🎯 Context

Last lesson you defined sustainability and saw what happened to the Newfoundland cod when catch limits were set from stock estimates that were far too high. This lesson is about how sustainability is measured: the indicators used to track it, the ecological footprint and biocapacity, the carbon and water footprints, and the volunteers who collect some of the data. By the end you should be able to use a named indicator to assess sustainability, judge whether a population is living sustainably by comparing its ecological footprint with its biocapacity, state what the carbon and water footprints measure and in which units, explain the role of citizen science, and calculate and compare your own footprint. Along the way you will use Country Profiles to compare countries' footprints for yourself.

Indicators: tracking sustainability over time

1.3.12 Sustainability indicators include quantitative measures of biodiversity, pollution, human population, climate change, material and carbon footprints, and others. These indicators can be applied on a range of scales, from local to global.

Consider the use of one named environmental indicator to assess sustainability.

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A sustainability indicator is a quantity measured in the same way again and again, so that you can see whether a system is moving towards or away from sustainability. Indicators cover biodiversity (for example the number of breeding birds), pollution (for example the concentration of nitrogen dioxide in city air), human population, climate change and footprints. The same kind of indicator can be used at very different scales: one tree, one lake, one country or the whole planet.

A useful indicator is:

measured in the same way each time, so that a change in the number is a change in the system and not in the method
recorded over a long period, so that a trend stands out from the ups and downs of single years
clearly linked to the thing you actually want to know about.

Local example: Geneva's official chestnut tree

Scatter graph of the date of the first leaf on Geneva's official chestnut trees each year from 1818 to about 2020, with dashed lines marking the change to a second, third and fourth tree

First-leaf dates of Geneva's official chestnut trees since 1818. The dashed lines mark each change of tree (marronnier). The pale horizontal line separates winter (hiver) from spring (printemps). Graph: Grand Conseil, République et canton de Genève.

Every year since 1818, the sautier of Geneva, an official of the cantonal parliament rather than a scientist, has recorded the day the first leaf opens on the Republic's official horse chestnut tree in the old town. The dates were written on parchment, and the record has never been interrupted.

A horse chestnut opens its leaves once late winter and early spring have been warm enough, so a warmer climate brings the date forward. Over two centuries the date has moved from April towards February, with a clear change after the 1950s. The earliest date ever recorded is 29 December 2002. The canton now calls the record "un véritable outil d'observation du réchauffement climatique": a real tool for observing climate warming.

The record has a weakness: it does not come from one tree. Four trees have been used: the first from 1818 to 1905, the second from 1905 to 1928, the third from 1928 until its last spring in 2015, and the fourth since 2016.

Sources: Grand Conseil, Le marronnier officiel; Grand Conseil, history of the official chestnut trees; swissinfo.ch, 25 January 2025.

1. Explain why the date on which a tree's first leaf opens can be used as an indicator of climate change. [2]

2. Using the graph, describe how the first-leaf dates of the first tree (1818 to 1905) differ from those of the third tree after 1950. [2]

3. Look at the dates recorded since the fourth tree took over in 2016. Suggest why changing the tree is a problem for anyone using this record as an indicator. [2]

Who collects the data: citizen science

1.3.16 Citizen science plays a role in monitoring Earth systems and whether resources are being used sustainably.

Citizen science has a role in the larger picture of scientific research on environmental systems. The information gathered is relevant to local problems and conditions, and can be used in research on global issues, such as climate change.

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