Mr. Hutchinson makes the mistake of assuming that nitrogen (N2), oxygen (O2) and carbon dioxide (CO2) behave the same in the atmosphere because they are all "colorless, odorless gases". The observation that they are colorless occurs because they do not absorb visible light. However, CO2 can also absorb infrared light (IR for short, such as you would get from a heat lamp). O2 and N2 cannot.

The physics behind this difference is not quickly explained (it has to do with the way that molecules absorb IR radiation), but suffice it to say that this is well understood and can easily be measured. And because of this difference, IR that is emitted from the earth's surface, light which would normally travel unabsorbed into space, can be absorbed by CO2 in the atmosphere, and some of this will be re-emitted back down to earth. The net effect is that the earth stays warmer.

CO2 is a very minor component of the atmosphere compared to N2 and O2. Thus, emissions into the atmosphere by burning fossil fuel can measurably change the CO2 concentration. And such increases have been measured. As to the relative contributions of man-made vs. natural sources, and whether or not recent climate changes are caused by this, are subjects for another discussion.

(1) the "greenhouse effect" is a faulty metaphor (at best) for clouds. clouds do several things: they absorb/radiate heat from/to the earth and the sun, and they reflect heat. the sum of these three effects determines whether the cloud cools or warms the earth; some clouds tend to cool the earth and some tend to warm it.

(2) okay, so how does it work? basically, imagine a bunch of heat (mostly in the form of visible light) arriving at the earth (from the sun). the atmosphere is transparent to visible light, so all of it hits the earth's surface. some of it is reflected right back at the sun (say, 30%). the remaining 70% is absorbed by the earth, which emits heat, too. if there's a layer of gas (like the atmosphere), some of that heat is absorbed by the gas and tossed back to the earth---the gas lets all of the energy in, but only some of it out. note that this "greenhouse effect" also explains why earth is habitable---with no atmosphere (and no greenhouse effect), the earth would have an average surface temperature of about -34 degrees F (instead of the actual +59) and this temperature would vary much more during the day.

(3) many who do not believe that global warming is happening, or that it is but humans are not to blame (including a prominent colleague of mine) will insist that, since water vapor contributes more to the overall greenhouse effect than carbon dioxide (CO2), CO2 is "not important." but we have little control over the amount of water in the atmosphere... so if by "important" we mean "that which we can control," then CO2 is more "important." also, warm surface temperatures tend to increase water vapor in the atmosphere (think muggy summers vs. crisp winters; hell, just think katrina) so if higher CO2 levels lead to warmer temps which lead to more water vapor... you get a positive feedback.

in short, if he thinks that there is "no understandable mechanism ... that explains how CO2 gas in the atmosphere increases heat on earth", i think he's just not trying hard enough to understand it.

Being fairly certain that Fred Hutchison is not a regular reader of Gristmill, I emailed him the answers that I received on the blog. His reply (I've resisted the urge to edit for spelling):

Sarah,

Your point about the CO2 absorption of infared rays is a good one. Someone else pointed this out to me.  However, the green house effect in green houses and the atmosphere comes down to whether the rate of absortion of heat by the ground exceeds the rate of escape of heat into space (or the escape of heat out the greenhouse windows is slower than the absorption of heat).  Thus, the CO2 slows down the rate of escape of heat into the atmosphere during the day.  However, there are no infrared rays at night, so CO2 can continue to release heat for escape into the atmosphere throughout the night.  I don't disagree with the logic of your point but I think that the net effect on the rate of heat escape by Co2 is less than you might suppose.

I do not deny that CO2 emmissions into the atmosphere have an effect on total CO2, but I think that effect is slight relative to the effect of CO2 from forest fires, and volcanoes.

I do not understand how some clouds can have a cooling effect.

Your point about the increased evaporation of ocean water by warmer air leading to increased cloud cover is interesting. But consider that rising columns of warm moist air over the ocean hits the cool breezes at higher altitudes and preciptiates as rain back to the oceans.  It is not clear to me that the cloud cover is increased.  Indeed, the hot ocean tropics have more sunny days than the overcast northern hemisphere.

I congratuate your alert thinking and remain open to your ideas.

Fred Hutchison

Rebuttal?

Dear Mr. Hutchison:

On clouds: Clouds are, arguably, the least-understood element of the climate. Depending on its altitude, a cloud can have a reflectivity of 30% (high clouds) to 70% (low clouds). Enough low clouds can cause the surface to be cooler than if there were no clouds, and enough high clouds can cause the surface to be warmer! Clouds are a big unknown, and since they are very reflective (the earth's overall reflectivity is only about 30%) they can have a big effect.

On the greenhouse effect: Okay, let's try this again. First, all objects radiate energy, all the time. You, for example, radiate at a rate of about 100 Watts. The rate of radiation depends on the temperature (and surface area) of the object: the hotter it is, the faster it radiates energy. Second, the rate of energy absorption by the earth does not "exceed" its rate of radiation: if that were true, the earth's temperature would be constantly rising! Rather, there is a fine balance between the energy in and the energy out.

