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What is Global Warming and the Greenhouse Effect?

Posted on 30 June 2010 by Kevin Judd

Guest post by Kevin Judd

Climate scientists are telling us that gases like carbon dioxide are causing global warming. Carbon dioxide is produced when petrol is burned in your car engine, or when coal and gas are burned at power-stations to make electricity. Carbon dioxide causes global warming because it contributes to the so-called greenhouse effect. So what is this greenhouse effect?

Winter is a good opportunity to observe the greenhouse effect in action. It should be obvious that sunlight heats the earth: it gets hotter when the sun shines, and colder at night when it doesn't. On clear dry nights it can get very cold indeed, but if the sky is cloudy, or overcast, then it doesn't get so cold. This happens because clouds trap heat. The more clouds there are, the warmer it stays overnight.

So how does this heat trapping work? And how does carbon dioxide come into play?

You know that radio stations have different frequencies. When you tune your radio to your favourite station, you are telling your radio receiver to block all radio-frequencies except those that the station uses. These frequencies are allowed into the radio's electronics.

Heat, just like radio-waves, has different frequencies, and clouds, just like a radio receiver, block certain frequencies of heat and allow other frequencies through. Without clouds most frequencies of heat escape into space and it gets very cold overnight. When there are clouds, some frequencies of heat are blocked from escaping into space, keeping it warmer.

It turns out that clouds and carbon dioxide trap heat differently, like radios tuned to different frequencies. In fact, carbon dioxide pretty much blocks precisely those frequencies that clouds would allow through. Add to this the fact that unlike clouds, which come and go, carbon dioxide is always there, its warming effect occurs even when the sky is clear and dry. You can probably understand now why climate scientists are so concerned about carbon dioxide. The more carbon dioxide in the atmosphere, the hotter the earth will become.

Of course, this is a simplified explanation of global warming, but the basic story I have just told you is correct. Scientists have known and understood this for over 100 years, and it has been confirmed in laboratory experiments. There is no doubt about the basic science behind global warming.

So does it really matter that driving a car and using electricity adds more carbon dioxide to the atmosphere? In my next segment, I will examine why scientists are convinced that if we are not careful, the earth will warm 2 or more degrees, and I will examine what the consequences of this will be.

The message for today, however, is that anyone who tells you that carbon dioxide does not cause global warming, either does not understand the basic science, or is being deliberately misleading.

NOTE: this post is also being "climatecast" by Kevin Judd on RTR-FM 92.1 around 11.30 AM WAST today. You can listen to a streaming broadcast of RTR-FM online via

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Comments 51 to 71 out of 71:

