Why do greenhouse gases warm the earth?

The earth receives heat from the sun at an incredible rate and, if it didn’t get rid of that heat, it would get hotter and hotter. To maintain a steady average temperature, the earth must radiate away heat just as fast as it receives that heat from the sun. In other words, the thermal radiation that the sun emits into empty space must be equal to the thermal radiation the earth receives from the sun.

Though they’re equal in amount, these two thermal radiations have quite different spectrums. Because the sun is extremely hot, its thermal radiation spectrum is largely visible light. The earth, on the other hand, is relatively cool and its thermal radiation spectrum is almost entirely invisible infrared light. While the sun’s thermal radiation is brilliantly visible, we can’t see the earth’s thermal glow or where it’s coming from, which is important because some of it comes from our atmosphere.

If the earth had no atmosphere, its average surface temperature would be approximately -18 °C. At that temperature, the earth’s surface would emit just enough thermal radiation to balance the thermal radiation it receives from the sun. If the earth were hotter, it would radiate away more heat than it receives and cool down. If the earth were cooler, it would radiate away less heat than it receives and warm up.


But the earth does have an atmosphere and that atmosphere contributes to these exchanges of thermal radiation. Although it’s relatively transparent to visible light, the atmosphere is able to emit and absorb significant portions of the infrared spectrum. As a result, a substantial fraction of the earth’s thermal radiation originates in its atmosphere.

Because of the atmosphere’s contribution to the earth’s thermal radiation, the average altitude at which the earth’s thermal radiation originates is not ground level. Instead, it’s 5 kilometers above sea level, the altitude at which the air temperature is about -18 °C. So the earth’s atmosphere shifts the -18 °C average radiating surface from sea level to an altitude of 5 kilometers above sea level.


If you’ve ever traveled up into the mountains and felt the air during your trip, you’ve probably noticed that air’s temperature decreases with altitude. The earth’s atmosphere has a natural temperature gradient of about -6.6 °C per kilometer upward. A simple explanation for that temperature gradient is that at higher altitudes, air must commit more of its overall energy to gravitational potential energy (energy stored in the force of gravity), leaving it with less for thermal energy (energy associated with temperature).

Since the air temperature increases by 6.6 °C each time you descend 1 kilometer, the air temperature at an altitude of 4 kilometers is -11.4 °C (-18 °C + 6.6 °C), at an altitude of 3 kilometers is -4.8 °C (-11.4 + 6.6 °C), …, and at sea level is 15 °C. Sure enough, the average historical air temperature at sea level is about 15 °C.

Which brings us to the question itself: “Why do greenhouse gases warm the earth?”

The 5 kilometer average altitude of origin for the earth’s thermal radiation actually depends on the atmosphere’s chemical composition. Some molecules in the air, notably nitrogen and oxygen, are remarkably transparent in the infrared and barely contribute to the earth’s thermal radiation. Other molecules, notably water, carbon dioxide, methane, and nitrogen oxides, interact strongly with infrared light and contribute significantly to the earth’s thermal radiation. Those thermally radiating gases are collectively known as “greenhouse gases.” The higher the concentration of those greenhouse gases in the atmosphere, the more the atmosphere contributes to the earth’s thermal radiation and the higher the average altitude of origin for the earth’s thermal radiation.

As human-produced greenhouse gases accumulate in the earth’s atmosphere, the average altitude of origin for the earth’s thermal radiation increases. If that average altitude were to rise from 5 kilometers to 6 kilometers, the average temperature at sea level would increase by another 6.6 °C to 21.6 °C. A rise of that magnitude would be catastrophic or even apocalyptic.

Alas, the average altitude of origin for the earth’s thermal radiation has already risen significantly since the industrial revolution and with it, a rise in the average temperature at sea level. The rate of temperature rise is alarming and the task of halting it or at least slowing it substantially cannot be put off for another generation. Even with serious international effort, it’s likely that many areas of the world will become uninhabitable by the next century, either because they are too hot for human survival or because they are under water as the result of rising sea levels.

My daughter did a school project in which we placed a thermometer inside cloths …

My daughter did a school project in which we placed a thermometer inside cloths of various colors. Black cloth showed the highest temperature, blue next, then red, and finally white. Why is that?

Since light carries energy with it, a cloth that absorbs light also absorbs energy. In most cases, this absorbed energy becomes thermal energy in the cloth. Because of this extra thermal energy, the cloth’s temperature rises and it begins to transfer the thermal energy to its surroundings as heat. Its temperature stops rising when the thermal energy it receives from the light is exactly equal to the thermal energy it transfers to its surroundings as heat. This final temperature depends on how much light it absorbs—if it absorbs lots of light, then it will reach a high temperature before the balance of energy flow sets in.

