Climate Change Science

A Really Basic Introduction to Climate Change Science for Non-Scientists

This page is for people who’d like to learn more about the science behind the climate change controversy. If you have already made up your mind one way or the other about climate change, then this page is not for you. This is for people who want to understand the issues, not partisans.

Most explanations of climate change issues jump straight into the arguments: the “hockey stick” argument and the “solar variability” argument and so forth. You don’t need yet another version of those arguments. Instead, I’m going to take you from the very beginning so that you understand the concepts behind it, the better to make your own judgement of the issues. So let’s get started!

First, exorcise yourself of the notion of “scientific proof”. There’s no such thing. Science never proves anything. Proof is something you get with mathematical theorems and logical puzzles. It doesn’t apply to science. If you drop an apple from a tree a billion times, and every single time it falls to the ground, that does not prove that apples fall to the ground. It means only that it is highly likely that a dropped apple will fall to the ground. You can’t be absolutely, 100% certain that the billion-and-onethe ball will fall to the ground. You can say that the chances that it won’t fall to the ground are extremely tiny.

This leads to another common mistake of non-scientists: black-and-white thinking. All too often I see arguments that boil down to simple-minded yes-or-no, either-or thinking. The real world is all in shades of gray, and so it’s usually a mistake to engage in all-or-nothing thinking. Instead, we think about everything in terms of probabilities, scale factors, and so forth. For example, I recall one young hothead who insisted that the sun’s brightness is not constant. Well, yes, that’s true – but the question is not “Is or is not the sun’s brightness constant?” That’s a black-and-white question. The better question is “How much does the sun’s brightness change?” We’ll be seeing lots of examples of this mistake later.

The next big idea I want you to understand is the concept of modeling. Science never really talks about the real world – it talks about models of the real world that produce results strikingly similar to what we see in the real world. For example, we can model the earth as a sphere with a radius of 3,962 miles. We can then use some simple mathematics to calculate that a trip all the way around the earth would cover 24,891 miles. And if you actually went on that trip, and measured your mileage, you’d measure a number very close to 24,891 miles. Hooray for science!

But wait – the earth isn’t really a perfect sphere. If you measure it more carefully, you’ll discover that it’s an oblate spheroid, which is just like a sphere, only slightly squashed. So we’d have to alter our calculation of the circumference of the earth to get an answer even closer to the correct answer.

Well, actually, I wasn’t being entirely truthful: the earth isn’t exactly an oblate spheroid. It’s sort of pear-shaped. The “pear-shapedness” is really slight compared to the oblateness, which in turn is pretty slight compared to the sphericity – but it’s definitely real. So if you wanted better accuracy, you’d need a more complicated formula that took into account the pearish shape of the earth.

And then there are all sorts of really tiny differences between mathematical perfection and the actual shape of the earth. There are continents and mountains and tides and lots of other things.

What we’ve gone through here is a sequence of models. The first-order model is the sphere. The second-order model is the oblate spheroid. The third-order model is the pear-shaped oblate spheroid. And the fifth, sixth, seventh, and so forth order models cover ever finer details.

The big idea here is that perfection is unattainable. You can never get an absolutely perfect formula for anything in the real world. The real world is just too messy. What you can do is get a model that’s close to the real world. How close? Well, that depends on how close you need to be. If you’re in the store and you are worried about whether the chair you want to buy will fit into your car, you don’t need to measure the chair or the car down to the last thousandth of an inch – if you can get it to an inch or two, that’s good enough. And we do the same thing with climate change science – get close enough to give useful answers. Not perfect answers – just answers that are close enough to help us make good decisions.

So let’s follow the same process in figuring out global warming. We’ll start with the first-order model of the earth and its temperature. In this first-order model, light from the sun heats up the earth. It’s that simple. And what is the resulting temperature of the earth? Well, it’s simple: the more time passes, the more light hits the earth, and the hotter the temperature gets. The earth just gets hotter and hotter. It melts, then it vaporizes, then...

Hold it right there! That’s a lousy model! It’s all wrong! The earth hasn’t been getting hotter and hotter for billions of years! It looks like our first-order model is junk. We need to go one step better, to the second-order model. That second-order model takes into account a crucially important factor: the earth also cools by sending light radiation out into space.

