How Do Greenhouse Gases Trap the Earth’s Heat?

I recently encountered a fascinating line of thought on the internet. Someone claimed that greenhouse gases are a myth, and he presented a simple argument to explain why. He said that if greenhouse gases actually trapped heat, then instead of warming the earth they would cool the earth — because they would prevent the heat of the sun from reaching the earth. To a certain extent this is a great argument, because it is simple, logical, and easy to understand — attributes that are desirable in any argument. Unfortunately, there are flaws in this argument, and as a result the conclusions are false.

This provides a wonderful opportunity to discuss why an argument could seem so logical and yet still be wrong — a trap that all of us fall into from time to time. Furthermore, it provides a great starting point for discussing some basic and yet often overlooked concepts about how energy gets from one place to another. These concepts will then allow us to understand how greenhouse gases trap the earth’s heat.

Simplicity Can Be Good — Or Not

Even though the argument above — denying the existence of greenhouse gases — is false, I greatly admire its elegant simplicity. Scientists often start their search for answers by investigating the simplest possible explanation for a particular phenomenon. Good science educators often have the ability to explain the essentials of a concept, so that what had appeared complex now appears simple. A key mental tool for creating simplicity is abstraction. As we search for the essentials of an explanation, we attempt to strip away everything that is not relevant to the current discussion. We do our best to boil the ideas down to their pure essence, discarding all the rest. In fact, all of us — when presenting a simple, logical argument — have usually gone through a similar process, whether intentionally or not.

But sometimes, in our efforts to create a streamlined mental model of a situation, we carry the abstraction a bit too far for the current circumstances. A simple mental model that works well in one context might fail us in a different context. And so it is with this argument against the existence of greenhouse gases. The argument lumps several different forms of energy into a single term — “heat”. But to understand the mechanism of greenhouse gases, it is necessary to understand that the energy from the sun changes form several times before it gets “trapped” by greenhouse gases. When speaking informally, we might sometimes refer to all these types of energy as “heat” — but this glosses over some crucial distinctions. A second flaw in the argument is based on the ambiguity of the word “trapped”.

The short explanation is this: The solar energy that strikes the earth is significantly different from the energy that the earth sends back into space — and this difference has a huge effect on what gets in or out. Furthermore, there are several different mechanisms by which energy comes in or out, and this also has a big impact on the result. So now let’s examine a more detailed version of this explanation. We’ll follow the energy from the time it leaves the sun until the point where it has the chance to get trapped by greenhouse gases.

When Sunlight Reaches the Earth

The first step is for the energy to travel from the sun to the earth. This is a long journey — about 93 million miles — but it only takes 8 minutes, because this energy travels at the speed of light. In fact, this energy is light. We sometimes say that the sun gives off “heat and light”, because sunlight feels warm and it also allows us to see — but this is a description of human perceptions rather than a description of the energy itself. All of this energy is really just light, also called electromagnetic radiation. However, light can exist in a wide range of wavelengths, from gamma rays to radio waves, and only a limited subset of these wavelengths is visible to the human eye.

Sunlight is not evenly distributed across this spectrum. Instead, sunlight is most intense in the visible part of the spectrum. Quite a bit of sunlight also occurs in the near infrared, which is the part of the infrared band that is closest to visible light. In fact, 95% of the energy that arrives from the sun is in the visible and IR (infrared) bands, with the other 5% in the ultraviolet:

Upon reaching the earth’s atmosphere, the sunlight begins to encounter matter. This matter includes the various gases in our atmosphere, along with clouds and various types of particulate matter. These tiny particles, often called “aerosols” by climate scientists, are tiny airborne bits of soot, dust, and other materials — some natural, and some caused by human activity. Each type of gas and each type of aerosol interacts differently with the incoming solar energy. Therefore sunlight has to run a gauntlet of obstructions before reaching the surface of the earth. These obstructions affect some wavelengths of sunlight more than others. In the graph above, the deep yellow area (roughly corresponding to the gray curve) represents the intensity of sunlight at the top of the atmosphere. The red area represents the sunlight that actually reaches the earth’s surface.

