Thursday, August 15, 2013

Criticize and Correct

The short explanation of the greenhouse effect found below was used in a survey about attitudes and knowledge of climate change.  Eli asks the bunnies to critically evaluate and suggest changes in the same.  Brevity is important.  The text is limited to no more than 400 words.  Eli, of course has offered his own explanations, as have many others.

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How does climate change (“global warming”) work? The mechanism of the greenhouse effect

[Or: “Why do some gases concern scientists –– like carbon dioxide (CO2) but not others, like oxygen?”]

Scientists tell us that human activities are changing Earth’s atmosphere and increasing Earth’s average temperature. What causes these climate changes? 

First, let’s understand Earth’s “normal” temperature: When Earth absorbs sunlight, which is mostly visible light, it heats up. Like the sun, Earth emits energy - but because it is cooler than the sun, Earth emits lower-energy infrared wavelengths.  Greenhouse gases in the atmosphere (methane, carbon dioxide, etc.) let visible light pass through, but absorb infrared light causing the atmosphere to heat up. The warmer atmosphere emits more infrared light, which tends to be reabsorbed perhaps many times before the energy eventually returns to space. The extra time this energy hangs around has helped keep Earth warm enough to support life as we know it. (In contrast, the moon has no atmosphere, and it is colder than Earth, on average.)

Since the industrial age began around the year 1750, atmospheric carbon dioxide has increased by 40% and methane has increased by 150%. Such increases cause extra infrared light absorption, further heating Earth above its typical temperature range (even as energy from the sun stays basically the same). In other words, energy that gets to Earth has an even harder time leaving it, causing Earth’s average temperature to increase producing global climate change.

[In molecular detail, greenhouse gases absorb infrared light because their molecules can vibrate to produce asymmetric distributions of electric charge, which match the energy levels of various infrared wavelengths. In contrast, non-greenhouse gases (such as oxygen and nitrogen, that is, O2 and N2) don't absorb infrared light, because they have symmetric charge distributions even when vibrating.]

Summary: (a) Earth absorbs most of the sunlight it receives; (b) Earth then emits the absorbed light’s energy as infrared light; (c) greenhouse gases absorb a lot of the infrared light before it can leave our atmosphere; (d) being absorbed slows the rate at which energy escapes to space; and (e) the slower passage of energy heats up the atmosphere, water, and ground. By increasing the amount of greenhouse gases in the atmosphere, humans are increasing the atmosphere’s absorption of infrared light, thereby warming Earth and disrupting global climate patterns.

Shorter summary: Earth transforms sunlight’s visible light energy into infrared light energy, which leaves Earth slowly because it is absorbed by greenhouse gases. When people produce greenhouse gases, energy leaves Earth even more slowly raising Earth’s temperature.

25 comments:

Geoff Wexler said...

I like the section in brackets but not this:

"...but absorb infrared light causing the atmosphere to heat up."

which might be misunderstood * because it may be that the correct simple explanation involves non-local effects or at least has to be done very carefully. I think it is easier to concentrate on the increase of global energy as you have done elsewhere.

Incidentally , I may have missed something but I think that the cooling of the stratosphere is actually simpler than the warming of the troposphere. I am not suggesting that the stratosphere should be mentioned here, but its discussion can be simplified by neglecting the greenhouse absorption compared to the UV absorption. So
more gh gases => more absorption and more emission of which the first term can be neglected => local cooling.
The approximation cannot be applied in the troposphere so it looks as if a vertically global discussion is needed. The ground warms more than before, the IR coming up from below increases, so does convection ...etc.
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* This might refer to the oxygen and nitrogen molecules equilibrating with the gh molecules but of course this is a two way process.

badger badger badger said...

"When Earth absorbs sunlight, which is mostly visible light, it heats up." -> "When Earth absorbs sunlight, which is mostly in visible wavelengths, it heats up."

...in order to eliminate a redundant "light" and emphasize the shift in wavelengths here:

"Like the sun, Earth emits energy - but because it is cooler than the sun, Earth emits lower-energy infrared wavelengths."

(Assuming talking about nonvisible light wavelengths isn't too technical already.)

Also, "perhaps" redundant here: "which tends to be reabsorbed perhaps many times before the energy eventually returns to space."

Art said...

A diagram showing the collisional energy cascade of an optical photon along with an infrared radiation diagram might demonstrate the complexity of the process without too much technical verbiage. Since equilibrium timescales are also involved an animated movie would be even better, to show the net energy gain of the planet.