A cloth’s color is determined by how it absorbs and emits light. Black cloth absorbs essentially all light that hits it, which is why its temperature rises so much. White cloth absorbs virtually no light, which is why it remains cool. Colored cloths fall somewhere in between black and white. Blue cloth absorbs light in the green and red portions of the spectrum while reflecting the blue portion. Red cloth absorbs light in the blue and green portions of the spectrum while reflecting the red portion. Since most light sources put more energy in the red portion of the spectrum than in the blue portion of the spectrum, the blue cloth absorbs more energy than the red cloth. So the sequence of temperatures you observed is the one you should expect to observe.

One final note: most light sources also emit invisible infrared light, which also carries energy. Most of the light from an incandescent lamp is infrared. You can’t tell by looking at a piece of cloth how much infrared light it absorbs and how much it reflects. Nonetheless, infrared light affects the cloth’s temperature. A piece of white cloth that absorbs infrared light may become surprisingly hot and a piece of black cloth that reflects infrared light may not become as hot as you would expect.

What is the relative insulating value of various levels of vacuum? For example, …

What is the relative insulating value of various levels of vacuum? For example, how insulating is 1/2 atmosphere as compared to full atmosphere?

Amazingly enough, air’s ability to carry heat doesn’t change much as you reduce its pressure and density as long as you stay above about a thousandth of atmospheric pressure and density. That’s because reducing the density of air molecules may leave fewer particles to carry heat, but it also allows them to travel farther before they collide with other molecules. The reduction in molecular density is almost perfectly cancelled by an increase in the mean free path those molecules travel between collisions—there are fewer heat carriers, but they can move more easily. It isn’t until you reach very low pressures and densities—so that the mean free path begins to approach the size of the enclosed gas—that reducing the air pressure and density begins to decrease the air’s ability to carry heat. That’s why even a small leakage of gas into a vacuum flask spoils that flask’s insulating characteristics. However, you can decrease the “air’s” ability to carry heat by increasing the mass of its molecules—heavier particles such as carbon dioxide or krypton travel more slowly than normal air molecules and don’t carry heat as well.

Is it possible to construct “home-made” thermal windows (double pan) so conden…

Is it possible to construct “home-made” thermal windows (double pan) so condensation can be avoided? I work in stained glass and want to make an energy efficient window. — JAA, York, PA

Yes, you should be able to make your own thermal windows. The value of having two vertical panes of glass that are separated by a narrow gap is that heat has trouble flowing across gap. While air is a poor conductor of heat, it carries heat reasonably well via convection. But with only a narrow gap of air between two vertical glass panes, convection doesn’t work well. Air heated by its contact with the warmer pane tends to flow directly upward, rather than toward the cooler pane. Similarly, air cooled by its contact with the cooler pane tends to flow directly downward, rather than toward the warmer pane.

But as you’ve anticipated, you may have trouble with condensation on the inside surface of the cooler pan. Your best bet at avoiding this problem is to completely seal the space between the two panes and to fill it with very dry air or even bottled nitrogen gas—which can be obtained cheaply from a local gas supply company. You’d have to blow the dry air or nitrogen in through one hole and allow the trapped air to flow out through another hole. After the trapped air has been replaced several times with dry gas and you’re sure there is little moisture left between the panes, you can stop replacing the air and seal both holes. But with stained glass, you have many potential gaps through which moisture can enter the trapped air, so achieving a seal could be very difficult. In that case, you might just put a desiccant at one edge of the window. Drierite is an inexpensive material that resembles little white pebbles and that can absorb quite a bit of moisture. If you put some Drierite between the two panes before you did your best to seal the space between them, I would expect the Drierite to remove enough moisture from the trapped air to avoid condensation problems. After a few years, enough moisture may have leaked in through cracks to cause trouble, in which case you would simply replace the Drierite. One useful type of Drierite is blue when fresh and turns pink when it has absorbed its fill of moisture.

What are firewalls? What are they made of?

What are firewalls? What are they made of? — RB

Firewalls are just insulating, non-flammable walls that prevent the heat from a fire on one side of a firewall from initiating combustion on the other side. As far as I know, most firewalls are made from masonry block. Ceramics such as stone or cement are already fully oxidized and can’t burn. Furthermore, most ceramics are poor conductors of heat and many can become almost white hot without melting. As long as a masonry wall is thick enough and sturdy enough, it can tolerate having a fire on one side without conveying that fire’s heat to the combustible materials on its other side. If there aren’t any holes or flaws in the firewall, it will prevent the spread of fire between adjacent buildings.

