As far as we know, Earth is the only planet in our solar system to harbor life. Scientists are still trying to figure out exactly why that is, but one reason might be that we have plenty of liquid water. The outer planets are very cold, so any water there would be locked up in ice. The inner planets are piping hot, so scientists think most water there would boil away.
Earth, however, falls right in the middle. It sits in a narrow band, known to astronomers as the Goldilocks zone, where scientists think liquid water can survive and life might be able to flourish. Like the bowl of porridge in the children’s story, Earth is not too hot and not too cold. It’s just right.
The distance between Earth and the Sun is also a good place to start when it comes to understanding Earth’s climate. It is the main factor affecting the planet’s average temperature, but it is not the only one. The temperature of the Earth is also influenced by the composition of the atmosphere, which contains heat-trapping greenhouse gases and other chemicals injected by volcanic eruptions and human activities.
Climate starts with the Sun
Aristotle was the first to attempt to explain weather and climate in his book Meteorology back in 350 BCE (Figure 1). He believed that there were four elements – fire, air, water, and earth – and that they interacted to produce the weather phenomena we experience on Earth. Working with these four elements, he was able to explain some things well, but not others. For instance, he correctly wrote that heat (fire) could evaporate water, and that clouds formed when water vapor condensed in the air. But he also wrote – incorrectly – that thunderbolts fell from the sky when it “exhaled” and that shooting stars were burning air.
Many of Aristotle’s explanations of climate, while creative, were proved wrong by the 2,000 years of scientific discoveries that came after him. But he got one major point right: The Sun is the most important factor in driving climate. Or as Aristotle put it, “The Sun's motion alone is sufficient to account for the origin of terrestrial warmth and heat” (Meteorology, Book 1, Part 3). We would amend this now to say that the Sun’s energy, rather than its motion, accounts for warmth and heat.
Indeed, the Sun is the primary source of energy at Earth’s surface. This energy is produced by nuclear fusion in the Sun’s core, a process that heats the core to approximately 15 million degrees Celsius. The heat created inside the Sun then makes its way through the star’s interior to the surface, where the temperature is a mere 5,800° C. From the surface, this energy radiates into space in the form of visible light and other kinds of energy along the electromagnetic spectrum (see Figure 2 for the complete solar emission spectrum). You can learn more about light and electromagnetism in our module on Light and Electromagnetism.
As this energy travels 150 million km across the solar system to our planet, its intensity decreases. To understand why, imagine a single light bulb burning in a large empty room. When you are right next to the lightbulb, it is very bright: You could read a book. But as you move farther away from the bulb in any direction, it becomes harder and harder to read because less light reaches your location.
In fact, the amount of light that makes it from the bulb to your eyeball decreases faster than the rate at which you move away from the bulb, and here’s why. When you stand close to the bulb, the energy that reaches you is spread around a sphere whose radius is the distance between you and bulb. When you move twice as far away, the light reaching you now is spread around a sphere with twice the radius. But because the surface area of a sphere equals 4πr2, that light is now spread over an area four times as large (see Figure 3 for an illustration of this concept).
As a result, planets closer to the Sun receive much more solar radiation than planets farther out in the solar system. The Earth receives 342 Watts per meter squared or W/m2; this received energy from solar radiation is called insolation. That’s about twice as much insolation as Mars and half as much as Venus (see Figure 4).
However, Earth doesn’t absorb all of the solar radiation that comes our way. About 30% of it reflects off of light-colored surfaces, like clouds, snow, ice, and sandy deserts. The fraction of reflected light is known as albedo, and it also varies between planets (see Figure 4). Mercury, which has virtually no atmosphere or ice, reflects only 10% of incoming radiation, while Venus, which is cloaked in a thick haze of carbon dioxide, reflects about 75%.
If incoming sunlight and albedo were the only factors involved, scientists have calculated that the average temperature of the Earth would be -18° C (approximately 0° F), shown as Tpredicted (or predicted temperature) in Figure 4. But clearly that’s not the case – that’s much too cold to maintain the liquid water that makes up Earth’s vast oceans. In fact, the observed average temperature (Tobserved in Figure 4) is around 15° C (59° F), well above the freezing temperature of water.
Venus has an even larger difference between the predicted and observed average temperature, while the predicted and observed temperatures for Mercury and Mars are close to equal (Figure 4). Therefore, some other forces must influence Earth’s climate besides just the Sun. And these forces must be greater on Venus and almost non-existent on Mars and Mercury.
The greenhouse effect
The first person to recognize the discrepancy between the temperature of Earth and the amount of energy received from the Sun was a French mathematician named Joseph Fourier. He had been studying heat flow in the ground, and concluded that although the interior of the planet was hot, it did not supply much energy to the surface. Mostly, the Sun provided energy for the surface, but that was not enough to explain observed temperatures. In addition, he proposed that the atmosphere might help warm our planet (Fourier, 1827).
