Band saturation

The core of the CO2 bend absorption band, between 600 and 800 cycles/cm, looks smooth rather than jagged and it follows a blackbody spectrum from about 220 K. This is about as cold as the atmosphere gets, and if we change the amount of CO2 in the atmosphere, the intensity of light in this range does not get any lower (Fig. 4.5). We call this phenomenon band saturation. You can see it in a series of model runs in which the CO2 concentration of the atmosphere goes up from 0 to 1000 ppm. The current concentration of CO2 in the atmosphere is about 380 ppm, as we will learn more in Part II. If there were no CO2 in the atmosphere, the atmosphere would be

Co2 Saturation Spectrum

Fig. 4.5 A demonstration of band saturation by CO2. The addition of (b) 10 ppm CO2 makes a huge difference to the outgoing IR light spectrum relative to an atmosphere that has (a) no CO2. Increasing CO2 to (c) 100 and (d) 1000 ppm continues to affect the spectrum, but you get less bang for your CO2 buck as CO2 concentration gets higher.

Fig. 4.5 A demonstration of band saturation by CO2. The addition of (b) 10 ppm CO2 makes a huge difference to the outgoing IR light spectrum relative to an atmosphere that has (a) no CO2. Increasing CO2 to (c) 100 and (d) 1000 ppm continues to affect the spectrum, but you get less bang for your CO2 buck as CO2 concentration gets higher.

200 400 600 800 1000

Atmospheric CO2 concentration (ppm)

Fig. 4.6 Band saturation viewed in a different way from Fig. 4.5. This is a plot of the total energy flux carried by all IR light, which is proportional to the area under the spectrum curves in Fig. 4.5. The outgoing energy flux is less sensitive to CO2 when CO2 concentration is high.

200 400 600 800 1000

Atmospheric CO2 concentration (ppm)

Fig. 4.6 Band saturation viewed in a different way from Fig. 4.5. This is a plot of the total energy flux carried by all IR light, which is proportional to the area under the spectrum curves in Fig. 4.5. The outgoing energy flux is less sensitive to CO2 when CO2 concentration is high.

transparent to light of around 700 cycles/cm, as it is in the atmospheric window. Adding the first 10 ppm of CO2 has a fairly noticeable impact on the shape of the outgoing light spectrum, but increasing CO2 from say 100 to 1000 ppm has a somewhat subtler effect.

I have plotted the total energy intensity Iout (in W/m2) as a function of the concentration of CO2 in the atmosphere (Fig. 4.6). Changes in CO2 concentration have the greatest effect if we were starting out from no CO2 and adding just a bit. The first 10 ppm of added CO2 changes Iout by as much as going from 10 to 100, or 100 to 1000 ppm. We can understand why by analogy to our murky pond or by looking back at Fig. 4.4. As we increase the murkiness of the water, we decrease the distance that a photon of light can travel before it is absorbed. It doesn't take much murk in the water to obscure the old tire on the bottom, shifting the depth to which we can see from the bottom at say 3 m to maybe only 1 m. If we make the pond a lot murkier we will only be able to see a few centimeters down into the water. Making it murkier still will limit our view to only 1 cm. The change in depth is getting less sensitive to the murkiness of the pond. In the same way, the changes in the temperature at which the atmosphere radiates to space get smaller as the CO2 concentration of the air gets higher. You just see the coldest light that you can get.

The band saturation for CO2 makes CO2 a less potent greenhouse gas than it would be if we had no CO2 in the air to start with. Let's revisit our comparison of the CO2 and methane as greenhouse gases. Methane had a disadvantage because its absorption band sort of fell in the suburbs of the earthlight spectrum whereas CO2 fell right downtown. Now we see the advantage shifting the other way. Methane has a much lower concentration in the atmosphere. You can see from the jagged edges of the methane peak in Fig. 4.3 that the methane absorption band is not saturated. For this reason, in spite of the suburban location of the methane band, a molecule of methane added to the atmosphere is 20 times more powerful than is a molecule ofCO2.

If the edges of the absorption bands were completely abrupt, as if CO2 absorbed 600 cycles/cm light completely and 599 cycles/cm light not at all, then once an absorption band from a gas was saturated, that would be it. Further increases in the concentration of the gas would have no impact on the radiation energy budget for the Earth.