  1. Clarifying RSVP's remark, the Earth is not "getting hotter all the time because of C02", it's going to become warmer until radiative balance against additional retained heat is achieved at which point temperature will stop trending upward due to the particular system of forcings influenced by C02 and will then dither and possibly trend upwards or downwards due to other causes, as it always has. Not only may we count the beans we have, we may propose removing or adding beans and then calculate how many beans will be available if or when such quantities of beans are actually manipulated. Different hands may reach into the pot and either add or remove beans. The hands we're speaking of here wear gloves marked "anthropogenic C02." Always watch the hands, don't look at the brightly colored scarves being waved about.
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  2. It's pretty straightforward, RSVP. Gasses aren't perfect blackbodies (nothing really is), and in fact N2 and O2 have pretty lousy absorptivity and hence emissivity (Kirchoff's law) at the IR wavelengths a blackbody would radiate at our surface temperature. They heat and cool very slowly with IR, much as a polished silver surface is a poor absorber/emitter of daylight. CO2 and water vapor, on the other hand, are handy at absorbing/emitting IR in the 4-15 micron thermal IR bands. They both absorb (raising electron states, heating up the molecule both electronically and vibrationally) and emit (dropping electron levels which are pumped by vibrational inputs, losing energy) quite well. When an air mass is losing IR to the surroundings (net output), the CO2 and H2O molecules will be on the whole cooler than the air mass due to radiation, repeated brought back to the air mass temperature through collisional interactions. When an air mass is heated by IR, the CO2 and H2O are hotter than average than the the rest of the air mass, with collisions warming the entire 100% of air molecules. At equilibrium (same coming in as going out) the GHG's will be at the same average temperature as the rest of the air mass. Granted, pure CO2 would heat/cool faster, with a much higher exchange rate with it's surroundings. But the fractional percentage of CO2 and H2O in air provide an energy pathway via IR exchange that would be nearly non-existent otherwise for the air mass. CO2 and H2O (and methane, ozone, etc.) are thereby preferential infra-red interaction molecules (IR antenna), heating or cooling the entire air mass through the net IR exchange - much as the water in your food acts as a receiving antenna in your microwave oven, heating the rest of the food through microwave absorption.
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  3. Sorry! John, could you wipe the two duplicates there? I hadn't realized I was running onto the next page! (Doh!)
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    Moderator Response: No problem. Operational detail: If you happen to post just before the comments thread is going to "roll" past the first page of comments, your view will remain on the page you started from but your comment will actually appear on the next page.
  4. RSVP, KR's comment with regard to the antenna aspect of GHGs took me a second, but then I got the metaphor that just as antennas are sensitive to certain wavelengths, dependent on the length of the wire, and much less to others, gases absorb and emit at specific wavelengths. I'll connect the dots regarding your engine example. Suppose that instead burning fuel in an engine, you simply burned the same amount of fuel directly. Is there any difference in the amount of energy released? None whatsoever. Does the engine's cooling system transport the energy to some higher altitude where it can be radiated off the earth with less impedance? No. Hence, my simplification to just fire. The heat from naturally occurring sources does not simply accumulate; so, it would take a bit of magic to make the heat from manufactured ones to do the same.
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  5. RSVP #48, Mmm, no, in a hydroelectric dam the energy out is the difference between the gravitational potential of the water going in versus the gravitational potential of the water going out. How the water got uphill from the dam can be any number of stories.
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  6. Chris G Combustion produces two IR radiative gases: water vapor and CO2. These two gases come out of a fire with a fairly high temperature. I have said nothing above about N2 or O2 getting hot. As far as heat radiating from the Earth, you cant have it both ways. Either N2 and O2 allow IR though or they dont. If they are transparent to IR, then you have to admit that convective cooling through a radiators (or home heating for that matter) is directly elevating temperature in a way that differs from fire and sun heated bolders.
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  7. By the way, I actually like the antenna analogy. Try transmitting 1000 M Watts of AM radio through a safety pin and see what happens!
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  8. RSVP - if you're talking about exhaust gases, yep, the energy starts with the CO2/H2O combustion products. I believe that by the time the exhaust has reached the end of the muffler, however, you're close to thermal equilibrium with the remaining O2 and N2. Your example, however, was an automobile radiator, which conductively heats ALL of the air mass, moving that energy out of the car via convection. And yes, yes, convective cooling in your house from a hot air register is different from radiant heat. Convection, conduction, radiation - all valid paths in some amount for heat energy, until you get to the top of the atmosphere and only radiation works. That's why the Trenberth diagrams and supporting measurements are so important - determining how much goes through each pathway. Whatever; GHG's act as effective antenna (absorb/emit) for thermal band IR. I think I have a mad scientist experiment for my next party now! I just have to get a (cheap) AM transmitter I don't care about...
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  9. A ~5.3 inch piece of wire suspended in an otherwise empty microwave oven will do nicely for the mad scientist experiment. Wear welding goggles.
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  10. #49 "...which as I said does not emit IR. " OK, maybe it doesn't emit IR (though IR covers a lot more spectrum than what we normally consider); I'm pretty sure the gases other than what we call GHGs emit energy at some wavelengths. Also, relatively excited molecules will transfer energy to their neighbors. This in true for all molecules; you must know that. The end result is that energy is still dissipated to the surroundings. Once the energy from the fire or engine is dissipated to its immediate surroundings, it makes no difference whether that happened through radiation or convection, , if the original molecule was CO2 or N2, or if what was hot was a rock or a piece of aluminum.
