Last lesson you judged progress on sustainability with the SDGs, and met the SDG wedding cake, a picture that shows the economy and society resting on the biosphere. That picture is a model, and so is almost everything else you have used in this course so far. This lesson is about models themselves: what they are, why every one of them is less accurate than the reality it stands for, and how to judge whether one is still useful. You will then apply that to the planetary boundaries model. By the end you should be able to define a model and name its different forms, explain why simplifying a model costs accuracy, use population projections to show how models can give very different results, identify which planetary boundaries have been crossed and what pushed them, and outline the uses and limitations of the planetary boundaries model.
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1.2.13 A model is a simplified representation of reality; it can be used to understand how a system works and to predict how it will respond to change.
A model may take many forms, including a graph, a diagram, an equation, a simulation or words. Models are used throughout the course to represent systems and processes.
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A model is a simplified representation of reality. Nobody can hold a whole forest, a whole economy or the whole Earth in their head at once, so we build something smaller that keeps the parts we care about and leaves the rest out.
Models do two jobs. They help you understand how a system works, by showing its parts and how they connect. They also help you predict how the system will respond to change: what happens to a planet's temperature if its sun brightens, or to a lake if more fertiliser washes into it.
A model can take many forms:
Some models are physical objects. Biosphere 2 was a real building, filled with real plants and animals, built to behave like the Earth's ecosystems.
You have been using models since the first lesson of this course, mostly without calling them that.
1. Explain why a systems diagram of a tree is a model. [2]
2. Fill in the table for the models you have already used in this course. The first row is done for you.
| Model | Where you met it | Form | What it helps you understand or predict |
|---|---|---|---|
| Systems diagram of a tree | foundation.2.1 and 2.2 | Diagram | How energy and matter enter and leave a tree, and where they are stored |
| Gaia hypothesis | foundation.2.2 | ||
| Biosphere 2 | foundation.2.2 | ||
| Daisyworld | foundation.2.3 | ||
| Strong sustainability diagram | foundation.3.1 | ||
| Ecological footprint | foundation.3.2 | ||
| SDG wedding cake | foundation.3.3 |
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1.2.14 Simplification of a model involves approximation and, therefore, loss of accuracy.
The simplification of a model can make it less accurate. The systems approach uses models throughout the course. For example, predictive models of climate change or projections of human population growth explain how a model may give very different results. Simplification will also affect how well a lab-based model ecosystem approximates to a natural ecosystem.
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Every model leaves things out, which is what makes it simple enough to use and also what makes it wrong in places. A model is only ever an approximation of the real thing.
In foundation.2.2 you met Biosphere 2 in Arizona as an example of a closed system. It was also a model: a set of small ecosystems, including a rainforest, a savannah, a desert and a coral reef, built to behave like the real ones. Its current deputy director, John Adams, says that none of its ecosystems was a perfect model of its real-world counterpart. Three of the things it left out, or got wrong, changed what happened inside.
Biosphere 2 is now run by the University of Arizona as a research facility, and the simplification has become its strength. Scientists can do things inside it that are impossible in a real forest, such as sending a whole grown rainforest into a 70-day drought on purpose and measuring how every tree responds.
Source: BBC Future, How the Biosphere 2 experiment changed our understanding of the Earth, 3 July 2025.
3. Identify two features of a natural ecosystem that Biosphere 2 left out. [2]
4. Explain how one of these simplifications made Biosphere 2 a less accurate model of a real ecosystem. [2]
5. Suggest why scientists still use Biosphere 2 to study how rainforests respond to drought, even though it is not a perfect copy of a real forest. [2]
The guide's second example is projections of human population growth. Demographers build models of how birth rates, death rates and migration will change, then run them forward to predict the population in the future. Three teams have asked the same question: how many people will be alive in 2100?
The IHME also published its own range of uncertainty: its population for 2100 could be anywhere from 6.83 to 11.8 billion. Every model simplifies billions of decisions that people have not made yet, and each team simplifies in a different way.
Projected world population in 2100 from three models, with the mid-2024 world population for comparison. Chart made for this lesson from the sources below.
Sources: UN News, World population projected to peak at 10.3 billion, July 2024; Vollset et al., The Lancet, 14 July 2020; The Guardian, 27 March 2023, reporting Earth4All. Chart made for this lesson.
6. Using the chart, describe the difference between the three projections for 2100. Use figures. [2]
7. Explain why three models of the same population give such different results. [3]
8. A government is planning schools and hospitals for the year 2100. Suggest how it should use three projections that disagree. [2]