CO2, the most saturated of the greenhouse gases, would stop changing climate after it exceeded some concentration. It turns out that this is not how it works. Even though the core of the CO2 band is saturated, the edges of the band are not saturated. When we increase the CO2 concentration, the bite that CO2 takes out of the spectrum doesn't get deeper, but it gets a bit broader.

The bottom line is that the energy intensity Iout in units of Watts per square meter goes up proportionally to the log of the CO2 concentration, rather than proportionally to the CO2 concentration itself (we would say linear in CO2 concentration). The logarithmic dependence means that you get the same Iout change in Watts per square meter from any doubling of the CO2 concentration. The radiative effect of going from 10 to 20 ¡atm pCO2 is the same as going from 100 to 200 ¡atm, or 1000 to 2000 ¡atm.

The sensitivities of climate models are often compared as the average equilibrium temperature change from doubling CO2, a diagnostic number that is called A T2x. Most models have a AT2x between 2 and 5 K, which is the same as between 2°C to 5°C. You can use AT2x to estimate a temperature change resulting from some change in CO2. Note that this is the ultimate temperature change, after hundreds or even thousands of years have passed (see Chapters 7 and 12). The equation is

where ln is the natural log, the reverse operation of the exponential function ex, The symbol "e" denotes a number which has no name other than simply "e". We will meet "e" again in Chapter 5. The exponential function is to raise "e" to the power of x. If ex = y (4.2)

Equilibrium temperature changes from changes in CO2, assuming various A T2x values, are shown in Fig. 4.7.

What happens to the energy balance of the Earth if we add a greenhouse gas to its atmosphere? If the energy budget was in equilibrium before, it isn't any more because the greenhouse gas has decreased the amount of energy leaving the Earth to space. We can see this visually as the big bite out of the spectrum going from the top to the middle diagram in Fig. 4.8. The decrease in energy flux is proportional to the area of that bite, the difference between (a) and (b) in Fig. 4.8. Referring to Chapter 2, the premise of the layer model is that the energy coming into and going out of the planet must balance, and the planet accomplishes this feat by adjusting its temperature. If we want to rebalance the energy flux after kicking it by adding CO2, we do that by increasing the temperature of the ground. Using the online model, we find that a temperature change of 8.5 K brings us back to the same energy output Iout as we had before. Looking at Fig. 4.8(c), we see that the new, warmer output spectrum has risen

Climate Parameter
Fig. 4.7 The average temperature of the Earth as a function of atmospheric CO2 concentration and the climate sensitivity parameter, A T2x.

everywhere compared to (b). Visually, we have cut some area out of the CO2 absorption band, and added it in the atmospheric window and other parts of the spectrum, until the overall area under the curve is the same as it was initially. Adding the CO2 caused the planet to warm.

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Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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Responses

  • ermias
    What concentration of methane does it take to begin to saturate the absorption in this band?
    9 years ago
  • saul
    What is band saturation?
    9 years ago
  • zemzem
    Does the band saturation effect prevent global warming from CO2 If not, what effect does it have?
    6 years ago
  • anna
    What concentration of methane in the atmosphere would cause ir flux saturation?
    6 years ago
  • Calimero
    How do you identify saturation of a band, on a spectrum plot)?
    6 years ago
  • SHIRLEY
    Where does methane absorb and saturate in the spectrum?
    6 years ago
  • Ottavia
    Why is the bend absorption band of co2 greenhouse climate?
    6 years ago
  • Mark
    Does the band saturations effect prevent global warming from CO2?
    6 years ago
  • ralf
    Does the band saturation effect prevent global warning from CO2?
    5 years ago
  • FALCO
    Why does CO2 band get broader when it saturates?
    5 years ago
  • patrick
    Does the band saturation effect absorb light?
    5 years ago
  • marcel
    Does band saturation effect prevent global warming from carbon dioxide?
    4 years ago
  • Christopher
    When does methane concentration saturate spectrum?
    4 years ago
  • Anja
    How to identify saturation of an absorption band?
    11 months ago
  • jill
    Is it accepted that co2 concentration is logarthimic with warming?
    3 months ago
  • karita
    Is the CO2 absorption in the atmosphere saturated?
    7 days ago

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