After posting some comments on various blogs and seeing the replies I realized that a page like this was necessary.
For people who’ve just arrived at this page, you might be asking:
-which in itself is one of the most important questions, but let’s not jump ahead..
The background is the series CO2 – An Insignificant Trace Gas? and especially the last post – which maybe should have come earlier! – CO2 – An Insignificant Trace Gas? Part 6 – Visualization
If you take a quick look at that last post you will find a few simple measurements that demonstrate that CO2 and other “greenhouse” gases have an effect at the earth’s surface.
In brief, simply that CO2 and other greenhouse gases add a “radiative forcing” to the earth’s surface. A “radiative forcing” means more energy and, therefore, heating at the earth’s surface. And more CO2 will increase this slightly.
At this stage, we have said nothing about feedback effects or even the end of the world.. The series on CO2 is simply to unravel its effect on global temperatures all other things being equal. Which of course, they are not! But we have to start somewhere.
Here are two graphics from Part Six showing energy up and energy down that are the basis for many many questions..
“TOA” = top of atmosphere.
The simple story these two graphics outline is that the earth radiates “longwave radiation” from its surface (because it has been heated by “shortwave radiation” from the sun).
The radiation from the earth’s surface is a lot more than the radiation leaving the atmosphere. Where does it go?
And why do we measure longwave radiation downwards at the earth’s surface. Where does that come from? And why do the wavelengths match those of CO2, methane and so on?
The answer – CO2 and other “greenhouse” gases absorb longwave radiation and re-emit radiation, both up (which continues on its journey out of the atmosphere) and down. The downward component increases the temperature at the earth’s surface.
The story sparked many questions on other blogs..
Questions like these are great, they clarify for me the common problems people have in understanding the “greenhouse” effect (always in quotes because it’s not really like a greenhouse at all!)
I’m not writing to try and change people’s minds. I’m writing for people who are asking questions and want to understand the subject. The only two things I ask:
- Be prepared to think it over
- If you have questions or comments, please ask these questions or make these comments (just remember the etiquette)
1. The Downwards Radiation is probably from the Sun
One commenter said:
You cite measurements of downward radiation. Were those measurements taken during the day or at night? Your link doesn’t say, and the answer is extremely critical to your argument.
I wasn’t clear why it really mattered so I asked. The response from the commenter was:
More than half of what we receive from the sun is already in the IR, so a daytime measurement is just measuring spectral lines by shining a light source through a gas. Anyone could do that in a lab with just air. The energy measured is just solar energy.
Anyone who has read the CO2 series on this blog, even just Part One, will have their hands in the air already..
Log plot of solar radiation vs terrestrial radiation by wavelength. The solar radiation is amount absorbed (i.e. takes into account typical albedo) and received at 45°.
Linear plot of the same data.
- 99% of the sun’s radiation has a wavelength less than 4μm
- 99.9% of the earth’s radiation has a wavelength greater than 4μm
There is almost no overlap, so if we measure what we conventionally call longwave radiation (>4μm) we know it comes from the earth. And if we measure what we conventionally call shortwave radiation (<4μm) we know if comes from the sun.
This simple fact is an amazing help in understanding the climate! But most people don’t know it!
Two related points have arisen, one of which is alluded to in the question above:
1. “Half of the radiation from the sun is infra-red, therefore..”
True but a red-herring. Infra-red means longer wavelength than visible light. Greater than 0.7μm. Not greater than 4μm.
2. “The sun’s energy is way way higher, so even though only 1% of its energy is greater than 4μm this will still overwhelm the earth’s energy above 4μm.”
This is true when we look at the energy at its source, but only a two billionth of the sun’s total energy is received by the earth. Alternatively, considering the radiation per m² the solar radiation is reduced by a factor of 46,000 (as a result of the inverse square law) by the time it reaches the earth.
The total energy from the sun’s radiation (at the earth’s surface) is very similar to the total energy radiated from the earth. (Actually no surprise otherwise the earth would rapidly heat up!)
