What Effect Does The Ground Temperature Have On The Shape Of The Outgoing Ir Spectrum

Answer these questions using the online model at http://understandingtheforecast.org/ Projects/infrared_spectrum.html. The model takes CO2 concentration and other environmental variables as input, and calculates the outgoing IR light spectrum to space, similarly to Figs. 4.3,4.5, and 4.8. The total energy flux from all IR light is listed as part of the model output, and was used to construct Fig. 4.6.

1. Methane. Methane has a current concentration of 1.7 ppm in the atmosphere, and it's doubling at a faster rate than is CO2.

a. Is 10 additional ppm of methane in the atmosphere more or less important than 10 additional ppm of CO2 in the atmosphere at current concentrations?

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Fig. 4.8 A demonstration of the greenhouse effect of CO2. (a) We begin with no CO2. Let's assume that the energy budget of the Earth was in balance at a ground temperature of 270 K. In (b) we add 1000 ppm CO2, decreasing the outgoing energy flux. (c) The ground and the atmosphere above it respond by warming up to 8.5 K. The total outgoing energy flux is restored to its initial value. The total energy flux is proportional to the area under the curves. CO2 takes a bite out of (a) to generate (b), but (c) bulks up everywhere to compensate.

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Fig. 4.8 A demonstration of the greenhouse effect of CO2. (a) We begin with no CO2. Let's assume that the energy budget of the Earth was in balance at a ground temperature of 270 K. In (b) we add 1000 ppm CO2, decreasing the outgoing energy flux. (c) The ground and the atmosphere above it respond by warming up to 8.5 K. The total outgoing energy flux is restored to its initial value. The total energy flux is proportional to the area under the curves. CO2 takes a bite out of (a) to generate (b), but (c) bulks up everywhere to compensate.

b. Where in the spectrum does methane absorb? What concentration would it take to begin to saturate the absorption in this band? (How do you identify saturation ofa band on a spectrum plot?)

c. Would a doubling of methane have as great an impact on the heat balance as a doubling of CO2?

d. What is the "equivalent CO2" of doubling atmospheric methane? That is to say, how many ppm of CO2 would lead to the same change in outgoing IR radiation energy flux as doubling methane? What is the ratio of ppm CO2 change to ppm methane change?

a. Is the direct effect of increasing CO2 on the energy output at the top of the atmosphere larger in high latitudes or in the tropics?

b. Set pCO2 to an absurdly high value of 10,000 ppm. You will see an upward spike in the CO2 absorption band. What temperature is this light coming from? Where in the atmosphere do you think this comes from?

3. Earth temperature. Our theory of climate presumes that an increase in the temperature at ground level will lead to an increase in the outgoing IR energy flux at the top of the atmosphere.

a. How much extra outgoing IR would you get by raising the temperature of the ground by 1°C? What effect does the ground temperature have on the shape of the outgoing IR spectrum and why?

b. More water can evaporate into warm air than cool air. By setting the model to hold the water vapor at constant relative humidity rather than constant vapor pressure (the default), calculate again the change in outgoing IR energy flux that accompanies a 1°C temperature increase. Is it higher or lower? Does this make the Earth more sensitive to CO2 increases or less sensitive?

c. Now see this effect in another way. Starting from a base case, record the total outgoing IR flux. Now increase pCO2 by some significant amount, say 30 ppm. The IR flux goes down. Now, using the constant vapor pressure of water option, increase the temperature offset until you get the original IR flux back again. What is the change in T required? Now repeat the calculation but at constant relative humidity. Does the increase in CO2 drive a bigger or smaller temperature change? This is the water vapor feedback.

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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

• arlene
What effect does the ground temperature have on the shape of the outgoing IR spectrum and why?
9 years ago
• Lochlan Wallace
Where in the spectrum does methane absorb?
9 years ago