The greenhouse effect is the process by which gases in the atmosphere absorb and then re-emit the infrared radiation from the ground, warming the Earth’s surface. Natural and indispensable, it raises the average temperature from −18 °C to +15 °C.
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How to measure it in class
The smartphone’s lux meter and camera make it possible to approach experimentally the two building blocks of the mechanism: infrared radiation and the albedo of surfaces.
Steps:
- Point a television remote control at the smartphone’s front camera and press a button: the LED appears on the screen even though it is invisible to the eye. The sensor sees near infrared, the eye does not.
- With FizziQ’s lux meter, measure the illuminance reflected by cardboard sheets of different colors lit in an identical way. Calculate the ratio to the incident illuminance: you obtain a relative albedo.
- Place a thermometer under a lamp behind a glass plate, then behind a thin plastic film, and compare the temperatures recorded in FizziQ’s experiment notebook.
- Reproduce Herschel’s experiment: disperse the light of a lamp with a prism and record the temperature just beyond the red, where the eye no longer sees anything.
- Record all the measurements in the experiment notebook to compare light and dark surfaces and discuss the role of albedo in the Earth’s energy balance.
Scientific activities on this topic
The greenhouse effect is difficult to demonstrate with simple experiments. Many proposals are available on the internet, but few are really reproducible and many give poor physical representations of the phenomenon.
- Radiative effect of CO₂: demonstrate experimentally the absorption of infrared radiation by carbon dioxide (high school and above).
- Solar constant by pyrheliometer
- Beer-Lambert law with a lux meter
We explain all this in our article on the greenhouse effect, where we propose 7 experiments that work and can be carried out with inexpensive equipment:
Possible extensions with FizziQ: visualizing infrared rays with a smartphone, how a surface absorbs or transmits infrared rays, black body radiation, calculating the Earth’s albedo, demonstrating the radiative effect of a greenhouse gas, identifying the infrared absorption frequencies of CO₂ and recreating the cooling effect of the stratosphere due to climate warming.
Learn more
The greenhouse effect is first of all a natural phenomenon, as old as the atmosphere. It keeps the Earth’s surface at an average temperature of about 15 °C, whereas a body receiving the same solar radiation without an absorbing atmosphere would be at about −18 °C. Without the greenhouse effect, liquid water and life as we know it would not exist. What causes the current climate warming is its intensification: by releasing carbon dioxide and methane, human activities increase the absorption of infrared radiation and shift the equilibrium toward higher temperatures. To understand this process properly, it is essential to explore several aspects: infrared rays, the transformation of visible rays into infrared, albedo, the radiative effect of greenhouse gases, the variability of this effect with frequency, and the cooling of the stratosphere.
Infrared Rays
Infrared rays (IR) are a form of electromagnetic radiation with wavelengths longer than visible light but shorter than microwaves. Discovered by the astronomer William Herschel in 1800, they are often called “thermal radiation” because they are associated with the heat we feel. Although infrared rays are not visible to the naked eye, they can be detected by thermal sensors and are emitted by all objects according to their temperature.
The Transformation of Visible Rays into Infrared Rays by the Black Body
When a surface, such as the Earth’s, is exposed to visible sunlight, it absorbs this energy and heats up. This phenomenon is described by the black body concept in physics, a theoretical object that perfectly absorbs all electromagnetic radiation. Once heated, this surface in turn emits radiation, mainly in the form of infrared rays. This re-emission of energy as infrared rays is a key element of the greenhouse effect mechanism.
Albedo and the Warming of Dark Surfaces
Not all surfaces return the same proportion of the sunlight they receive. This reflected fraction is called albedo: it is indeed a reflection of the incident light, not to be confused with the re-emission of infrared by the heated body, which belongs to thermal radiation. Albedo is the measure of a surface’s reflectivity. It varies between 0 (no reflection, totally absorbing surface) and 1 (total reflection, completely reflective surface). Dark surfaces, such as oceans or forests, have a low albedo and absorb more sunlight, thus contributing to greater warming of the Earth. Conversely, light surfaces, such as ice and snow, have a high albedo and reflect a large part of the sunlight, contributing to localized cooling. This difference in the absorption of sunlight plays a significant role in regulating the Earth’s temperature.
The Radiative Effect of Greenhouse Gases
Greenhouse gases, such as water vapor (H₂O), carbon dioxide (CO₂) and methane (CH₄), play a central role in the planet’s energy balance. The atmosphere is almost transparent to visible solar radiation, which therefore reaches the ground and heats it. The ground re-emits this energy as infrared radiation, which these gases absorb and then re-emit in all directions, including downward. It is this asymmetry between an atmosphere that is transparent in the visible and absorbing in the infrared that constitutes the greenhouse effect. Water vapor really is a gas, not to be confused with the liquid water droplets of clouds and mist, and it is the leading natural contributor to the greenhouse effect. Its quantity in the atmosphere, however, is not directly controlled by human activities: it depends on temperature, which makes it an amplifier of the variations caused by CO₂ rather than an initial cause.
Infrared Absorption Frequencies
The absorption of infrared rays by greenhouse gases is not uniform and depends on the frequency of the radiation. Each gas has specific absorption bands where it is particularly effective at absorbing infrared energy. For example, CO₂ absorbs strongly at wavelengths around 4.3 and 15 micrometers. Water vapor, for its part, absorbs in several bands, notably around 2.7, 6.3 and 19 micrometers. This variability means that different greenhouse gases contribute differently to climate warming depending on their spectral characteristics. Understanding these differences is essential for predicting and modeling the impact of the various gases on the climate.