E Shumard said...

d) Should be modified to something like: "being absorbed at a higher and hence colder place in the atmosphere means that the absorbed energy is radiated at a slower rate".
There is no greenhouse effect with a uniform atmosphere.

Ian said...

My main issue is with "radiated at a slower rate"...
Only while out of equilibrium is this true - when back to equilibrium (at a higher overall temperature) the radiation rate is back to that before increased CO2. It is heat energy being retained that is the problem, not it being more slowly radiated.

Rattus Norvegicus said...

I was going to make the same point as E. Shumard, but by the time I got to work he had already made it.

Tom Curtis said...

"... slows the rate ..." in (d) is technically correct. The photons all have approximately the same wavelength. Therefore, for less energy to be emitted from the cooler, higher molecules, those molecules must emit fewer photons per second - ie, emit at a slower rate. The term, however, invites the confusion that is found in (e). It is not the slower passage through the atmosphere that causes the greenhouse effect, but the reduced energy emitted in the wavelengths at which CO2 is absorbed/emitted. That confusion is made more probable by only speaking of absorption in (d).

Gary B said...

Eli, can I post this at another (pro science) site?

Ian said...

re Tom Curits: Certainly while less energy is emitted, the rate of radiation is less, but this is only while energy equilibrium is being reestablished. Once (assuming CO2 levels ever reach a steady level again) energy equilibrium is attained the rate is the same as before, the difference is that the temperature is higher because more heat energy has been retained.
While warming increases, the rate of emission is less, but the greenhouse effect as such is due to retained heat energy.

EliRabett said...

Sure Gary, but you have to come to your own decision as to posting where it came from. Eli had a particular reason not to (to be revealed in time)

Tom Curtis said...

Ian, the rate of emission of photons with a wavelength around 15 micrometers escaping to space (the primary CO2 absorption/emission band) is reduced even at equilibrium. Consequently the rate of emission at the surface needs to increase so that at wavelengths which are not absorbed, more energy escapes; and that is accomplished by increased surface temperature.

As it happens, the two atmospheric windows are at shorter wavelengths, and hence carry more energy per photon. Ergo the increased rate of emission at those wavelengths can be less than the reduction at absorbed wavelengths and still reach equilibrium; but that is not relevant to the main point.

Anonymous said...

Comments here are all about details (and all correct, BTW), which I suspect completely misses the point.

In my estimation, this explanation of the GHE is guaranteed to confuse most of the somebunnies of the non-scientific bent. It's useless, as far as its intended audience is concerned. Too technical, with phraseology that will go way over most heads.

It needs to be re-written from scratch/first principles into about just 2 paragraphs that summarise the science without going into so much physical/quantum level detail. I know I can do a lot better, given a long Sunday afternoon and a few beers. Should I even bother, or is this a sort of Turing test from the Top Bunny?

--metzomagic

Susan Anderson said...

Metzomagic, you are so right. There are quite a few good lay explanations around here and there, but this (and all suggested improvements), ain't one of them.

Now I'm a bit better than most laypeople at understanding this material, but it's too technical for me. I'd make the effort, but I'm not your typical punter.

Kevin O'Neill said...

Except for high-dollar political campaigns, liberals in general tend to fall down in the art/science of messaging. You 'sell' an idea by testing multiple messages and finding out which one most strongly resonates with the target audience.

We could craft a half-dozen alternative texts - but their effect on the target audience is all that matters. The most effective wording for a high school-educated, evangelical audience is probably not the most effective wording for a different audience. It's best to define the target audience, then test multiple wordings and let the data tell us the best possible wording.

Russell Seitz said...

Try translating it into French and back

Martin Vermeer said...

From John Tyndall it's all downhill

Anonymous said...

I had an idea to aid understanding by characterising IR radiation as a gas - a "photon gas" of massless particles travelling away from the earth at the speed of light.

The term "photon gas" is in Feynman's Red Book talking about the pressure of radiation, so it has some physical respectability.

It takes care of visualising why such thinly represented gases in the atmosphere as CO2, CH4 and H2O (compared to N2 and O2) have such an effect.

It is difficult to hit a grid of sparse targets with a single-shot rifle, or a bow. However, it is easy with a bank of rapidly firing machine guns. And the more targets the more hits. Similarly, the more photons the more hits.

I did not work out the idea quantitavely - just too lazy or preoccupied with other matters. Perhaps someone who reads this might do so.

Toby

rab said...