For more on this, see The Sun and Max Planck Agree – Part Two.
2. Energy has to Balance (at the Top of Atmosphere)
If you measure all the energy going in at TOA vs all the energy going out at TOA, you will find that they net to zero over time.
This is true. Everyone agrees. The “greenhouse” gas theory doesn’t make a claim that contradicts this well-established fact. In fact, it relies on it!
Let’s clarify the numbers because I gave the “clear sky” results in the graphic, but most of the time it’s cloudy and then the numbers are lower.
The average over the globe and over a year at the top of atmosphere for incoming and outgoing radiation is about 240W/m2. Strictly speaking it is incoming radiation absorbed, because about 30% of “incoming” is reflected by clouds and the earth’s surface. Check out the numbers in The Earth’s Energy Budget. This is all measured by satellites.
Note - what is very important about this is that the radiation in and out at the top of atmosphere balance. Down at the earth’s surface many other effects are going on – convection, latent heat (evaporation and condensation of water) as well as radiation. Energy in = Energy out is true everywhere that no heating or cooling is going on. But it’s not necessarily true that Radiation in = Radiation out at the surface or in the atmosphere, as other ways exist of losing or gaining energy. At the top of atmosphere there is no convection and no water vapor, so energy can only be moved by radiation.
Hopefully that makes sense. Read on..
3. The most popular – You are “Creating” Energy!
Since there is no NEW energy being put into the system, and the amount of energy being put in will, over the long term, equal exactly the amount of energy coming out, all you get at most is a short term fluctuation. If I am wrong, then you have invented perpetual motion.
This is a common theme and a recurring one. Many people think that the theory is effectively claiming that CO2 is creating energy.
Obviously that doesn’t happen.
Therefore, QED, the “greenhouse” effect doesn’t occur! The defence rests.
Well.. let’s take a look.
I’ll first give an analogy. This is an illustration not a proof.
You have a house without a roof. It has a heater on the floor and there aren’t any other sources of energy. The temperature being measured is around 10°C, it’s a bit chilly. Someone puts a roof on the house, what happens to the temperature? It goes up, maybe now it is 15°C.
No, it can’t have gone up. The roof doesn’t create energy so the temperature must still be 10°C!
Of course, no one reading this is confused. But when I gave that example I had people still trying to demonstrate that this analogy wasn’t valid.
Suppose there was no energy source. The roof – or insulation – wouldn’t create any heat.
True – and if the earth had no sun heating it, CO2 wouldn’t have any heating effect at the earth’s surface either.
What is the theory claiming for CO2?
- It isn’t creating energy
- It isn’t adding energy to the climate system
- It is absorbing and re-emitting “energy”
So instead of all the radiation from the earth’s surface simply heading up and out of the top of atmosphere, instead, some proportion is being “redirected” back down to the earth’s surface.
Like a roof but different.
The point is, there is no violation of energy conservation or any other law of thermodynamics.
The longwave energy being re-emitted back down to the earth’s surface – as you can see in the 2nd graphic above – simply increases the surface temperature. It increases the surface temperature above what it would be if this effect didn’t exist. (Like a roof on a house).
4. Your Radiation Numbers are Wrong
Referring to my “Upwards longwave radiation from the surface of the earth is around 390W/m2.”
Nyet. At 0 C, radiance is about 320 watts/m2. At 30 C, its about 550 watts/m2. You can’t just average the numbers from low to high across the globe and get the right answer either. you get a curve with a peak or high about mid day, but you also get a curve with a peak at the equator as compared to the poles. The average between the lows and the highs is NOT the average of the curve.
This is a very good point and worth covering in a little detail.
How do we come up with the number 390W/m2 in the first place?
There is a relationship between temperature and radiation, which is very well established, known as the Stefan-Boltzman law. You can see it at the start of the maths section in Part One.
Energy radiated is proportional to the 4th power of (absolute) temperature
Yuck. Before you skip forward, here are some example numbers (I used the amazing and recommended spectralcalc.com).
