The greenhouse effect, at the origin of the phenomenon of global warming, is difficult to show by simple experiments. Many proposals are however available on the internet but few are really reproducible and many give poor physical representations of the phenomenon [1] . This article offers practical methods and accessible tools to help teachers and educators explain this phenomenon in an engaging way.
A popular greenhouse effect experiment - The challenges of the greenhouse effect in a bottle - Measuring instruments - Infrared radiation - Conduction and absorption - Black body radiation - Albedo - Identification of greenhouse gases - CO2 absorption spectrum - Cooling of the stratosphere
A popular greenhouse effect experiment
What is the ideal experiment to demonstrate the greenhouse effect? By typing "greenhouse effect" into a search engine, a suggestion often comes up: compare the temperatures of two glass jars, one open and the other closed, each containing a black sheet of paper at the bottom and exposed to sun. When one of the jars is closed with glass or plastic, the temperature in the closed jar increases more quickly than that of the open jar, reaching up to 4 to 5 degrees warmer.
It is well established that this simple experiment does not offer a realistic representation of the phenomena at work in the terrestrial greenhouse effect. Indeed, if the temperature is higher in the closed jar, it is essentially because of the suppression of the convection currents which cool the air in the open jar and not from the radiative effect responsible for the warming of our planet. . This argument had also been proposed as early as 1774 by the Swiss scientist Horace-Bénédict de Saussure and confirmed in 1909 by the scientist Robert Williams Wood who showed that even by replacing the glass of a greenhouse with halite, transparent to infrared, the temperature increased similarly, proving that the observed effect is mainly related to convection.
If this experiment continues to be presented in class, it is not because it offers a good representation of the phenomenon, but because it provides a simple and visual analogy useful for introducing the basic concepts of the greenhouse effect. , particularly aimed at the youngest. It is useful from an educational point of view but does not prove anything regarding global warming.
In the following we will see that it is possible to offer more accurate representations of the greenhouse effect and, with a little commitment, to show experimentally the radiative effect of greenhouse gases. This sometimes somewhat difficult task is largely rewarded by the intimate and precise understanding that we can then have of the phenomenon. John Tyndall thus recalls in one of his articles that he carried out more than 10,000 different experiments to finally prove in 1859 that carbon dioxide was a greenhouse gas [4] .
The challenges of the greenhouse effect in a bottle
The greenhouse effect phenomenon which creates global warming is well known. It is due to the fact that certain gases in the Earth's atmosphere, such as carbon dioxide (CO₂), methane (CH₄) and water vapor, absorb and re-emit infrared radiation emitted by the Earth's surface. It is this radiative effect that the educator or young scientist seeks to highlight through experimental activities. However, this effect is difficult to demonstrate with simple experiments, what we would call a greenhouse effect in a bottle.
The first obstacle that the observer encounters is that the transmission of heat results from three different phenomena: conduction, convection and radiation. These phenomena coexist in most greenhouse experiments, making it very difficult to isolate individual effects with simple equipment[2] .
This difficulty is increased by the fact that the heating due to radiation is relatively weak compared to other mechanisms. A radiative effect experiment involving a 20 cm layer of CO₂ would at best produce only one degree of warming, mostly less than the impact of convection or conduction [3] .
Due to the small increase in heat caused by radiative phenomena, the reproducibility of the experiments is difficult. The smallest variations in the distance from light sources, in the choice of materials, in the power of the lamps, in the concentration of the gases used, can have a significant impact on the results obtained.
Finally, the Earth's atmosphere is a complex environment structured into several distinct layers. Each layer has specific characteristics that contribute to the greenhouse effect, and significant convection phenomena distribute heat evenly around the globe. Modeling such an environment in a bottle is impossible [6] .
For all these reasons, there is no simple and irrefutable experiment that can be carried out in class or at home which demonstrates the “climatic” greenhouse effect as a whole. However, it is possible to show different aspects of global warming by analogy or by measuring specific effects as we will discover.
Measuring instruments
What measuring instruments should we use to show the greenhouse effect and carry out the experiments that we describe in the rest of this article?
The simplest instrument to use and available is the thermometer. For more precise and long-term measurements, a digital thermometer, ideally connected to a computer or a smartphone to carry out EXAO (Computer Aided Experimentation), is preferable. The thermometer, however, has several limitations: its reaction time is long, its placement in the oven is crucial and it does not allow the radiation to be measured directly.
