How Was the Greenhouse Effect Discovered?

The greenhouse effect is central to the issue of global warming. But do you know how it was revealed? A look back at the history of climatology reveals unexpected elements.

On this evening of 1859, the rain fell on the cobblestones of Piccadilly Circus (a famous square located in London, England) on which long silvery reflections, emanating from the gaslights, stretched out. A carriage arriving from Shaftesbury Avenue broke the silence of the night. It took Regent Street, where it vanished, leaving only the fading echo of the noise of hooves in the maze of streets behind it. Nearby, on Albemarle Street, was a large building. Under the pediment, supported by heavy columns and on which letters carved into the stone read, “The Royal Institution of Great Britain,” a window was still lit despite the late hour. Under the flickering light of Bunsen burners (a piece of laboratory equipment that permits the production of a flame with gas), a scientist examined a strange machine that he had just finished assembling. He then went near its pipes and turned stopcocks; a hiss was heard, and carbon dioxide filled a long tube. The goal of this experiment was to solve one of the greatest mysteries of science at the time. It was known that Earth had gone through ice ages; how was it possible that the climate had altered so drastically? Science already had addressed this issue. In 1824, Joseph Fourier, a French physicist, proposed that these climate changes stemmed from changes in the composition of the terrestrial atmosphere. How did he reach this conclusion? Researchers knew that our planet, considering the heat that it receives from the sun, should be so cold that its entire surface could be covered with ice. It was therefore postulated that something in the atmosphere retained heat. Joseph Fourier carried out the following experiment: he put a box on which stood a window pane in the sun. He observed that the temperature rose inside this system. According to him, the atmosphere played the same role for the globe as the glass for the box. This experiment would later inspire the name of the phenomenon that he discovered: the greenhouse effect.

THE GREENHOUSE EFFECT IS ILL-NAMED!

Even so, this analogy is wrong. The reason that the temperature increases inside the box or the greenhouse is that the air is confined. One can realize this by opening a window in a greenhouse: the temperature then decreases until it is identical to the one outside. As regards the greenhouse effect of the atmosphere, it comes from the fact that the latter lets sunlight in but absorbs infrared radiation, as we shall see below. A pane has no such properties. The greenhouse effect is therefore ill-named!
However, this model was then accepted as valid; the question was how the atmosphere acts as a glass. The experiments developed by a scientist at the Royal Institution, John Tyndall, were precisely designed to address this issue.

John Tyndall

The first step was to extract gases constituting the air as oxygen, nitrogen, water vapor, and CO2. Carbon dioxide was obtained by distilling the air (distillation is a process that allows the separation of chemical substances by heating). A question that arises then is the following: the sun’s rays pass through the atmosphere and heat Earth. If this energy came from the universe, why does it not completely return there so that it warms the world? The reason for this is that the ground and oceans will cool by emitting what Joseph Fourier called “chaleur obscure” (”dark heat”). But the latter will be trapped by the atmosphere.

AN INVISIBLE LIGHT

What is this mysterious heat? In 1800, a discovery revealed its true nature. An English composer and astronomer, Sir Frederick William Herschel, was doing an experiment with a prism (a transparent object that disperses light into different colors. The same phenomenon can be observed with the raindrops when there is a rainbow).

Here we see a prism (the pyramid). On the right side, at the bottom, a ray of light hits it. A second beam is reflected and moves upward, whereas another part goes through it and is separated into colors.

By chance, a thermometer was placed on his desk, but beside the spectrum, i.e. the dispersion of light into colors. To this scientist’s great surprise, the temperature indicated by this instrument increased even though the light was not reaching it! Sir Herschel then realized that the thermometer was right next to the red part of the spectrum—which we see on the image at the top of the shaft of light transmitted through the prism. He deduced from that that there must be, beyond the red, an invisible light that carries heat. That is what we currently call infrared light. Though this light is not visible to humans, it is perceptible by other animals like certain snakes thanks to receptors located on their heads. Since Earth gives off the heat that it receives from the sun in the form of infrared light, John Tyndall had to use the latter to measure the heat absorption of the gases found in our atmosphere. How to generate such a light? The answer is surprisingly simple: any warm body gives out this light.

Your pet gives off light! Here, a cat seen in infrared. The light parts—the eyes, mouth, and ears—are those that emit the most, i.e. those that are warm or have significant heat losses.One also observes that its nose is relatively cold compared to the rest of the body.

This is the reason that some reptiles can detect infrared light: this enables them to perfectly locate their prey, even in the darkest night. But let’s get back to John Tyndall’s experiment. For the generation of infrared light, he would therefore create a warm body by pouring heated water into a container. Such an apparatus is called a Leslie’s cube because it had been developed by an English physicist, Sir John Leslie, in 1804.

View of the instruments used by John Tyndall for his experiment.