On average, about 70% of the incoming sunlight is absorbed by the earth or its atmosphere. The other 30% bounces back into space without being absorbed. This can be further broken down as follows:

• 50% of the incoming sunlight is absorbed by the land and the sea
• 20% is absorbed by the atmosphere and clouds
• 20% is reflected by clouds back into space
• 5% is reflected back into space by land and sea surfaces
• 5% is backscattered into space by the atmosphere

Note that these are round numbers, representing averages for the entire earth.

Any light that is reflected or scattered back into space continues to move along at the speed of light, and therefore it instantly leaves the vicinity of the earth. On the other hand, any light that is absorbed — by either the earth or the atmosphere — is no longer light. Instead it becomes heat (also called thermal energy) — a completely different form of energy. Heat is (in essence) the vibratory motions of molecules of matter. The hotter the matter becomes, the more the molecules vibrate. In contrast, the energy that travels from the sun to the earth consists of massless photons of light. When those photons reach the earth and are absorbed by matter, then the photons disappear, and the energy is converted into heat — molecular vibrations.

During the day, the surface of the earth absorbs about 90% of the solar energy that strikes it. Therefore this surface continues to grow warmer for nearly the entire day, until the sun sets too low in the sky to be effective. In short, the energy that arrived in the form of light has been converted into heat.

Some of the Energy Returns to Space

Now here is a really key point, one that most people don’t realize. All objects gives off radiant energy (light), all the time, even though we usually can’t see it (except with special equipment). The amount of light given off, and the wavelengths of that light, both depend primarily on the temperature of the object. Hotter objects radiate a lot more energy than cooler objects. Warmer objects give off a wider range of wavelengths, extending more and more into the shorter wavelengths, which carry more energy. An object that is cooler than about 1000 degrees Fahrenheit will not give off any visible light on the basis of its temperature. (When a cooler object gives off visible light, it is for a different reason altogether.) Therefore the objects that surround us on earth primarily give off their radiant energy in the form of infrared radiation, which has a longer wavelength than visible light.

The upshot is that the earth is constantly radiating energy back into space. During the day there is a net gain of energy, because the incoming solar radiation is much stronger than the outgoing radiation. But at night there is a net loss of energy — because the outgoing radiation never stops — and therefore the surface of the earth cools down at night. Furthermore, the outgoing energy has much longer wavelengths than the incoming radiation, mostly in the mid-infrared section of the spectrum:

These longer wavelengths mean that the outgoing radiation interacts differently with the gases in the atmosphere than does the incoming solar radiation. The gases in our atmosphere are quite transparent to visible light, which is why the air looks clear and colorless. Clouds can obviously block some of the visible light, which is why we can’t see through them, and why it gets darker when a cloud passes overhead. Aerosols such as smoke and dust can dim the sky and make it hazy, again interfering with visible light. But the gases in the air don’t block our vision, indicating that they don’t have much effect on the visible part of the incoming solar radiation.

In contrast, some of these gases absorb a lot of energy in various parts of the infrared band. The incoming IR light is partially absorbed by water vapor in the atmosphere, and to a lesser extent by other atmospheric gases. But the outgoing energy is much more seriously affected. On average, 90% of the energy radiated by the surface of the earth is trapped before it can escape, absorbed by the atmosphere and clouds. Water vapor and carbon dioxide are the two gases that trap the most outgoing energy, but several other gases (such as methane) also play a role.

What Does “Trapped” Mean?

Now let’s clarify the ambiguous term “trapped”. This word can result in several different mental models, some of which can lead to misconceptions. One possible model is to think of the clouds and greenhouse gases as impermeable barriers, preventing energy from passing through. This was the mental model used to justify the concept that if greenhouse gases really existed, then they would prevent the “heat” of the sun from reaching the earth. This model actual works quite well when we discuss reflected energy, but it falls apart when we discuss absorbed energy. Earlier we saw that 20% of the incoming solar energy is reflected back into space by clouds — a significant barrier. This is because white surfaces reflect a lot of light. Landscapes covered in snow also reflect a lot of energy back into space.