You may be a little skeptical about this. After all, you don’t see the earth radiating anything into space. Where are the antennas for it, you might ask. Well, it doesn’t take antennas. All you need is a warm body. Sit in the warm sunlight and you can feel the heat of the light on your face. Sit next to a fireplace and you can feel the warmth of the fire, even if the air itself is moving away from you and up the chimney. That warmth you feel is infrared radiation. If you’ve ever been close to a big fire, like a burning building or a forest fire, you can appreciate the power of infrared radiation. But it’s not just fires – everything that’s warm emits infrared radiation. Remember the night-vision viewers they show in the movies? The bodies of people always stand out as brighter than the buildings and the trees, because people are warmer than the buildings and the trees. They emit more infrared radiation. And the same thing goes for everything else. ALL objects emit infrared light. They emit more of it if they’re hotter, and less of it if they’re colder, but they all do it.

Think about that. It means that everything on this earth is emitting infrared radiation. And where does it go? Well, it goes in every direction equally – which means that half of it goes up and half goes down. The half that goes up – it goes into the sky and straight on out into space. That heat it’s carrying goes with it. So, in the same way that sunlight carries heat into the earth, the infrared radiation carries heat out of the earth.

By the way, you already have the proof of this in your own experience. Have you ever noticed that it gets cold at night? Have you ever wondered why? Some people might say, “Because the sun went down, silly, and the sun is the source of heat!” Which is true, but that doesn’t explain why it should get colder; it predicts only that things should not get any hotter after the sun goes down. It gets colder because the earth continues to radiate heat out into space after the sun goes down. You’re losing heat into space and not getting any from the sun.

Now it’s time to consider the balance between the heat coming in from the sun and the heat going out in infrared radiation. I said earlier that hotter things emit more radiation than cooler things. The sun is obviously a lot hotter than the earth, so it emits much, much more radiation. But we’re 93 million miles away from the sun. And I can tell you with great confidence that the amount of radiation that the earth emits is almost exactly the same as the amount of radiation that it receives. How do I know that? Simple. Go back and re-read the first-order model. If sunlight comes in, and nothing goes out, then the earth heats up indefinitely. But we know that the earth’s temperature is, by and large, pretty constant. That means that the amount going out has to be pretty close to the amount coming in. This is only an approximation, of course, but it gives us the basis for our second-order model.

In our second-order model, the earth gets light coming in from the sun, and emits light going out as infrared radiation. In this model (and remember, it’s only a model, not reality), the earth is getting exactly the same amount of energy coming in as goes out. The absorption of light from the sun and the emission of light are perfectly balanced.

Now, if you have a properly skeptical mind, you’re probably asking, “Hold on; how does the earth know exactly how much infrared light to emit to make everything balance? Is there some big computer somewhere that calculates all the light coming in and figures out how much light to send out? That’s silly!”

And you’re quite right. But the earth doesn’t need to compute anything. It’s all done with a natural balance based on (take a deep breath here) the Stefan-Boltzmann equation. Gadzooks, doesn’t that name sound so intimidatingly scientific? Doesn’t it sound just like the kind of thing that would make an awful question on a science test? Hey, don’t blame me – blame Mr. Stefan and Mr. Boltzmann for not having names like Smith and Jones.

Just for kicks, I’m going to show you the equation in all its awe-inspiring glory. You don’t need to actually read or even attempt to understand this equation. I just want to show off how smart I am:

F = sigma x T**4

So, uh... what’s it mean?

The “F” stands for the radiant flux – how much energy is emitted every second from every square meter of surface area. The sigma is of course, the Stefan-Boltzmann constant (did you expect it to be called Smith-Jones constant?), which, as you learned in your 7th grade toy science class, is equal to 5.67 * 10**-8 joules per second per square meter per degree Kelvin to the inverse fourth power. See? That wasn’t so hard, was it?

So, again you ask: what’s it mean?

Well, the basic idea is that, as an object heats up, it emits more infrared radiation. In fact, that “4” in the equation means that, if it heats up just a tiny bit, it emits a lot more radiation. If the temperature of the earth increases by just 1%, then its infrared output will increase by 4%.

So now at last you’re ready to “get” a big idea: the natural balance of the earth’s temperature. Suppose that the sun gets hotter gets 1% brighter. That means that there’s 1% more energy reaching the earth. As a result, the earth will heat up. As it heats up, it emits more infrared radiation. At some point, the increase in emitted infrared radiation matches the increase in absorbed solar radiation. At that point, the earth is in balance again: the amount going out matches the amount coming in, and the temperature stops changing.

This works backwards, too. Suppose that the sun is having a bad day and emits 1% less radiation. So at the earth, there’s now less radiation coming in than going out. This causes the earth to cool. As it cools, the Stefan-Boltzmann equation says that it will emit less infrared radiation. At some point, the decrease in outgoing infrared is exactly equal to the decrease in incoming solar, and the system is back in balance again.