THE GREENHOUSE EFFECT MECHANISM CLARIFIED

In this picture, on the left-hand side of the piece of furniture on the left, we notice a cube, underneath which there is a Bunsen burner. This is a Leslie’s cube (the ”rod” striking out of it is a thermometer to measure the temperature of the water). A similar piece of equipment is mounted on the right side of the long horizontal tube. Inside the latter is the gas of which one wants to study the heat absorption. Its ends are sealed with a transparent material in order to confine the fluid of interest while letting the infrared light produced by the second Leslie’s cube go through it. We notice on the right-hand table an object composed of a stand and two cones. It is a thermopile. What is that? Inside it there is a special coil of wire that has the property, if one heats part of it up, of creating electricity. This type of battery has some applications today, such as to supply electricity to industrial processes requiring rapidly large quantities of energy. But this phenomenon is not used on a big scale because of its poor efficiency. In the experiment, the infrared light coming from the cubes will enter the thermopile through the cones. The greater the temperature difference between the two sides, the more electricity will be generated. It is connected by wires to another instrument placed on the stool; this is a galvanometer, a device capable of measuring electrical currents. So the more heat is absorbed by the gas inside the tube, the greater the temperature difference inside the thermopile and the more current it generates. Therefore, this equipment permits the measurement of the heat absorption of a gas. (Regarding the other instruments used for this experiment, one sees a screen on the right of the thermopile. The others are used to generate the fluids in question).
What were the results of this experiment? Among the perfectly translucent and invisible gases constituting our atmosphere, some absorb heat when infrared light goes through them. This is the case for water vapor and carbon dioxide. The mechanism of the greenhouse effect was thus clarified some 150 years ago!

TOWARDS A TEMPERATURE RISE?
Though this theory was seen at the time as a way to explain the past glaciations by variations of atmospheric CO2 concentration, it did not appear interesting regarding the contemporary climate. It was not until 72 years later, in 1896, that two Swedish scientists, a geologist, Arvid Högbom, and a chemist, Svante Arrhenius, made estimates and came to the conclusion that the CO2 generated by burning coal could lead to an increase in the terrestrial temperature. But this did not appear to be of concern given the fact that, with the quantities of carbon dioxide that were discharged, a long time would have been necessary to reach a problematic situation. Furthermore, for Nordic scientists, an increase in temperature would have been rather welcome. The idea arose of deliberately changing the Earth’s climate. The German physicist Walter Nernst, a Nobel Prize winner, fantasized about setting fire to useless coal seams in order to warm the climate!
In the years that followed, CO2 continued to be perceived as harmless. Indeed, many thought that the oceans could soak up the carbon dioxide that was released into the atmosphere. Soon another regulatory mechanism of nature was proposed: if the oceans contain more CO2 than the atmosphere, the same is true for living matter. It was therefore considered that even if the water could not absorb all the CO2 released, vegetation would take it up. The idea is that an increase in the atmospheric carbon dioxide level causes plants to grow faster. The latter using CO2 for photosynthesis, more plants would mean that more CO2 would disappear from the atmosphere and finally everything would return to normal. Yet in 1938, an English engineer named Guy Steward Callendar realized that the carbon dioxide concentration and the temperature increased. This discovery would revive scientific interest in this issue, and the measurements of atmospheric CO2 concentration would be improved progressively from 1960 onwards. Terrestrial temperature records would also get more sophisticated. Since the late 70s, satellites have been used for this task. New areas of research would open up such as the study of past atmospheric CO2 variations and their implications for the climate.

THE MAGNOLIA’S MEMORY
For instance, the 90s saw works dealing with plants that haven’t evolved much since the dinosaur era, such as the magnolia.

A magnolia flower. As these organisms evolved while bees did not exist, they are designed to be pollinated by beetles! (Beetles are insects, such as ladybugs, that are distinguished by their special wings. Today, this is the animal order that numbers the greatest number of species).

If these plants are exposed to significant levels of CO2, the shape of their leaves will be different. Fossils revealed such modifications. Given the fact that the climates of the dinosaur age were generally warmer than the present ones, this confirms that large carbon dioxide concentrations were connected with them.
The perception of the greenhouse effect has become more complex with the passing of time, though the one popularized by the media since the 60s is equivalent to the Joseph Fourier’s. More precisely, the infrared radiation given out by the soil will be gradually filtered during its passage through the atmosphere, so some of the light is absorbed by the air that thus heats up. The latter itself begins to give off infrared light in all directions, which in turn warms the surrounding air, etc. One can visualize the atmosphere as a juxtaposition of layers. The infrared light escapes into space only at the level of the uppermost layer of the troposphere, a layer of the atmosphere extending from the surface to an altitude of about 6 to 20 km (4-12 miles) depending on the geographic location.(The upper layers play an insignificant role in these phenomena.) What happens if we increase the CO2 level in the atmosphere? This last layer will contain more carbon dioxide, and thus infrared light will have more difficulty going through it. This reduces the amount of this light emitted by Earth. This layer therefore heats up, giving out infrared light that warms the layers beneath it, which themselves start to emit infrared light…This is how the whole atmosphere warms.
This model, which is used in the computer simulations of the temperature evolution in the future, shows that concerning warming, everything is conditional upon the uppermost layer of the stratosphere. The latter determines how much infrared light leaves the globe. The warming or the cooling of our blue planet depends on this layer.

Gaëtan Dübler