The Cooling of the Stratosphere
One of the consequences of the greenhouse effect is the cooling of the stratosphere, the layer of the atmosphere located above the troposphere. This effect was predicted by the first climate warming models (1967, Syukuro Manabe and Richard Wetherald) but was only proven, thanks to weather satellites, more than twenty years later. The increase in greenhouse gases in the troposphere retains more heat near the Earth’s surface, which reduces the amount of heat reaching the stratosphere. As a result, while the troposphere warms, the stratosphere tends to cool. This phenomenon was predicted by climate models and confirmed by satellite observations. Stratospheric cooling is a characteristic signature of human influence on the climate.
A misleading name
The expression “greenhouse effect” is an analogy, and it is physically inaccurate. A real garden greenhouse warms up mainly because its walls prevent warm air from escaping: they block convection. The glazing does absorb a little infrared, but that is not the dominant mechanism. The atmospheric greenhouse effect, for its part, is purely radiative: there is no wall above the atmosphere, and cooling takes place by radiation toward space. The analogy was proposed in the 19th century, before the physics of radiation was established, and the name has stuck. Fourier in 1824, Tyndall in 1859 and then Arrhenius in 1896 laid the foundations of the real mechanism.
Orders of magnitude
Solar constant at the top of the atmosphere: 1,361 W/m². Average albedo of the Earth: about 0.30, which gives an equilibrium temperature without greenhouse effect of −18 °C. Actual average temperature at the ground: about 15 °C, that is a natural greenhouse effect of 33 °C. Albedo of fresh snow: 0.80 to 0.90; of an ocean: 0.06; of a forest: 0.10 to 0.15. CO₂ concentration: 280 ppm before the industrial era, more than 420 ppm today. Radiative forcing associated with human activities since 1750: about 2.7 W/m². Warming observed since the pre-industrial era: about 1.2 °C.
Formula
The flux radiated by a surface depends on its temperature according to the Stefan-Boltzmann law:
P/S = σ × T⁴
where:
- P/S: power radiated per unit of surface (W/m²)
- σ: Stefan-Boltzmann constant, 5.67 × 10⁻⁸ W·m⁻²·K⁻⁴
- T: absolute temperature of the surface (K)
The wavelength of the emission maximum is given by Wien’s law:
λ_max × T = 2.90 × 10⁻³ m·K
where:
- λ_max: wavelength of the emission maximum (m)
- T: absolute temperature of the body (K)
The radiative balance of a planet without an absorbing atmosphere is written, at equilibrium:
σ × T_e⁴ = (1 − A) × E / 4
where:
- T_e: equilibrium temperature of the planet (K)
- A: average albedo of the planet (dimensionless)
- E: solar irradiance received, 1,361 W/m² for the Earth
- the factor 4 comes from the ratio between the surface of the sphere and that of the intercepted disk
For the Earth, this calculation gives T_e ≈ 255 K, that is −18 °C. The 33 °C gap with the actual temperature measures the natural greenhouse effect.
Application examples
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The Earth receives 1,361 W/m² and reflects 30 % of it: its calculated equilibrium temperature is −18 °C, compared with the +15 °C observed, the difference being due to the greenhouse effect
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Venus, whose atmosphere is composed of 96 % CO₂ at 92 bar, has a surface temperature of 465 °C, well above that of Mercury, which is nevertheless closer to the Sun
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Mars, with a very thin CO₂ atmosphere, benefits from a greenhouse effect of only about 5 °C
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The Earth’s ground at 15 °C radiates at a maximum around 10 µm, in the far infrared, exactly where CO₂ and water vapor absorb
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The Sun, at 5,800 K, radiates at a maximum around 500 nm, in the visible, which the atmosphere lets through
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A clear, dry night is colder than a cloudy night: without water vapor or clouds, the ground radiates directly toward space
FAQ
Q: Is the greenhouse effect a harmful phenomenon? A: No, it is natural and indispensable. Without it, the Earth’s average temperature would be about −18 °C and liquid water would not exist at the surface. The climate problem comes from its intensification by human emissions of CO₂ and methane, which add a few degrees to an equilibrium that had been stable for millennia.
Q: Why does the atmosphere let sunlight through but not the infrared from the ground? A: Because the two radiations do not have the same wavelengths. The Sun, which is very hot, emits mainly in the visible, where atmospheric gases hardly absorb at all. The ground, which is much colder, emits around 10 µm, a range where CO₂ and water vapor have absorption bands.
Q: Does a garden greenhouse really work like the atmospheric greenhouse effect? A: No, and that is what makes the name misleading. A greenhouse heats up mainly because its walls prevent warm air from escaping, that is, by blocking convection. The atmospheric greenhouse effect is a radiative mechanism, with no wall at all.
Q: If water vapor is the main greenhouse gas, why do we mostly talk about CO₂? A: Because their roles differ. The quantity of water vapor in the air is set by temperature and adjusts within a few days: it amplifies a variation, it does not trigger it. Emitted CO₂, for its part, remains in the atmosphere for centuries and acts as the control knob of the system.
Q: Can the greenhouse effect be demonstrated with a simple bottle filled with CO₂? A: With great caution. Many commercial setups actually measure a difference in convection or thermal insulation, not a radiative effect. To be convincing, the experiment must isolate the transfer by infrared radiation, which is what the activity proposed above does.
Related concepts
Illuminance - Luxmeter - Visible Spectrum - Beer-Lambert Law - Thermal Convection