To measure infrared radiation, we prefer to use a thermopile, invented by the Italian physicist Leopoldo Nobili at the beginning of the 19th century . Composed of several thermocouples in series, they allow the temperature of a surface to be measured remotely for temperatures ranging from -20°C to 350 °C. Thermopiles are present in infrared thermometers, but also available as an external sensor for completely reduced prices.
Another instrument for measuring infrared rays but more expensive is the infrared camera, capable of analyzing the radiation of objects in the form of an image. Infrared cameras can today be plugged into a smartphone to produce infrared photos or videos. These new, extremely practical devices make it possible to analyze phenomena on video.
In addition to temperature measurements, humidity sensors, CO₂ sensors can be used to measure concentrations, and photoelectric cells to compare the light intensities received or measure the albedo of materials.
All these sensors exist independently but are generally quite expensive, especially for EXAO. One solution is to use sensors for hobbyists, connected to a microcontroller such as an Arduino, ESP32 or Microbit. Data can be easily analyzed by connecting the controllers to the FizziQ app via Bluetooth , allowing data to be recorded and analyzed in experiment notebooks.
For those who are not comfortable with microcontrollers, we have developed the FizziQ Connect environment, which allows EXAO analyzes to be carried out at a reduced cost compared to other educational solutions. FizziQ Connect uses M5 Stack sensors , which are inexpensive to purchase and sufficiently precise in this context. A wide selection is available from other manufacturers like Seed Studio .
Infrared radiation
To understand the mechanism of the earth's greenhouse effect, it is essential to better understand infrared radiation, or "heat radiation" as the British astronomer William Herschel called it in 1800. By conducting an experiment with a prism to decompose sunlight into a spectrum of colors, Herschel observed that the temperature increased beyond red light, in an area where no visible light was present.
Although infrared radiation is not visible to the naked eye, we can feel it as heat through thermoreceptors located in our skin . Some animals, such as vipers, pythons and boas, have heat-sensitive pits on their heads, allowing them to detect infrared heat emitted by warm-blooded prey. This gives them a significant advantage when hunting at night. Smartphone camera sensors are also sensitive to infrared rays, but to produce an image close to what the human eye sees,manufacturers add infrared blocking filters . However, some smartphones have less efficient or broader spectrum filters, such as many low-end Android smartphones. An opportunity to visualize infrared rays!
In an area with moderate lighting, open your smartphone's camera app and point it at the infrared emitter on a TV remote control. Press a button on the remote control while viewing the transmitter through the smartphone screen. You will see a flashing light coming from the transmitter, visible on the screen even though it is invisible to the naked eye. If your smartphone does not detect infrared, this means that the camera's infrared filter is calibrated to give an image as close as possible to what the human eye perceives.
Why do remote controls use infrared rays rather than other types of radiation? Several reasons explain this choice: technological simplicity and reduced cost, but also specific characteristics of infrared rays. These are invisible to the naked eye, harmless to health, have a limited range and are sufficiently directional to allow precise control of the devices without interfering with other nearby electronic devices.
Conduction and absorption
One of the fundamental advances in the theory of global warming is the discovery of the interaction between certain gases, called greenhouse gases, and infrared rays [14] . We will see in other experiments how to demonstrate this phenomenon, but to understand it, it is simpler to experiment with solid surfaces. Indeed, while certain materials such as glass or Plexiglas block infrared rays (or rather absorb them), others such as transparent low density polyethylene (LDPE) allow infrared rays to pass through. Some materials even allow infrared rays to pass through while blocking visible radiation.
To highlight these notions of transparency, we can carry out the following experiment with a thermopile or an infrared digital thermometer [5] . A cup is filled with hot water and a surface is inserted between the thermopile (MLX90614 circuit) and the infrared radiation source. We choose three different materials: glass plate, transparent packaging bag and trash bag. The results in the experiment we conducted are as follows: without material: 23.9 degrees, with glass: 18 degrees, with transparent plastic: 23.2 degrees, and with the trash bag: 21.8 degrees. The polypropylene of the cling film therefore allows 90% of infrared rays to pass through, while the colored one of the garbage bags only allows 65% to pass and the glass is opaque.
This experiment allows us to understand how the atmosphere lets visible rays through, but blocks ultraviolet rays, dangerous to humans, and certain frequencies of infrared rays.
Black body radiation
The Earth's atmosphere allows visible rays (and some infrared rays) to pass through, which are then absorbed by the Earth's surface. The latter then re-emits infrared radiation, according to the black body principle. A black body is a theoretical object in physics that perfectly absorbs all incident electromagnetic radiation, without reflecting or transmitting any. It emits electromagnetic radiation called black body radiation, which depends only on its temperature and not its composition. This radiation follows Planck's law, describing the spectral distribution of the energy emitted.