With no atmosphere, the earth receives energy  (S0), and radiates the same amount of energy (E0) at its surface temperature. Like I said, with no atmosphere, the earth would be cold as a witch's you-know-what: -34 degrees F.

With an atmosphere, the earth receives the same amount of energy (S0), and radiates energy (E). But this time, the atmosphere absorbs all of the earth's energy (E), and radiates energy up to space (A) and down to the earth (A). So now earth's energy balance at the surface is this: energy in (S0+A) balances energy out (E). Since the energy in is larger, the energy out (and the temperature) is larger.

So, it's best not to think of the greenhouse effect as "slowing down" the rate at which heat escapes from the earth. It is better to think of it as this: the atmosphere returns some of the earth's energy, allowing the earth to be warmer (lucky for us carbon-and-water based life forms).

On positive feedback: I did not make any claims about cloud cover. I did say that a warmer surface could lead to more water vapor, which causes the largest part of the overall greenhouse effect. If (as you point out) warmer water leads to more water vapor rising and condensing (which happens largely because of expansive cooling, not by encountering cool air, as you state), this will heat the atmosphere even more---as you know if you've gotten burned by condensing steam, when water condenses, it gives off a lot of heat.

On cloud cover: "the hot ocean tropics have more sunny days than the overcast northern hemisphere"? I'm not sure where you got this information, but it's neither relevant nor true: most cloud cover is concentrated between 30 and 60 degrees latitude in both hemispheres, but over the oceans. The least cloud cover is between 10 and 30 degrees (over the deserts); there's about the same cloud cover over Canada as there is over the Amazon (source: International Satellite Cloud Climatology Project, ISCCP).

On CO2 from forest fires and volcanoes: I'm not even going to get into this.

It is helpful to keep in mind that when it is night where you are, it is day on the other side of the Earth and there are plenty of rays, infrared and otherwise, to soak up.

I'm inclined to just offer to show Fred how to visit Gristmill and respond on his own, but my first post was so snarky, and he's been so civil in his responses, I would feel bad. Anyway, here's what he has to say to mihan's post: I do not understand why high clouds have more reflectivity than low couds, but it is an interesting tidbit. Your blogger friend seems to be saying that high reflectivity clouds offsets the greenhouse effect of clouds more than low reflectivity clouds.  True?  It is interesting the complexity and subtlety of how clouds effect earth temperature - a water cycle phenomenon.

I, of course agree that the heat on earth is a grand equilibrium and not a perpetual rising of temperature.  However, I still feel that there is something not quite right about emphasizing the radiation of heat from CO2 back to earth more than how CO2 influences the rate of heat escape.  This seems like the confusion of a minor factor for a major factor.  In a greenhouse, the heating of solid objects from the sun is a major factor in heating the green house.   The radiation of heat from warm objects is a major factor. The rate of heat escape through the windows is a major factor. The heating of solid objects by warm air in the greenhouse is a minor factor. Therefore, to explain the greenhouse effect as the warming of inside objects from the warm air is a muddle.  Now, if you were to add a gas to the greenhouse which slowed the rate of escape of heat through the windows and said the green house is 5 degrees warmer as a result, I would believe you.  If you told me the gas does not effect heat escape but increases the rate by which heat can be transferred from the air to solid objects, and then said it would cause temperature to rise 5 degrees, I would be skeptical.  We must work out the major forces before we factor in the little things.

When I was living in Cleveland, Ohio, it seemed overcast most of the time.  However, Barbados, a tropical island near the equator has sunny days 300 days of the year. Yes, deserts are sunny and dry.  The south pacific, which is far larger than deserts is sunny and tropical.  The rising warm air from oceans in the tropics does not produce overcast as effectively as northern climes, because warm, moist, rising  air from tropical oceans so quickly precipitates.

Fred

Keep it coming, Gristy smart people, I'm enjoying this conversation a lot.

I have to agree with mihan overall.  I am taking that exact basic weather/climate class this semester, different university and book though.

Though going along with the 'faulty metaphor' in an attempt to help out Mr. Hutchison I think I can offer some simple help to get the brain wrapped around it.  The gases in the atmosphere, CO2 and H2O, ARE the glass in the Greenhouse Theory.  So in the second response, 'Now, if you were to add a gas to the greenhouse which slowed the rate of escape of heat through the windows. . ."  adding gas to the greenhouse is the wrong way to think about it.  It would be more like making the glass thicker and just as transparent as the original glass, thus allowing in the same amount of energy but increasing the insulation factor (the R value like in the fiberglass in your attic) which keeps more energy in thus increasing the temp of the green house.  So more CO2 = thicker glass in the greenhouse metaphor.

If Carbon Dioxide has an atomic mass of 42 and air has an atomic mass of 28 or so, how does CO2 get high enough in the air to create a greenhouse effect?