However, greenhouse gases absorb energy — they don’t reflect it. And as they absorb energy, they heat up. Reflected energy is immediately lost to space, but absorbed energy stays right here in the atmosphere of the earth. Therefore, instead of blocking solar energy from affecting the earth, greenhouse gases actually help to sop up the incoming energy, warming the earth. Still, the effect of greenhouse gases on the incoming radiation is small compared to the effect on the outgoing radiation. That’s because greenhouse gases absorb infrared energy (also called longwave radiation). A relatively small proportion of the incoming radiation is infrared, but ALL of the outgoing radiation is infrared.

Balancing the Energy Budget

Here’s another subtle but crucial point: If the temperature of the earth is to stay essentially the same from year to year, then the amount of incoming solar radiation must be exactly balanced by the amount of outgoing radiation from the earth and its atmosphere. If the amounts are out of balance, then the earth will either warm up or cool off.

Just as we itemized what happens to the incoming solar energy (reflected by clouds, absorbed by the surface of the earth, etc.), we can analyze the ultimate source of all the energy that is sent back into space:

• 30% of the outgoing energy is reflected or backscattered solar radiation
• 10% is emitted by the surface of the earth
• 60% is emitted by the atmosphere and clouds

Notice that only 10% of the radiation that returns to space is emitted from the earth’s surface. And yet the surface of the earth emits a huge amount of infrared energy — but most of this energy is absorbed by the atmosphere before it can escape. Likewise the atmosphere emits a huge amount of energy, but more than half of it is sent downward to the surface of the earth, with the rest escaping into space. In other words, the earth’s surface and its atmosphere are constantly trading energy, in large part by radiating infrared light back and forth. (There is also energy transfer via conduction, evaporation, and other mechanisms.)

In fact, everything on the surface of the earth is constantly trading energy with other nearby objects. In our informal speech, this thermal radiation is often called radiant heat, but in fact it is not really heat at all — it is light. However, in some ways the term “radiant heat” actually makes sense, because it is a key mechanism by which heat is transferred between objects. Object A gives up some of its heat energy by emitting infrared light. Object B, sitting nearby, absorbs some of that IR light, thereby increasing its heat energy.

Earlier we noted that water vapor is the greenhouse gas that most affects the outgoing infrared radiation. This means that water vapor is an important factor in models of the earth’s energy budget, but it is not a major factor with regards to anthropogenic (human-caused) changes in the climate. That is because we live on a water planet, 2/3 covered by huge oceans of water — which means that the atmosphere has always had access to plenty of water.

On the other hand, human activity is having a dramatic effect on the amount of carbon dioxide in the atmosphere, which is the second most important greenhouse gas. Furthermore, carbon dioxide has a major absorption band right in the middle of the infrared wavelengths that the earth and the atmosphere radiate into space. This means that carbon dioxide has a huge potential to decrease the amount of radiation that returns to space, thereby causing the planet to warm. This potential has been known for a long time, but predicting the exact result of any increase in carbon dioxide is devilishly difficult. We know enough to say that the current increase in carbon dioxide is indeed having a significant effect, and that further increases will have further effects. But precise, reliable details are harder to come by. That said, each year we get better and better at measuring the current effects and predicting the future effects.

Conclusions

In summary, greenhouse gases are quite real, and they play an important role in the balance between incoming solar radiation and outgoing infrared radiation. Our atmosphere has always had greenhouse gases, so their mere presence is not the problem. The issue is that human activity is causing gases such as carbon dioxide and methane to become more abundant in the atmosphere. These higher concentrations are affecting the balance of incoming and outgoing energy, leading to an overall increase in atmospheric energy, which produces climate change. While the broad outlines of these changes are now clear, the precise future effects are still subject to some scientific debate. But even if we could predict all the future effects with high precision, we would still have a big political debate about the policy consequences — which is perfectly fine, because refining public policy is what a good political debate is all about.

For a more detailed discussion of this same topic, see the chapter “Global Warming” in my book The Stickler’s Guide to Science in the Age of Misinformation.