What is polyester and how does it work insulating clothing?

What is polyester and how does it work insulating clothing? — PGF, Seabrook, SC

Polyesters are a class of polymers, extremely long molecules that are commonly known as plastics. Each polyester molecule is several thousand atoms long, so that a polyester fiber resembles a tiny rope made of microscopic spaghetti strands that are all entangled with one another. Like most electric insulators, polyester plastic is a poor conductor of heat. But in clothing its main insulating effect is to trap air. While air is a terrible conductor of heat, it tends to undergo convection and convection allows it to transport heat pretty effectively. However, when air is trapped by countless tiny fibers, convection is inhibited and the air becomes a great insulator. That’s why polyester fibers are such good thermal insulation—they trap air and let the air act as the real insulation.

How can heat be trapped?

How can heat be trapped? — PR, Brooklyn, NY

You can prevent heat from moving about with the help of insulation. The three principal mechanisms of heat transfer are conduction (the passage of heat through a stationary material), convection (the passage of heat in a moving fluid), and radiation (the passage of heat as electromagnetic waves or light). Good insulation doesn’t conduct heat well, doesn’t support convection, and blocks radiation. Wool is a good example: its hair and trapped air don’t conduct heat well, the trapped air can’t really undergo convection well, and thermal radiation can’t travel through the wool along a straight path. As a result, wearing a wool sweater keeps you from losing heat quickly—you stay warm. Wool has the added benefit of carrying water away from your skin.

How does a vacuum flask operate?

How does a vacuum flask operate? — MA, Altamonte Springs, FL

A vacuum or “Dewar” flask is a double-walled vessel with a vacuum between the two walls. It’s effectively a bottle inside a bottle, with their only contact at their mouths. Since there is no air in between their bodies, no heat can flow from one bottle to the other by either conduction or convection. The only way in which heat can move between the bottles is via thermal radiation. By coating the surfaces of the two bottles with highly reflective materials such as aluminum, even thermal radiation can be almost prevented from transferring heat between the bottles. Since there is virtually no way for heat to flow into or out of the vacuum flask, except through its mouth, the flask can keep hot things hot or cold things cold for extremely long times.

Some friends and I are having a debate. They maintain that if a person sleeps on…

Some friends and I are having a debate. They maintain that if a person sleeps on an unheated waterbed, heat might be drawn from their body to the point that hypothermia would occur. Is it possible for a waterbed to do this? — JS, College Park, MD

The answer depends on how cold you allow room temperature to become. Without a heater, the water temperature in the bed will be very close to room temperature. When you then lie on the bed, you will be in contact with a surface that’s at room temperature and heat will flow out of you and into the water. Your heat will warm the water and it will tend to float upward and remain at the top surface of the waterbed, forming an insulating layer that will slow your heat loss. However, heat will continue to diffuse into the water as a whole and you will continue to lose heat. As long as the water isn’t too cold, your metabolism will be able to replace the lost heat and you’ll stay warm. But if the room and waterbed are very cold, your temperature will begin to drop. I’m not sure how cold the water would have to be for this to happen, but if the room and water were almost ice cold, you’d probably have trouble.

Why is it that when you have water on your skin and an air current travels over …

Why is it that when you have water on your skin and an air current travels over it, your skin gets cold?

Whenever water is exposed to air, the water and air begin to exchange water molecules. By that, I mean that water molecules leave the surface of the liquid water to become water vapor in the air and water molecules that are already vapor in the air leave the air to become liquid water. If the relative humidity of the air is less than 100% (meaning that the air can still hold more water vapor), more water molecules will leave the liquid water than will return to it and the liquid water will gradually evaporate into water vapor. If the relative humidity of the air is greater than 100% (meaning that the air is holding more water vapor than it can tolerate), more water molecules will return to the liquid water than leave it and the water vapor will gradually condense into liquid water.

For a water molecule to leave the surface of liquid water, it needs a substantial amount of energy because it must break several hydrogen bonds which are holding it to its neighbors. It obtains this extra energy from nearby molecules and they become colder. Whenever a water molecule returns to the surface of liquid water, it returns this energy to the nearby molecules and they become hotter. Thus whenever liquid water is evaporating, the water molecules that leave the liquid water are taking away its energy so that it becomes colder. And whenever water vapor is condensing, the water molecules that return to the liquid water are giving it energy so that it becomes hotter.

When your skin is wet and water is evaporating from it, your skin also becomes colder. Blowing additional air across your skin prevents any build-up of humid air near its surface so that far more water molecules leave your skin than return to it. The evaporation then proceeds rather quickly and your skin feels quite cold.