At temperatures below 500 degrees Celsius, a black body emits infrared rays invisible to the naked eye. However, as the temperature increases, the amount of radiation emitted in the visible spectrum also increases, making the black body visible. Wien's law allows us to determine the wavelength at which radiation is maximum.
To visualize the black body effect, we can illuminate a black cardboard with an LED lamp and measure the temperature of the cardboard and the lamp with a thermopile (or an infrared temperature detector). It can be seen that the temperature of the lamp is slightly higher than that of the atmosphere. Indeed, LED lamps produce very little heat, and are therefore very efficient. On the other hand, the temperature of the black cardboard is higher than that of the lamp, because the cardboard has absorbed all the visible rays and re-emits infrared rays.
Albedo
A black body absorbs all radiation, but in reality only part is absorbed by physical bodies. This ability to reflect incident light is albedo. Used primarily in astronomy and climatology, albedo is between 0 and 1, where 0 means the surface absorbs all light and 1 means it reflects all light. A material with a high albedo, such as snow or ice, reflects the majority of light, contributing to local cooling. In contrast, a surface with a low albedo, such as the ocean or an asphalt road, reflects less light and absorbs some of it. This fraction of absorbed light is converted into heat, thereby increasing the surface temperature. It is for this reason that the IPCC (Intergovernmental Panel on Climate) states that “painting roofs white would save 1Gt/year of greenhouse gas emissions, the equivalent of 250 million vehicles. A very old solution since in ancient times the Egyptians painted their buildings white to reflect the heat of the sun, and the Romans used marble and other reflective materials in building structures.
To understand the effect of albedo on temperature, gather a powerful lamp (60W), connected thermometers or sensors, and materials of different colors (yellow, red, orange modeling clay, and gray and black aluminum bottles filled with water). Place the materials under the lamp keeping the same distance, tilt and orientation. Turn on the lamp for 10 minutes. Use the FizziQ app to measure luminance, a measure of reflected light, and calculate albedo as the ratio of the luminance of a surface compared to a sheet of white paper. After exposure, measure temperatures and observe that dark materials (red, black) absorb more light and heat more than light materials (yellow, gray), demonstrating the impact of albedo on temperature. This experience is described in detail on the website of our partner La main à la pâte by following this link .
From the above we can estimate the albedo of the earth. By proposing a distribution of the different colored surfaces of the terrestrial globe, we can get an idea of the shape of the earth. We will compare this value to the estimate of 0.3 which is generally used to estimate what the temperature of the globe would be in the absence of a greenhouse effect, i.e. -18 degrees.
Identification of greenhouse gases
In 1856, the experimentalist Eunice Foote published a paper in the annals of the American Association for the Advancement of Science in which she compared the relative heating of jars filled with air, CO2 and water vapor. She notes that jars filled with CO2 and water vapor heat up more quickly and concludes with a prophetic sentence: "An atmosphere filled with this gas (CO2) would give our Earth a higher temperature" [13] .
We know today that the experiment as carried out does not make it possible to explain the "climatic" greenhouse effect, and is mainly explained by the differences in different density between air, CO2, and conduction and convection effects in glass jars absorbing infrared rays. However, this pioneer's intuition was correct and CO2 is identified as a greenhouse gas, that is to say a gas which absorbs certain infrared rays and rebroadcasts them.
Many other experiments found on the internet also claim to be able to demonstrate the effect of CO2 as a greenhouse gas. Many of these experiments are not reproducible or give false results . In these experiments the effects of convection and conduction are not evaluated although they are dominant compared to the radiative effect due to the absorption of infrared rays for greenhouse gases [2] [7] [10] .
On the other hand, the following protocol gives completely interpretable results. It consists of comparing two gases having similar physical characteristics but one of which is not a greenhouse gas. We will then have three measurements at our disposal which will also allow us to evaluate the convective effect. A commonly used gas is argon, which is an inert gas and has characteristics close to those of carbon dioxide. The temperature differences due to the radiative effect are of the order of a few tenths of degrees, the precision of the measurements is therefore very important.
In the example in the photo above, we took a plastic bottle cut and lined with black paper, illuminated by a 100 W spotlight at a height of 60 cm. A probe is placed inside, protected by a piece of aluminum to avoid direct radiation from the lamp and an exterior probe is placed 1.5 m away as a reference. At equilibrium, we measure the temperature difference ΔT between the internal probe and the reference. We note a radiative effect of 0.5°C and a convection effect of 0.4°C.
