How Does The Greenhouse Effect Work?

Question

How many of us actually understand how the greenhouse effect works? The simple physics explanations you find on the Internet are usually quite wrong. With this in mind, I've decided to attempt a very simple explanation of the physics behind Earth's greenhouse effect. Bear in mind that this is a simple radiative model, there are actually many more parts to the greenhouse effect than I present here, they're just unnecessary for such a basic explanation.


This is just my current understanding of the process, so I'd be curious to know if there's anything I'm misunderstanding here.

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First, an important idea is that hot objects tend to cool off relatively quickly, while cool objects cool off more slowly, that is, the rate that energy flows from an object is dependent on the object's temperature. This means that a hot object in the presence of a constant energy source will tend to cool down until the amount of energy it emits equals the amount of energy it receives, while a cool object will warm up until the amount of outgoing energy equals the amount of incoming energy. 


Get it? Got it? Good. Moving right along.

A planet is in thermal equilibrium when the amount of energy radiated from the planet to space equals the amount of radiation the planet receives from the sun. Due to its position relative to the sun, the Earth receives about 240 Watts per square meter (W/m^2) of solar energy, which means that in equilibrium, the planet must radiate 240 W/m^2 back out into space.


Some rough calculations would show that if Earth was absorbing heat from the sun and then radiating it directly out into space, the average temperature of the planet would be about 0ºF—well below freezing.

Happily for us, this isn't the case. There are certain trace gases in the atmosphere that are transparent to sunlight but opaque to radiation at wavelengths emitted from Earth's surface. These gases are called the "greenhouse gases" (although they have nothing to do with greenhouses). The heat radiated from the Earth's surface must pass through these greenhouse gases before it can radiate into space. 


The atmosphere is free to radiate heat in all directions (but for our simple model, we can think of it as radiating either "up" or "down"). So while the atmosphere radiates 240 W/m^2 into space, it also radiates 240 W/m^2 back toward the ground. So Earth's surface receives both 240 W/m^2 from the sun and 240 W/m^2 from the atmosphere for a grand total of 480 W/m^2*.

However, unlike the atmosphere, the ground can only radiate heat in one direction: up. So the surface of the planet radiates radiates a total of 480 W/m^2 up into the atmosphere, and this heat is absorbed by the atmosphere, rather than escaping straight into space. And viola! We have a greenhouse effect that heats up the planet's surface! In fact, the greenhouse effect I just described keeps Earth's surface at a cozy average of about 15ºC (59ºF).

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So that's my current (and amazingly long winded) understadning of how the whle shebang works. I look forward to your thoughts, comments, and, most importantly, criticisms. 

—EP

* [i]If you're having trouble imagining this part, try thinking in terms of this simple progression. The planet gets 240 W/m^2 from the sun, and radiates the same back out. The heat is absorbed in the atmosphere, where 120 W/m^2 is radiated out to space, and another 120 W/m^2 is radiated back toward the ground. Now the planet has absorbed 240 W/m^2 from the sun and 120 W/m^2 from the atmosphere, which it reradiates. Now the atmosphere absorbs this 360 W/m^2 and radiates half toward the Earth, and half out to space. Now the Earth receives 240 W/m^2 from the sun, and 180 W/m^2 from the atmosphere. You can continue this progression until the atmosphere is absorbing a total of 480 W/m^2, at which point the energy flux "balances out". (Whew!)[/i]

gjtudor, think of it this way:

At time=0 there is X energy from surface, X absorbed in atmosphere. 0.5X upwards, 0.5X downwards.

That means at time=1 the surface is now getting 1.5X, 1.5X absorbed in atmosphere. 0.75X upwards, 0.75 downwards.

That means at time=2 surface is now getting 1.75X

Notice that the increments are decreasing. The surface temperature at time T is 1 +(1-0.5^T)

That converges on 2 and no more, this doesn't go on perpetually.

If you're still having trouble visualizing it, try drawing out a diagram of the process on paper. That's how I figured it out.

Also note Trevor's correction. The rate isn't 240 W/m^2 as I said, it's actually 340 W/m^2.

Trevor, thanks much for the correction. I'm not sure now where the 240 figure came from.