However, scientists hadn’t yet invented the tools that would allow him to test his idea quantitatively. So, instead, he used the analogy of a simple device called a heliothermometer, or a “hot box,” to explain how the process might work. A hot box consisted of an insulated box, painted black on the inside, with a glass lid that enclosed a thermometer (the solar ovens of today are basically modified heliothermometers).
The device was invented by the Swiss physicist Horace de Saussure, who wanted to understand why temperatures are cooler on mountaintops than in valleys. De Saussure thought he could use the temperature of the thermometer to measure solar insolation, which he thought might decrease with altitude, resulting in colder temperatures. However, his experiments revealed that solar radiation actually increases at higher elevations, while air temperature decreases. (Learn the correct explanation for the change in temperature at higher elevation in our module The Composition of Earth's Atmosphere).
But Fourier used the hot box for another kind of experiment – a thought experiment. He knew that most solar radiation, which peaks in the visible part of the electromagnetic spectrum (see Figure 2), passed through the pane of glass unimpeded. The black walls of the box then absorbed this energy and heated up. The walls of the box then emitted long-wavelength infrared energy (seen to the right of the visible spectrum in Figure 2), which was known in Fourier’s day as “dark heat” because it is invisible to the human eye.
In this process, the hot box converted energy in the visible part of the spectrum to infrared energy. But Fourier also knew that glass is mostly opaque to infrared energy – it blocks it the same way a brick wall blocks visible light. So as sunlight continued to enter the box and the walls continued to warm, heat built up inside and increased the temperature. He thought the same thing might happen in the atmosphere if it, too, was transparent to visible light but blocked Earth’s outgoing infrared radiation.
This idea would later be called the “greenhouse effect,” because the glass walls of greenhouses also warm the air inside them. However, both greenhouses and de Saussure’s hot box are imperfect models for how greenhouse gases actually behave in Earth’s atmosphere. That’s because they are both enclosed spaces that physically trap warm air, and this accounts for much of the observed warming (in the same way that a car heats up on a hot day, even though it doesn’t have a transparent roof). In reality, Earth’s atmosphere absorbs outgoing infrared radiation, heats up, and radiates it in all directions, including back down toward the surface (more on this below).
Still, by conducting this thought experiment, Fourier identified two important features of the greenhouse effect. The first is that the atmosphere is basically transparent to visible light but absorbs infrared energy. The second is that visible light can be transformed into infrared energy by being absorbed and re-emitted at Earth’s surface.
Fourier’s ideas sounded promising, but no one could actually test them until 1859, more than 30 years after Fourier published his ideas, when an English physicist named John Tyndall set out to determine whether the atmosphere really does absorb infrared radiation. Over the course of two years, he devised an instrument that would let him measure how much energy was lost after passing through a 1.2-meter-long tube of air (his instrument is shown in Figure 5).
To seal the tube, he placed slabs of rock salt on both ends. Why rock salt? Because unlike glass, salt is transparent to infrared radiation. He then set a pot of boiling water or hot oil at one end, which produced a source of infrared radiation with a constant temperature, and thus, a constant wavelength. He measured how much energy came out the other end by detecting very small changes in temperature using a homemade sensor.
When Tyndall filled the tube with dry air, pure oxygen, or pure nitrogen, he did not detect any change in the amount of energy that passed through the tube. He tried every gas he could get his hands on, and when he finally added ethylene gas (C2H4) – the gas emitted by fruit when it ripens – he saw that much of the radiation was absorbed between the entrance and exit of the tube. This surprised him. He wrote:
The gas was invisible, nothing was seen in the air, but the needle [of the detector] immediately declared its presence… Those who like myself have been taught to regard transparent gases as almost perfectly diathermanous [permeable to heat], will probably share the astonishment with which I witnessed the foregoing effect. (Tyndall, 1861)
He continued with his experiments and documented the absorption of infrared energy when the tube was filled with several other chemicals. It turns out that Tyndall had just discovered greenhouse gases, the gases that absorb infrared radiation in the atmosphere, refining Fourier’s hypothesis. We now know that the most important greenhouse gases are water vapor, carbon dioxide, methane, and nitrous oxide, all of which absorb energy at specific wavelengths in the infrared region, shown in in Figure 6.
When he recognized how powerful these greenhouse gases were, Tyndall speculated that even small changes in the concentration of these gases in the atmosphere could exert a strong influence on Earth’s climate:
It is not, therefore, necessary to assume alterations in the density and height of the atmosphere to account for different amounts of heat being preserved to the Earth at different times; a slight change in its variable constituents would suffice for this. Such changes in fact may have produced all the mutations of climate which the researches of geologists reveal.
Tyndall turned out to be correct. Small changes in the concentrations of greenhouse gases do alter climate dramatically. However, it’s important to note that there is a key difference between water vapor and the rest of the greenhouse gases.
