In a time when scientific truths are regularly challenged with “alternative facts,” being able to explain “climate basics” becomes ever more important. For this reason, I thought I would share some of my own attempts to explain “climate basics” and because it’s so important, I’ll start with the greenhouse effect, which I usually explain with some assistance from two white cats …
... actually, one white cat and one black one would be better, but both of my cats are white, so I make do.
There are three factors that control the climate. These are the energy reaching the Earth from the Sun, the albedo effect and the greenhouse effect.
Energy reaching the Earth from the Sun
Because they like to be warm, cats tend to curl up in sunbeams. Unfortunately for cats, sunbeams move (relative to the cat). Have you ever seen a cat curled up on the floor beside a sunbeam that has moved? The cat, when it was in the sunbeam, was absorbing energy from the Sun (unlike the cat that is no longer in the sunbeam).
The energy that the cat absorbs is radiative, meaning that it moves. It is emitted by the Sun and, eight minutes later, it reaches the cat. Most of it is visible light – sunlight. The cat absorbs some of this energy (black cats are better at doing this), its temperature rises, and it radiates energy back into the room, but not as visible light. You can test this, by placing a cat that has just been curled up in a sunbeam in a dark room (it will be grumpy if you do this). You will notice that the cat does not glow in the dark. Instead, the energy that radiates from a cat is heat. You can test this too, by stroking the cat. It will feel warm (and it will like this more than being placed in a dark room).
The Earth is (in this respect) similar to a cat (albeit quite a lot bigger). The Earth absorbs energy from the Sun. It too, would have done it better if it had been perfectly black – a so-called blackbody. Then its surface would have absorbed all of the radiative energy that reached it from the Sun and its temperature would have been raised from minus 270 degrees Celsius (which is the temperature of outer space) to 6 degrees Celsius. The perfectly black surface of the Earth would then have radiated exactly the same amount of energy back into space, but as heat, not as visible light. This is because the Earth (and the cat) is colder than the Sun. Cooler bodies (the Earth, cats) radiate heat, whereas hotter bodies (the Sun) radiate visible light.
The Albedo Effect
The Earth is (of course) not a blackbody and nor are either of my cats. My cats are both white, and the Earth is multicoloured. Even a black cat is not perfectly black. Because of this, neither my cats, nor the Earth, nor even a (nominally) black cat, absorb all of the radiative energy (sunlight) reaching them from the Sun. Instead, they reflect some of the sunlight which reaches them. This mirror-like behaviour is called the albedo effect.
A white cat has high albedo, meaning that it reflects most of the sunlight that reaches it back into space. A black cat does the opposite. It has low albedo, meaning that it absorbs most of the sunlight that reaches it. The Earth’s albedo is somewhere between that of a white cat and that of a black cat. The result is that about one third of the sunlight that reaches the Earth is reflected back into space as sunlight. This reduces the average temperature at the surface of our no-longer-blackbody Earth by twenty-four degrees to minus 18 degrees Celsius. The cats (especially the white ones) will be somewhat cooler too.
The Greenhouse Effect
Clearly the Earth’s average temperature is not minus 18 degrees Celsius. This is because of the greenhouse effect, which is far easier to explain if we begin with one of our cats.
A cat that has absorbed energy from the Sun by curling up in a sunbeam will want to keep that energy. This is because cats like to be warm. The problem (for the cat) is that it radiates the energy that it has absorbed back into the room (as heat) as soon as the sunbeam moves on. A good solution (for the cat) is to curl up beneath a blanket. The blanket traps the heat that is radiated by the cat, returning some of it to the cat. This is why cats curled up beneath blankets tend to purr.
This is how the greenhouse effect works, but with one important difference. The Earth’s “blanket” – its atmosphere – is transparent (“see-through”). The atmosphere is made of different gases. Some of them are greenhouses gases. These are transparent to sunlight, meaning that they allow sunlight to pass through them. We know this, because we can see the Sun. If greenhouse gases did not allow sunlight to pass through them, we would not see the Sun. However, greenhouse gases behave differently with heat. They do not allow some kinds of heat to pass through them. Some of the heat that would otherwise have radiated from the Earth’s surface back into space is “trapped” by greenhouse and some it is returned to the Earth’s surface. This is the greenhouse effect. It makes the Earth’s surface warmer. Indeed, the greenhouse effect warms the Earth’s surface naturally from minus 18 degrees to 14 degrees Celsius.
Climate warming
It is important not to confuse the natural greenhouse effect with the artificial one caused by humans. The natural greenhouse effect is mainly caused by water vapor and carbon dioxide. The artificial one is caused by our emissions of carbon dioxide, methane and nitrous oxide. It is like adding a blanket when one is not needed. If you did this to a cat, it would creep out from beneath the blanket. We cannot creep out from beneath the atmosphere.
Instead, our emissions of carbon dioxide, methane and nitrous oxide create an artificial greenhouse effect that causes climate warming. At the time of writing, the artificial greenhouse effect caused by humans has warmed the Earth by 1.6 degrees. This might seem like a small number, but it is not. It is the kind of change that would take many thousands or millions of years to happen naturally. This kind of change in less then a century is far too fast for plants and animals to adapt to, putting them at risk of extinction. Passing 1.5 degrees equates to committing 2% of all species to extinction.