This second-order model is much better than the first-order model – it predicts a stable temperature for the earth. Unfortunately, in order to apply the Stefan-Boltzmann law as we did, we need to assume that that earth is a tiny pinpoint that is perfectly black. You may have noticed that the earth is a lot bigger than a pinpoint and that it’s not charcoal black. So now we need to correct for those realities in our model. On to the third order!

For the third order, we’ll take into a new concept: albedo. That’s the secret word that scientists use to mean “reflectivity”. Pure charcoal black has an albedo of 0.0; pure white snow has an albedo of 1.0. Instead of 0 and 1, it’s easier to think of albedo as a percentage of light reflected. So a piece of charcoal reflects 0% of the light that hits it (meaning that it absorbs 100% of that light), while pure white snow has an albedo of 100% (meaning that it absorbs none of the light that hits it.)

So, how does albedo enter into our model? Well, actually, it doesn’t, at least not immediately, because the albedo works just the same for emission of radiation as it does for the absorption of radiation. So if the earth had an albedo of 0.01, then it would absorb 99% of the light hitting it, making it hotter than a white earth – but it would also emit infrared radiation with 99% efficiency. Meanwhile, a white earth would reflect away most of the sunlight, making it cooler, but it would also emit infrared less efficiently, which would make it warmer – and the two effects cancel each other out perfectly. So albedo, all by itself, makes no difference to the planet’s temperature.

So why did I go to all this bother to teach you about something that doesn’t matter? Because the fourth-order model uses albedo in a way that DOES matter. It relies on a kind of selective albedo. You see, not everything is like charcoal or snow, reflecting or absorbing all kinds of light equally. Most things are quite selective about what kinds of light they reflect or absorb. For example, blood absorbs most light, but it reflects red light. That’s why it looks red – white light hits it but only red light is reflected away.

So now adjust that idea in one other way: imagine a gas that is transparent to most light, but absorbs one color of light. Most light would pass right through this gas unchanged, but that one special color wouldn’t get through. Looking through this gas would be like looking through a special glass filter that let everything through EXCEPT that one special color.

Ta-da! You’ve reached the end of the journey, the summit of the mountain, the cheese at the end of the maze. Because now at last you can understand the whole scientific basis for climate change. There really is such a gas: carbon dioxide (actually, there are others, but they’re part of the fifth-order or higher-order models). Carbon dioxide (also written as CO2) is a transparent gas EXCEPT for infrared light, which it absorbs. Normal sunlight passes right through CO2, but infrared isn’t so lucky: some of the infrared is intercepted by the CO2 and absorbed. After the CO2 holds onto it for a little while (a VERY little while – like, a few milliseconds), ithe CO2 re-emits it. But here’s the catch: the CO2 re-emits the infrared light in a random direction – not necessarily the same direction that the light was originally headed.

So, imagine this picture: here’s all this sunlight coming in to the earth (first-order model). It passes right through the CO2. The earth gets hot and emits infrared light, some of which heads up into space (second-order model). But before it reaches space, some of that infrared light is intercepted by CO2 in the atmosphere and absorbed, then re-emitted in random directions. Since it’s all random, half goes in an upward direction into space and half goes back in a downward direction to the earth (just like flipping coins).

Thus, only half of that infrared radiation that the earth emits actually makes it out into space. The rest gets bounced right back to the earth. So the earth can’t cool as efficiently as before. Which means that the temperature rises. And of course, when the earth’s temperature rises, we know from the second-order model that it will increase the amount of infrared radiation it emits. That extra radiation is enough to stabilize the earth’s temperature at a new, higher level.

Thus, if you put more CO2 into the air, then the earth will get hotter. The more CO2 you put into the air, the hotter the earth will get. It’s that simple.

You’ll notice that I haven’t discussed ANY of the things you hear about in the arguments. No hockey stick, no solar variability, no water vapor, none of those things. Why not? Because none of them are necessary in the first four models. You can add them in further down the chain if you want, but they’re not as important as the first four models. Here, I’ll show you a few of these less-important factors:

Solar variability

In the first-order model, I talked about sunlight heating the earth. What if the amount of sunlight changed? If the sun were getting brighter, then that would make the earth hotter, wouldn’t it? In other words, the warming trend that we’re seeing could be due to increases in the sun’s brightness rather than increases in CO2.

So we measure the brightness of the sun with satellites that are extremely precise. Guess what? The sun hasn’t gotten brighter, at least nowhere near enough to explain the increase in temperature that we’re seeing. It is variable, and the variations in brightness should be included somewhere in our model – but they belong somewhere around the tenth order. They’re not as important as the effect of the CO2.