6 t. A 25% reduction saves 0.6 t from food and 0.4 t from shelter, so the same percentage effort on the larger slice saves more. Reduction plans should start with the largest components.
Explain the tragedy of the commons using a fishery, and give three strategies that prevent destruction of a common resource.
Each boat gains the full value of every extra fish it catches but shares the cost of a shrinking stock with all boats, so each keeps adding effort until the stock collapses. Strategies: set a total catch limit below the regeneration rate; allocate quotas or permits so each user has a stake in the future; monitor catches and enforce penalties larger than the gain from cheating; and enable communication so users agree and trust the rules.
A town wants to reduce its transportation emissions. Propose one land-use change and one transportation change, and explain how each also affects air quality and biodiversity.
Land use: build new housing compactly near the center and shops (mixed use) rather than on farmland at the edge, so daily trips are short. Transportation: add frequent bus service and safe cycling routes on main corridors. Both reduce vehicle-kilometers, which lowers nitrogen oxides, particulates and ozone precursors, and compact development leaves farmland, hedgerows and stream corridors as habitat instead of subdividing them.
Give one reason the claim "sustainability means giving up modern life" is wrong, with an example from energy or transportation.
Sustainability requires using renewables within their regeneration rate and non-renewables efficiently, not abstaining. An insulated, well-oriented home uses far less energy for the same comfort and costs less to run; a compact neighborhood with transit gives the same access with less driving. Most measures reduce costs, which is the economic dimension of sustainability.
In the token fishery, the stock doubles after each take up to a cap of 20. Why is taking 10 per round sustainable while taking 12 is not?
Taking 10 leaves 10, which doubles back to 20, so the same 10 is available every round: the harvest equals the regeneration. Taking 12 leaves 8, which doubles to 16; taking 12 again leaves 4, which doubles to 8; the next take of 12 exceeds the stock. Harvest above the regeneration rate shrinks the stock each round until it collapses.