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  11. RSVP at 00:24 AM, your point 2 is exactly correct. The work done by each molecule is what is important, not the presence of the molecule itself. The crucial difference is that the H2O molecule changes state twice each time it cycles, whereas the CO2 molecule does not. The change of state requires the absorption of heat energy and the dissipation making the H20 molecule very suitable for transporting heat energy. Thus the less time each H2O molecule resides in the atmosphere the greater the amount of heat energy transferred to the atmosphere from the surface. Under the conditions that the greenhouse effect provides, CO2 is not capable of performing this role. Given the total amount of water vapour in the atmosphere remains at about 2%, 20,000ppm, if the average residence time of a H2O molecule decreases then the amount of heat transferred from the surface, and the amount of precipitation would also increase. The clouds that form as part of the cycle are also always present varying between 64% and 69% coverage globally. On the other hand what actual work does a CO2 molecule do in the atmosphere? We do not see CO2 forming clouds of dry ice do we?It does not change state under the conditions that exist in the atmosphere. It's cycle requires it to remain close enough to the earths surface to be sequestered by plants or absorbed by the ocean, broken down into it's components and then reformed before it can be released to complete another cycle. Those CO2 molecules that reside in the atmosphere are obviously lost, like street urchins who are unaware that there is a purpose for their existence, and a real role for them to play if they can just get back into the system again. It's seems that there is some confusion about how powerful CO2 is as a greenhouse gas. There is a need to separate the direct effect, and the nett effect as a forcing agent acting on water vapour. Any warming due to the greenhouse effect is thus directly attributable to H2O, and only indirectly to CO2. The direct effect of CO2 itself is small and requires amplification.
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  12. johnd - we discussed relative amounts of energy exchange at quite some length over on CO2 is not the only driver of climate. As you might (or might not?) recall, radiative exchange is about 4x that of conductive/evaporative energy levels - 396 W/m^2 versus (24 convective + 78 latent heat)=102 W/m^2. Please re-read that thread if you still have issues with this; I really don't want to debate it again. The *measurements* show it. As to the "confusion about how powerful CO2", there's really NO confusion whatsoever as to the effect of CO2, or whether that effect exists, also here. There are certainly questions about the level of positive/negative feedbacks with water vapor and the like, but CO2 forcings with changing concentration are really the easiest to account for - and quite significant.
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  13. Hey Doug, Testing my long unused physics: If a typical microwave operates at 2.45GHz, wouldn't that correspond to a wavelength of about 12.2 cm ~= 4.75 inches. So, shouldn't the optimal bit of wire be that long?
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  14. Chris G, you're right, I took the 2200mHz number off the top of my head and remembered incorrectly. Middle-age fog=Bad mismatch!
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  15. At that point, RSVP, I will be using my own hot air on this topic to heat my house, much to the displeasure of my lovely spouse.
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  16. Nvd, I'm forgetting velocity factor, and possibly other stuff.
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  17. Chris G I think you only need the antenna to be half the actual wavelength... KR Wait a minute. You wont need to heat your house. It will already be globally warmed, and if not, maybe this will bring you closer together! Its a win, win situation.
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  18. Re the car radiator theory What about an evaporative air conditioner? This adds water vapour to the air, and in the process, it cools you more than a fan does. why? Because the process of converting liquid to gas locks up a lot of heat energy (latent heat). This latent heat is trapped in the molecular structure of the water vapour, and is re-released when the vapour condenses (another reason why it is warmer on cloudy nights). So the radiator in a car would work much more efficiently on a foggy morning, and water vapour is not the only gas in the amosphere that can absorb heat. The alternative is that the heat is simply radiated in all directions, with a fair amount being lost in space. So yes, the car radiator analogy does have merit even though it is an artificially amplified system (same as enhanced global warming actually)
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  19. #67, I believe you are correct - distance from peak to trough, or some harmonic thereof. Sounds right.
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  20. Anyhow. Back to the original idea. This is exactly the kind of explanation that will work with a non-scientific audience. And the more people in the scientific community who can come up with these ideas the better. Or talk them through with PR or advertising types who can simplify and condense complex messages, the easier it will be to have a more informed community.
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  21. re #10: clouds reflect light and some very shortwave infrared, but the droplets/crystals are almost black (absorbing some 90 %, like soot for light) for longwave infrared. Clouds don't reflect LWIR, they just absorb it. (Well, they may reflect a per cent or two.) A simple graphics for SW (solar) and LW (terrestrial] radiation is in a NASA fact sheet. And re the main Kevin's article: water vapour does indeed let some wavelengths go through rather easily and is supplemented by other trace gases to make the full existing greenhouse effect. But condensed water (liquid or solid) absorbs all wavelenghts over two micrometers completely, within a layer of several millimetres. Any such radiation can penetrate a cloud only in case it misses all its particles (it is possible for clouds through which we see the Sun sharply).
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  22. I just have a question, as CO2 is heaver than air how does it get up into the higher atmosphere? When there is a volcanic eruption there is often warnings for people to keep out of the lower areas due to build up of CO2.
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  23. Signmaster - simple answer is that CO2 is well-mixed in the atmosphere, because parcels of air rise as they are heated. Temperature gradients between warm and cool places creates wind which helps to mix CO2 in the atmosphere. Scientific explanation would cause your eyes to glaze over. As for volcanic eruptions these are localized events, and the rate of CO2 emissions can be very intense. You will note that when out-gassing finishes CO2 levels can drop rapidly as CO2 is carried away by winds/mixing. Fossil fuel combustion, by comparison, is a global phenomenon. Here's a pic of CO2 satellite measurements well above ground level:
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