Also, I'm not sure I understand your last paragraph, would you mind clarifying?

gjtudor, Exactly. The equilibrium point is reached then because that's the point at which it can send the full amount it recieves fro mthe sun back to space.

The most significant point, that I've learned from this simple model, is that, contrary to the explanations you'll see on the Internet, the greenhouse effect doesn't work by, "trapping heat in the atmosphere." Rather, it works by affecting the rate at which energy =leaves= the atmosphere.

Answer 1

That's pretty much what happens although I'm confused about the part where you say the atmosphere radiates 240Wm²yr to Earth and the same to space.

The solar constant averages out at 1366Wm²yr (the total of ALL electromagnetic solar radiation reaching the outer atmosphere), of which 340Wm²yr reaches Earth's surface in the form of solar radiation, this is part a greater energy exchance that is sometimes referred to as the Earth's Energy Budget.

The amount of energy absorbed by the atmosphere from thermal radiation (reradiated solar radiation from Earth's surface) is a very small amount - can't recall the figure offhand.

There appears to be a mathematical error in your calculations - total of 480 comprising 240 from the sun and 240 (the sum of progressively halving) from the atmosphere, the 240 from the atmopshere is the same 240 as originated from the sun prior to reradiation. Maybe I'm misunderstanding the point you're making but it seems like an error to me (PS it's 340, not 240).

But yes, that's the gist of it.

Answer 2

EP, let me confirm from the outset that I know very little on this subject, so, despite our disagreements in the past, I’m *not* contradicting you here – just trying to understand.

I’m also confused by the part you highlighted.

If the planet received 240 W/m^2 of energy from the Sun, how can the atmosphere “radiate 240 W/m^2 into space, [and] also radiate 240 W/m^2 back toward the ground”?

The law of conservation of energy states that energy can not be created or destroyed, but, according to your description, the atmosphere is receiving 240 W/m^2, but reradiating a total of 480 W/m^2.

At first glance this seems to be impossible.

What am I missing that makes this work?

:::EDIT:::

Oh, so the “radiate 240 W/m^2 into space, [and] also radiate 240 W/m^2 back toward the ground” quote is the position *after* equilibrium is reached? So it’s the situation at the end point, not the starting point?

Answer 3

Building A Greenhouse Plans Easiest!

Answer 4

How Does Greenhouse Work

Answer 5

Simply, the greenhouse gases in the air trap heat. The suns thermal energy "heat" is normally reflected off the earth's surface and allowed to escape through the atmosphere. The atmosphere normally holds some heat in to regulate temps. However greenhouse gases trap this excess heat. When this is combined with melting of glacier packs at the poles (the snow reflects the thermal energy, but with less snow there is less reflection) the heat builds and the average global temps rise.

Answer 6

Simple answer. The greenhouse effect is a term used to describe the effects of the building up of carbon dioxide in the earths atmosphere which causes the retention of heat thereby warming the ambient temperature.

Answer 7

well this is how i understand it...

The greenhouse effect is the rise in temperature that the Earth experiences because certain gases in the atmosphere (water vapor, carbon dioxide, nitrous oxide, and methane, for example) trap energy from the sun. Without these gases, heat would escape back into space and Earth’s average temperature would be about 60ºF colder. Because of how they warm our world, these gases are referred to as greenhouse gases.

Answer 8

I am certainly not a physics major, but that sounds pretty good except for one point. When you refer to the ground only being able to radiate heat in one direction, particularly the ocean. And while the radiative portion of earths energy budget is intuitive from a physics perspective, there is a significant amount of energy unaccounted for by convection, evaporation and condensation. This portion of earths climate that is generally written off as weather is chaotic and difficult to model, but eventually must have some portion of the 240 watts per square meter assigned to it, if we are to fully understand earths energy budget.

Answer 9

There is a big HOLE in the Greenhouse THEORY. Actually, the hole is in the O-Zone, it grows and shrinks almost daily. In a True greenhouse, heat is trapped and cannot escape, this is especially useful when growing plants in the winter...However, it has nothing to do with the earth's climate..Why..The HOLE...If you put a hole in a greenhouse that can expand and contracted to let gasses out, the greenhouse itself is useless.

Answer 10

like greenhouse, light come in, don't come out.