CO2 absorption spectrum
The CO₂ molecule absorbs infrared rays due to its vibration modes, including asymmetric stretching and bending vibrations. When the atoms in the molecule vibrate in a way that changes the dipole moment, they can interact with infrared radiation. These vibrations allow the CO₂ molecule to absorb and re-emit infrared energy. However, the CO2 molecule does not absorb all frequencies. As John Tyndall showed with the development of the first absorbance spectrum of different gases, CO2 has different absorption bands in the mid and far infrared (4 micrometers and 15 micrometers). Other compounds in the atmosphere such as water vapor also contribute to global warming by absorbing other frequencies of infrared rays, notably mid-infrared around 6.3 micrometers.
To visualize this absorption we can carry out the following experiment [11] . We inflate a balloon with CO2 using baking soda and vinegar, and we inflate another balloon with air. Then we measure the temperature of a candle flame placed behind the balloon using an infrared camera. We note that the maximum temperature of the flame decreases when using a balloon filled with CO2, due to the fact that infrared radiation is absorbed by the CO2 present in the balloon.
It is also possible to reproduce Tyndal's experiment by constructing a chamber closed at one end and in which a thermopile is placed. In front of this device is placed a candle which creates infrared rays. Carbon dioxide or air placed in a balloon is brought into this chamber. We compare the impact of air and CO2. This precise equipment allowing perfectly reproducible conditions is the ideal way to study the effect of infrared rays on CO2.
Cooling of the stratosphere
One of the most convincing predictions from modeling the effect of global warming is the cooling of the stratosphere.
The stratosphere is the second layer of the Earth's atmosphere, located above the troposphere and extending from 10 to 50 kilometers above sea level. It is characterized by a progressive increase in temperature with altitude, due to the absorption of ultraviolet (UV) rays by ozone. This layer is essential for protecting life on Earth because it contains the ozone layer, which absorbs the majority of harmful UV rays from the sun. Unlike the troposphere, the stratosphere is relatively stable, with little vertical air movement.
In 1967, scientists Syukuro Manabe and Richard Wetherald carried out the first computer modeling of the impact of a doubling of the concentration of CO2 in the atmosphere. They calculate that the greenhouse effect would cause a warming of the troposphere but also, and more surprisingly, a cooling of the stratosphere. Indeed, if there are more greenhouse gases in the stratosphere, it, which behaves like a black body, will emit more infrared radiation both towards the sky and towards the earth. But as less heat reaches the stratosphere since this heat is captured in the layers of the troposphere, this atmospheric layer cools because it emits more radiation than it receives.
This contrast between the warming of the troposphere and the cooling of the stratosphere is a clear signature of the impact of human activities on the climate and the predictions of Manabe and Wetherald have been confirmed by measurements by satellites and sounding balloons.
To experiment with this particular effect we can use with a solar pool. A solar pool, or solar pool, is a container in which highly salty water has been placed and which is placed in the sun. The density of water increases with salinity because dissolved salts add mass to the water without significantly increasing its volume. Layers in the water column naturally stratify according to their density, with denser (more salty) layers at the bottom and less dense (less salty or fresh) layers at the top. This stratification creates a stable situation where the heavier layers remain at the bottom, preventing convective movements that could mix the layers.
In solar basins, this stratification is exploited to create a stable thermal gradient. The upper layer, not very salty, acts as thermal insulation. The middle layer, with a salinity gradient, prevents convection movements, thereby trapping heat. The lower layer, very salty and dense, absorbs and stores solar heat. This configuration allows the solar basin to effectively retain heat, preventing layer mixing and maximizing the absorption and storage of solar energy. This trapped heat can then be extracted using heat exchangers and used for various applications such as space heating, industrial processes or power generation. The solar pond captures and retains solar energy simply and efficiently, providing an effective method for storing and using thermal energy.
What we see and which allows us to better understand the cooling effect of the stratosphere is that if we increase the concentration of salt, at constant illumination, the temperature of the water surface decreases. This is because more energy is trapped at the bottom of the pool, and less energy is available to heat the pool surface.
Conclusion
Understanding and demonstrating the greenhouse effect, the key to global warming, is a major experimental challenge. Although many experiments are available, few are faithfully reproducible. However, it is possible with inexpensive equipment to carry out completely realistic experiments which make it possible to show different aspects of the greenhouse effect phenomenon [11]. [12] . Combined approaches and analogies make it possible to grasp the essential aspects of global warming, highlighting the importance of rigorous and varied scientific teaching.
Learn more
[1] http://climatechangeeducation.org/hands-on/difficulties/heating_greenhouse_gases/sound_examples/