- -20°’C (253K) or -4°F – 232 W/m2
- -10°C (263K) or 14°F- 271 W/m2
- 0°C (273K) or 32°F – 315 W/m2
- 10°C (283K) or 50°F – 364 W/m2
- 20°C (293K) or 68°F – 418 W/m2
- 30°C (303K) or 86°F – 477W/m2
So our commentor was correct as to the method. If you want to work out how much energy is radiated from the surface of the earth, you can’t just assume you can use the earth’s average annual global temperature (15°C) to get the average radiation of 390W/m2.
This is true because the relationship is non-linear – see how the radiation increases more and more for the same 10°C or 10K rise in temperature.
Luckily this work has already been done for us, it doesn’t actually change the result that much.
Average annual global radiation from the earth’s surface = 396 W/m2 (See note 1 at end)
What’s very interesting about this number is that it is nowhere near 240W/m2 – that number would represent a temperature of -18°C (about 0°F).
So in fact, energy radiated upwards from the earth’s surface is a lot higher than energy radiated out of the top of atmosphere. What’s going on?
5. If your numbers are correct, which I doubt, the earth will ignite
I’m not going to go check your numbers but just consider what you are saying. Your claim is that 156 w/m2 is being retained as extra energy kept inside the atmosphere over the long term. If you are right the planet should ignite in a few days.
So, we saw that the energy out of the system – from the top of atmosphere – is only 240W/m2
And energy radiated up from the earth’s surface is 396W/m2. So if the claim is correct that this “missing energy” is re-radiated back down to the surface, then simple arithmetic demonstrates that the energy will keep “piling up” and the earth will ignite.
Obviously that won’t happen. QED, the theory or the measurements are wrong. The defence rests.
Except.. let’s look a bit closer. The measurements are right, of course. So in fact, anyone disputing the theory needs their own theory to explain the numbers..
If we add extra radiation to the surface of the earth what happens? Simple – it heats up. As the surface heats up it radiates more energy back out. So it keeps heating up until the energy being lost is balanced by the energy coming in.
The point at which the earth’s temperature will stop changing is the value at which the outgoing radiation from the top of the atmosphere is balanced by the sun’s incoming radiation absorbed.
Well, that’s why the earth’s temperature is not -18°C. With no greenhouse effect it would be.
If there was nothing absorbing the upwards longwave radiation, and re-radiating some of it downwards, the radiation from the surface of the earth would only be 240W/m2 – a surface temperature of around -18°C (0°F).
To many people it seems like a wacky theory easily refuted by common sense, the basic laws of thermodynamics or the fact that the earth hasn’t apparently heated up for a decade. (I didn’t comment on that one, the climate is very complex, many factors affect climate).
The theory wasn’t invented by the IPCC or Al Gore (he only invented the internet). And it wasn’t formed from a desire to understand why the earth warmed up over much of the 20th century.
The theory was developed by physicists going back to the start of the 20th century (well, probably before but I’m haven’t studied the history of the subject). Thousands of physicists have studied the subject, dissected it, written papers on it and improved on it.
Even the many “skeptics” of what has become known as AGW in its IPCC form are not skeptics of these concepts. (e.g. Lindzen, Roy Spencer, John Christy)
I don’t want to try and pull the “argument from authority” because I don’t really accept it myself. But pause for thought if you are still not convinced, if you still think this theory magically creates energy from somewhere or violates the 2nd law of thermodynamics – and ask yourself:
If 99.99% of physicists past and present believe this effect is real and measurable, how likely is it that none of them realized there is a basic error in the theory?
The value of 396W/m2 is calculated in Trenberth and Kiehl’s 2008 update to their 1997 paper: Earth’s Annual Global Mean Energy Budget. In the 2008 paper they comment that the upwards radiation from the surface cannot be assumed by averaging the temperature arithmetically and then calculating the radiation. So they take data on the surface temperature around the globe and re-calculate. Depending on exactly the method the values come in at 396.4, 396.1, 393.4. They stick with 396W/m2.