Climate

In this episode, the characters find themselves at the site of a climate protest, in which one of the four is involved. Their opinions differ on the question of the origin of global warming, which one of them believes could also be natural. They turned to a physicist present at the demonstration, whom they questioned on the subject. She invites them to carry out experiments in her laboratory to demonstrate both the concept of infrared radiation and the influence of CO₂ on it. Later, the characters stage these experiments in a street show at the heart of a new climate demonstration. They propose a representation of the Earth system's radiation balance, making it possible to pose the problem of the link between the addition of CO₂ , infrared radiation, and the consequences for temperature.

All the experiments depicted in the webcomic are made available in the form of photos and videos (in the editable documents associated with the proposed sequence in the last section). The choices that make up the proposed approach are based on pupils' points of view and difficulties identified in didactic research on the subject. By starting with experiments on the fundamental phenomena behind global warming, this approach aims to help build confidence in knowledge about the climate, ahead of the more technical considerations about climate models and simulations. Indeed, despite the scientific consensus on the entirely human origin of global warming, this is still questioned by a considerable proportion of the population.

Climate change cause and public opinion

According to an international EDF-IPSOS survey conducted in 2023 (24,000 people in 30 countries), 8% of those polled still deny the very existence of climate change, and 27% consider that it is mainly of natural origin. The proportion in even bigger (32%) for the 16-24 age group in 2022. In addition to this 32%, 10% of young people deny the reality of global warming. This total of 42% climate skepticism among 16-24 year-olds is particularly alarming for the teaching community.

Difficulties in understanding the greenhouse effect

A number of problems in interpreting greenhouse effect diagrams have been identified, some of which are due to problems inherent in the choice of representations themselves. These include the idea that greenhouse gases (GHGs) form a kind of 'layer' or 'lid' in the atmosphere, forming a 'barrier' against which some of the heat or radiation emitted by the Earth 'bounces' back towards it, thus 'trapping' 'energy from the Sun'. This is sometimes reinforced by the representation of infrared radiation "returning to Earth" in the manner of an optical reflection (which has nothing to do with the phenomenon of radiation emission by the atmosphere). This notion implicitly implies an idea of chronology, with radiation leaving the ground, being prevented from leaving by the atmosphere and then returning to the Earth's surface, helping to warm it up "again". This chronological type of reasoning is incompatible with the diagrams most often associated with instantaneous balances, i.e. where the values of the radiation powers are considered at the same time, and where the temperature is constant. Another problematic aspect, noted both by students and in popularisation resources, is the failure to distinguish between different types of radiation (solar, terrestrial infrared, atmospheric infrared). This contributes to the idea that radiation from the ground can 'bounce back' or be 'trapped'.The confusion between the different types of radiation can be linked to the difficulty of introducing the concept of infrared radiation, a point on which proposals are made later.

Can gases really interact with infrared radiation?

To demonstrate the emission and absorption capacity of infrared radiation, the most common method is to use images from infrared cameras with objects that are more or less hot and more or less transparent. However, these observations are only made with solid objects. From a physical point of view, gases do not share all the properties of solids, so why should they have the same properties when it comes to interactions with radiation? To what extent is extrapolation from solids to gases acceptable to students? A survey was carried out on the subject. After an introduction to the infrared radiation emitted or absorbed by solids, the students were asked for their a priori opinion of the capabilities of gases. The results were virtually the same for emission and absorption: more than 3/4 of secondary school pupils (N=208) and 2/3 of high school pupils (N=280) replied either that gases can neither emit nor absorb radiation, or that they did not know. Given this finding, the question arises of how to introduce these phenomena without giving them away, given the crucial nature of this point in making the link between CO2 emissions and global warming.

The first historical experiments highlighting the interaction between CO₂ and infrared radiation date back to the mid-19^ème century, independently by Eunice Newton Foot in the United States and John Tyndall in England. The first quantitative estimate of the possible influence of CO₂ on global temperature was proposed by Svante Arrhenius, a Swedish chemist, in his article "On the influence of carbonic acid in the air upon the temperature of the ground" (Arrhenius, 1896). It is based on the determination of the absorption spectrum of the atmosphere from observations of the Moon at different wavelengths, assuming that the Moon has an emission spectrum close to that of the Earth's surface.