Biological and physical processes (including human activity) can produce and consume greenhouse gases like carbon dioxide, methane, and nitrous oxide, changing their concentrations in the atmosphere and thus causing climate to change. In contrast, the concentration of water vapor in the atmosphere is controlled by the temperature of the planet: When the atmosphere is warmer, it can (and does) hold more water vapor, and the opposite is the case when it’s cold. Thus, even though water vapor is the most powerful greenhouse gas, it does not cause climate changes. It responds to these changes and amplifies them.
Other atmospheric components
Compared to gases like nitrogen and oxygen, which together make up 99% of the atmosphere, greenhouse gases make up just a tiny fraction of air (see our module Composition of Earth's Atmosphere for more information). Today, the concentration of carbon dioxide in the atmosphere is about 400 parts per million and the concentration of nitrous oxide is about 325 parts per billion! In addition to greenhouse gases, other minor components of the atmosphere also affect climate, like aerosols.
Aerosols are tiny particles that float in the air, and they usually have the opposite effect of greenhouse gases: as the concentration of aerosols increases, surface temperatures decrease. That’s because aerosols generally reflect incoming sunlight, increasing Earth’s albedo. However, in some cases, dark-colored particles, like soot, can absorb light more efficiently, and lead to warming.
Aerosols can include dust and microscopic droplets of liquids like sulfuric acid, which get ejected into the atmosphere after large volcanic eruptions. Such eruptions demonstrate the effect of aerosols on climate; global average temperatures dipped briefly after each of the major eruptions of the 20th century, as shown in Figure 7, including the 1991 eruption of Mount Pinatubo.
However, aerosols only remain in the air for a few years, and are not evenly distributed like greenhouse gases. There are always some floating around in the atmosphere, because natural processes continually produce them. But bursts of aerosols from big eruptions only affect climate temporarily as shown in Figure 7, where you can see that temperature drops sharply after very large volcanic eruptions and then returns to the previous average only a few years later.
The first climate model
One way to determine if you have taken into account all of the factors that influence a system is to build a model that combines them and see if it matches observations (see our module Modeling in Scientific Research for more information). The first scientist to take Fourier and Tyndall’s results and put them into a quantitative climate model was the Swedish chemist Svante Arrhenius, who is perhaps best known for his work on the rates of chemical reactions.
Arrhenius set out to account for all the energy coming into and leaving the Earth system – a kind of energy budget (Arrhenius, 1896). That required tallying up all the sources of energy, the ways energy could be lost (known as energy sinks) and the ways energy could be transferred (known as energy fluxes). Arrhenius did not include an illustration in his 1896 paper, but it is useful here to put his ideas into a diagram, shown in Figure 8.
On the incoming side of Arrhenius’ equation was solar radiation (thin black arrows in Figure 8). On the outgoing side was the long-wave infrared radiation emitted by the Earth’s surface (thick red arrow), plus reflected sunlight (thin gray arrows). However, Arrhenius knew that he also had to account for greenhouse gases in the atmosphere, which Tyndall had shown interfered with outgoing radiation.
Arrhenius reasoned that if the atmosphere was absorbing infrared radiation, it too was heating up. Thus, he added another level of complexity to his model: an atmosphere that could absorb and radiate heat just like Earth’s surface. For simplicity, he treated the whole atmosphere as one layer. The atmosphere absorbed outgoing radiation emitted by the surface (thick red arrow), and then emitted its own radiation both up to space and back down to Earth (thin red arrows).
This was an important realization, because it showed that the atmosphere didn’t block outgoing radiation as Fourier had proposed. It absorbed it. Then, like the hotbox, it heated up and emitted infrared energy. The atmosphere emits this energy in all directions, including back toward the earth. This flux of energy from the atmosphere to the surface represents another important source of heat to Earth’s surface, and it explains the real mechanism behind the greenhouse effect.
The Goldilocks zone and the search for extraterrestrial life
In 2009, NASA launched the Kepler Space Telescope with the goal of finding other potentially habitable planets in our galaxy. So far, scientists have found and confirmed more than 1,000 so-called exoplanets. Of these, twelve lie within the Goldilocks zone, where water can exist as a liquid.
We don’t yet know whether they might harbor life – at the moment, they are just faraway objects whose very presence is barely detectable. But, from what we’ve learned about our own solar system, we know that it’s not enough to know how far these exoplanets sit from their stars. Earth’s greenhouse effect helps make the planet more habitable. But on Venus, carbon dioxide makes up 96% of the atmosphere, and the greenhouse effect heats the planet 500 degrees Celsius above its predicted temperature (see Figure 4), making it hotter than Mercury. Therefore, while scientists have started the search for extraterrestrial life by looking for planets that are the right distance from their star to have host liquid water, they must consider the composition of the atmospheres of those planets and use their understanding of the greenhouse effect that researchers discovered here on Earth.
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