I like it; I like everything but the first 4 words. Imagine you received a survey that started "Republicans tell us that..." I would just knock off the first 4 words. If that seems to presume too much, give some standard references as footnotes.

rick

DWhite said...

What do you say when deniers come back with the "but CO2 is a tiny percentage of the atmosphere" argument?

Art said...

Laugh in their faces and call them the innumerate idiots that they are.

Mal Adapted said...

DWhite, you can always direct them here.

EliRabett said...

Point out that small amounts of poison can kill them, or somewhat less aggressively, that small doses of medicines can cure them.

For giggles, the exposure limit for CO is 50 ppm. Skeptical Science has some useful pointers on this sort of thing. Coby Beck has a good series from early on

grist.org/series/skeptics

Anonymous said...

What do you say when deniers come back with the "but CO2 is a tiny percentage of the atmosphere" argument?

You remind them that such a statement lumps the non-radiatively active gases ( N2, O2, and Ar2 ) in with the radiatively active gasses ( H2O, CO2, CH4 ). Among the radiatively active gasses, CO2 is a significant portion.

Eunice

dhogaza said...

"What do you say when deniers come back with the "but CO2 is a tiny percentage of the atmosphere" argument?"

Feed them 1,000 mics of led and spend the next 12 hours telling them ... "but the lsd you ingestion is but a tiny percentage of your body mass".

n-g said...

Lessee...some of these have been mentioned before, but rather than cross-reference I'll just lay them out. I'll also ignore inexact translations such as "light" for "radiation".

1. Quote: "...Earth emits lower-energy infrared wavelengths." Technical error: Infrared wavelengths aren't necessarily lower energy, they are merely quantized in smaller-energy chunks. Solution: don't even mention relative energies. [net change: -9 words]

2. Quote: "...(methane, carbon dioxide, etc.)..." Technical error: left off water vapor. Solution: don't provide a list. [net change: -4 words]

3. Quote: "...let visible light pass through..." Technical error: So? They would cause heating (a la black carbon) if they absorbed visible light too. Solution: omit this phrase. [net change: -6 words]

4. Quote: "...but absorb infrared light causing the atmosphere to heat up." Technical error: this is not the main reason that the atmosphere heats up. Compared to radiative exchange with the Earth's surface, the atmosphere gains much more energy by absorbing solar radiation (mostly by water vapor; cf. points 2 and 3) and through latent heat release. Greenhouse gases are the main reason that the atmosphere cools off, and it is the way they cool the atmosphere that matters for greenhouse warming. Solution: simply say "Greenhouse gases in the atmosphere emit infrared light, allowing the atmosphere to lose to outer space the energy it gains from the Sun and the Earth's surface." [net change: +13 words]

5. Quote: "The warmer atmosphere...as we know it." Technical error: relies on radiation as the primary means of energy transfer within the atmosphere (it's not), and claims that the time required for a quantum of energy to make its way to space is what matters. It doesn't. Most H2O and CO2 molecules that emit IR radiation in the atmosphere happened to obtain their energy through "collisions" with other molecules rather than by absorbing an IR photon. Those that absorb lose their energy mainly through collisions too, and what was a quantum of energy for one species of molecule can end up subdivided among many forms of energy among many molecules in very short order. Solution: replace with "A warmer atmosphere emits more infrared light, and the Earth's surface must be warmer still to transfer its heat to the atmosphere. This has helped keep Earth warm..." [Net change: -6 words]

6. Quote: "...cause extra infrared light absorption..." Problem: as noted earlier, the issue is emission, not absorption. Solution: change to "Such increases cause the energy that escapes to space to originate from higher, cooler parts of the atmosphere, reducing its intensity and causing the climate system to not lose as much heat as it gains from the Sun, producing global climate change." [net change: -8 words]

7. Summary item (a): That the Earth absorbs "most" of the sunlight it receives is new information so doesn't belong in a summary and is irrelevant anyway. Solution: combine (a) and (b): "Earth loses the energy it gains from the Sun by emitting infrared energy to space." Then... "(c) These emissions come mostly from greenhouse gases." [net change: -11 words]

8. As commented above, there's that "speed" issue in (d) and (e). Transform to: "(d) When greenhouse gases increase, emissions come from a higher, cooler part of the atmosphere and are not as strong; (e) The atmosphere, water, and ground warm until as much is again emitted as is absorbed from the Sun." This makes the last sentence of the paragraph unnecessary. [net change: -14 words]

9. Since the summary is 25 words shorter, the even shorter summary is unnecessary. [net change: -37 words]

Total change: -82 words.