 The first climate models emerged in the 1950s, and have since been perfected, taking into account more and more variables and the relationships between them. The relationship between the power of infrared radiation and the level of greenhouse gases is described locally at every point in the atmosphere using the so-called "radiative transfer" equations, which take into account the temperature at a given altitude. These equations, together with those for the other phenomena involved, are incorporated into the climate models. Using these models, it is possible to simulate the past climate without taking into account certain factors, in order to determine their influence relative to others. In particular, when we simulate climate change since 1850 without taking into account the increase in GHG and aerosol levels due to human activities, we obtain a globally stable mean temperature (see the blue curve in the figure below). When human factors are taken into account, the simulations obtained are very close to the change in mean temperature observed since 1850 (orange curve). These simulations show that all of the current warming is attributable to human activity.

The aim of the episode is to introduce the problem of the origin of global warming (chapter 1), and to provide some key elements to build the answer (chapters 2 and 3). The teaching approach chosen, both for the comic and the additional content, is based on three main choices, different from those most commonly used in teaching material on the subject.

Radiation balance between the Earth system and Space

The analysis of the Earth's radiation budget has been restricted to that of the Earth system as a whole, i.e. at the interface with Space.      

This is a different choice from many existing representations of the greenhouse effect, which also schematise the interactions between the atmosphere and the earth's surface. With such diagrams, it can be shown that it is not possible to draw any qualitative conclusions about the influence of the addition of greenhouse gases on temperature (see appendix A below). As well as being more easily accessible, the radiation balance at the interface with space means that we can avoid neglecting the many phenomena in the physics of the atmosphere that involve energy transfers other than those by radiation.     

Use of the concept of radiation power

Focusing on the balance of radiation entering and leaving Space allows a line of reasoning in which the concept of energy is not necessary. The notion of "radiation power" is in fact sufficient to reach a conclusion, and can be linked more easily to an empirical reference, by generalising the notion of luminosity. This didactic choice avoids adding to the complexity of the content the known difficulties in understanding energy. However, the proposed approach is compatible with a reformulation or a more in-depth study in terms of energy, in contexts where this is appropriate.

Identification of interactions between infrared radiation and CO₂

The core of the proposed teaching approach, set out in Chapters 2 and 3, is based on experiments to demonstrate the influence of CO₂ on infrared radiation, depending on its temperature. These experiments were carried out using a laboratory infrared camera whose sensitivity range (3-5 micrometres) overlaps part of the absorption/emission spectrum of CO₂ (which is not the case with the most common cameras sometimes available to schools). These observations make an empirical reference accessible to students with little or no knowledge of the concepts of wavelength and spectral intensity, based on the qualitative interpretation of infrared camera images, similar to those obtained for solids. The experiment is proposed using balloons of air and CO₂  at a lower temperature than the background fabric so as to create a situation analogous to the Earth system, where the CO₂ in the atmosphere is on average much colder than the Earth's surface. 

A more precise physical interpretation of the infrared observations of the balloons is available in the downloadable document below (see Appendix B).

The key step in the teaching approach is to extrapolate from these observations the influence of adding CO₂ to the atmosphere on the Earth's radiation balance. This extrapolation requires us to consider the potential effect of other characteristics of the atmosphere, relative to the situation of the balloons (for example, the fact that the atmosphere already contains a certain amount of CO₂ , as well as water vapour, or that its temperature decreases with altitude). Extrapolating the observation on the case of the cold CO₂ balloon relative to the case of the atmosphere therefore implies making the assumption that these differences do not qualitatively modify the direction of variation of the outgoing infrared power, even if all these characteristics influence the quantitative value of this decrease. While it is not necessarily possible to discuss all of these differences with the students, they can all be justified from a physical point of view, some of them in a fairly accessible way, as presented in Appendix C.

Radiation balance and conclusion on the temperature

Appendix A B C

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Introduction to infrared radiation

Since this concept is central to the proposed approach, how can it be introduced to pupils who are not familiar with it? Although they are generally familiar with infrared camera images, they generally think that this one allows them to "see the temperature". The observation below casts doubt on this interpretation: if this were the case, the glass and plastic cover should appear blue.

Pupils sometimes assume that the glass and plastic cover have been heated by the warmer body nearby. Videos of movements show that this is not the case, such as those available in the slides of the proposed teaching sequence below.

