First,

  Let’s get one thing straight.

The greenhouse effect and global warming
ARE NOT the same thing.

Silly copy

There is a greenhouse effect, but, if there were not, we would all be dead!

It is becoming increasingly clear that we are also experencing global warming, but, that is a different matter.

The greenhouse effect is the name applied to the process which causes the surface of the Earth to be warmer than it would have been in the absence of an atmosphere. (Unfortunately, the name, greenhouse effect is a misnomer --- more on that later.)

Global warming is the name given to an expected increase in the magnitude of the greenhouse effect, whereby the surface of the Earth will amost inevitably become hotter than it is now.

This page only treats the greenhouse effect --- not global warming.

Second,

  Let’s establish why there is a greenhouse effect.

The surface of the Earth is warmer than it would be in the absence of an atmosphere because it receives energy from two sources: the Sun and the atmosphere.

The atmosphere emits radiation for the same reason the Sun does: each has a finite temperature. So, just as one would be warmer by sitting beside two fireplaces than one would have been if one fireplace were extinguished, so, one is warmer by receiving radiation from both the Sun and the atmosphere than one would be if there were no atmosphere.

Curiously, the surface of the Earth receives nearly twice as much energy from the atmosphere as it does from the Sun. Even though the Sun is much hotter, it does not cover nearly as much of the sky as does the atmosphere. A great deal of radiation coming from the direction of the Sun does not add up to as much energy as does the smaller portion of radiation emitted by each portion of the atmosphere but now coming from the whole sky. (It would take about 90,000 Suns to paper over the whole sky).

So, it isn't even as if our atmosphere had only a minor influence on the surface temperature; it has a profound one. In the absence of an atmosphere the Earth would average about 30 Celsius degrees (about 50 Fahrenheit degrees) lower than it does at present. Life (as we now know it) could not exist.

Third,

  Let’s examine some of the nonsense frequently offered in the name of science.

 Is the greenhouse effect a good thing?

Well, yes, if you appreciate living.

 Does the atmosphere (or any greenhouse gas) act a blanket?

At best, the reference to a blanket is a bad metaphor. Blankets act primarily to suppress convection; the atmosphere acts to enable convection. To claim that the atmosphere acts a blanket, is to admit that you don't know how either one of them operates.

 Does the atmosphere trap radiation?

No, the atmosphere absorbs radiation emitted by the Earth. But, upon being absorbed, the radiation has ceased to exist by having been transformed into the kinetic and potential energy of the molecules. The atmosphere cannot be said to have succeeded in trapping something that has ceased to exist.

 Does the atmosphere reradiate?

One often hears the claim that the atmosphere absorbs radiation emitted by the Earth (correct) and then reradiates it back to Earth (false). The atmosphere radiates because it has a finite temperature, not because it received radiation. When the atmosphere emits radiation, it is not the same radiation (which ceased to exist upon being absorbed) as it received. The radiation absorbed and that emitted do not even have the same spectrum and certainly are not made up of the same photons. The term reradiate is a nonsense term which should never be used to explain anything.

Sometimes diagrams are drawn which show the radiation from the Earth's surface rising into the sky and being reflected off of the atmosphere (or clouds, or greenhouse gases). This too is nonsense. The radiation was not reflected, it was absorbed and different radiation was subsequently emitted.

 Does the atmosphere trap heat (in producing the greenhouse effect)?

Alas no. As rapidly as the atmosphere absorbs energy it loses it. Nothing is trapped. If energy were being trapped, i.e. retained, then the temperature would of necessity be steadily rising. Rather, on average, the temperature is constant and the energy courses through the system without being trapped within it.

 Does the atmosphere behave like a greenhouse?

The name, greenhouse effect is unfortunate, for a real greenhouse does not behave as the atmosphere does. The primary mechanism keeping the air warm in a real greenhouse is the suppression of convection (the exchange of air between the inside and outside). Thus, a real greenhouse does act like a blanket to prevent bubbles of warm air from being carried away from the surface. As we have seen, this is not how the atmosphere keeps the Earth's surface warm. Indeed, the atmosphere facilitates rather than suppresses convection.

One sometimes hears the comparison between the greenhouse effect in the atmosphere (not in real greenhouses) and the interior of a parked car which has been left in the summer Sun with its windows rolled up. This comparison is as phony as is the comparison to real greenhouses. Again, keeping the windows closed merely suppresses convection.

Whether the topic is a real greenhouse or a car, one still hears the old saw that each stays warm because visible radiation (light) can pass through the windows, and infrared radiation cannot. Actually, it has been known for the better part of a century that this has very little bearing on the issue.

Finally,

  What does one tell one’s students?

The correct explanation (as offered above) is remarkably simple and easy to understand, namely:

The surface of the Earth is warmer than it would be in the absence of an atmosphere because it receives energy from two sources: the Sun and the atmosphere.

But don’t ever teach nonsense by claiming that the radiation is trapped, or that the atmosphere reradiates, or that the atmosphere behaves as a greenhouse (or parked car), or that greenhouse gases behave as a blanket.

Bad Greenhouse FAQ

Before writing me with a question about this page, please check the  Bad Greenhouse FAQ  to see if the issue has already been addressed satisfactorily.