These observations show that what the camera detects has two things in common with light: the ability to reflect (1) and to pass through certain materials (2). These points in common allow us to introduce a new concept: "radiation" as a generalisation of "light". Light can then be considered as a case of "visible" radiation (in the sense that it is perceptible by our eyes), whereas the type of radiation emitted by an object is only perceptible with this type of camera. Just as light can be more or less powerful, invisible radiation can also be more or less powerful. The notion of "luminosity" for light (or "luminous power") can be generalised as "radiation power". The case of a metal heated to incandescence illustrates the link between visible and invisible radiation:

Above a certain temperature, the metal emits both visible radiation (1) and invisible radiation (2), which is detected by the camera. The further you move away from the flame, the darker red the light emitted (3).  After this red zone, only the invisible radiation (2) remains, which is called "infrared" radiation. The camera used to detect this radiation can therefore be called an "infrared camera". It measures the power of the infrared radiation at each point in the image, which is represented by a colour hue. The power of the infrared radiation emitted by an object increases with its temperature.

Photos and videos of the various infrared experiments featured in the comic are available in the teaching sequence slides below.

Prerequisites

The approach proposed aims to be accessible with a minimum of physics pre-requisites. It requires knowledge of the following phenomena:
- Knowledge of the mechanism of vision (light going from an object to the eye), which we know is not self-evident for young pupils (de Hosson & Kaminski, 2006). An understanding of this phenomenon is necessary if the images obtained with the infrared camera are to be interpreted in a similar way.
- Understand the Earth system and outer space: the fact that the atmosphere is a mixture of gases, including CO₂ , on average colder than the surface, and that outer space is empty of matter.
- CO₂ is a gas emitted into the atmosphere by human activity, primarily through the combustion of fossil fuels.

Be able to make the qualitative link between a colour scale, the values of a physical quantity (radiation power) and the width of an arrow representing this quantity.        

To get started

A teaching sequence can start with individual reading of the whole episode before the first session, or just chapter 1 in class as a group reading.

Chapter 1 & Position of the problem

The rest of the work will revolve around the following question, posed in Chapter 1: how can we explain the role of CO₂ in global warming? This question can be split into two intermediate questions:
- What are the possible causes of object warming in general?        
- How could CO₂ influence these causes?

The first question introduces the concept of infrared radiation and the radiation balance, while the second allows you to hypothesise about the effect of adding CO₂ on this balance, leading on to the experiments on the CO balloon₂ observed in infrared.

Introduction to infrared radiation & chapter 2

Following a gradual construction of the concept as presented above (complementary resource), reading chapter 2 in class can provide other experimental situations for the pupils to study, with glasses of cold and hot water (all the experiments represented in the comic strip are available in the form of photos and videos in the editable documents associated with the suggested sequence). This situation is an opportunity to get the pupils to work on representing the power of radiation in the form of an arrow.

At the end of Chapter 2, the balloon experiment is introduced. The students can be asked to interpret these as an exercise. The beginning of chapter 3 can then be seen as a correction or repeat of this exercise, before deducing the consequences for the Earth's temperature.

Representation of the Earth System's radiation balance

In order to answer the question posed in Chapter 3, we need to look at the concept of radiation balance. To do this, we can return to the initial question, applied to the Earth: what are the possible causes of global warming? We know that ground temperature depends on exposure to solar radiation, and also that there is a link between the temperature of an object and the power of the infrared radiation it emits. We can ask ourselves what the link is between these different types of radiation and temperature. This leads us to draw up a balance of all the radiation emitted and received by a system, called a "radiation balance", in order to compare their power. The analogy and experiment presented above (conceptual approach section) help us to understand the link between the nature of the balance (balanced or unbalanced) and changes in the system's temperature.

Chapter 3 & progressive clues to answer the final question

All the elements introduced so far help us to understand the influence of CO₂ on the Earth's temperature. The final pages of chapter 3 (p.81-90) provide progressive clues as to the consequences of adding CO₂  to the atmosphere. One way of working on this last part is to do a group reading of this last chapter (episode projected on the blackboard), leaving the pupils to find the answer, and gradually adding additional information to help them if they are unable to do so. To do this, the pupils should have the intermediate conclusions of the sequence available.   

Editable file to download [coming soon!]