geography worksheet 2

Molecular Motions and the Greenhouse Gases H2O and CO2

2349cm-1 667cm-1

Okay – let’s review what we learned in the previous week. We learned that there is a different way to describe greenhouse gases using our knowledge of quantum mechanics. First, greenhouse gas molecules can absorb and emit radiation in the infrared range (Earth’s outgoing radiation) by creating a specific molecular motion (bending, stretching, rotating, etc). Secondly, in order to interfere with electromagnetic energy, those molecules must perturb the electrical field as well. Thus, greenhouse gases must have asymmetric molecular movement or be electrically lopsided (electrically imbalance).

https://cimss.ssec.wisc.edu/sage/meteorology/lesson1/AtmAbsorbtion.htm

You learned that water vapor is a powerful greenhouse gas, because such water molecules are naturally lopsided and have an electrical imbalance. Therefore, water vapor shows wide and numerous absorption bands as seen in this figure.

NASA, Robert Rohde - http://earthobservatory.nasa.gov/Features/EnergyBalance/page7.php en:NASA Earth Observatory

You also learned that although not as powerful as H2O, carbon dioxide is also an important greenhouse gas because it absorbs a wavelength range that water vapor does not absorb (red arrow).

Molecular Motions and the Greenhouse Gases H2O and CO2

2349cm-1 667cm-1

Important!

And at that particular frequency of energy, 667 cycles per centimeter, carbon dioxide bends! Thus, this is an important molecular motion of carbon dioxide as a greenhouse gas.

�if the quantity of carbonic acid increases in geometric progression, the augmentation of the temperature will increase nearly in arithmetic progression� (1896)

ΔF (Wm-2) = α ln(C/C0) α = 5.35 Then, ΔT(K) = λ*ΔF

Svante Arrhenius �Climate Sensitivity�: change in global mean temperature in response to a doubling of CO2 volume mixing ratio. Arrhenius’s greenhouse law

One thing I would like to add is that based on the Greenhouse Law of Svante Arrhenius, temperature change is a function of change in carbon dioxide concentration in the atmosphere. Based on his equation, temperature will have the same impact in response to a change in carbon dioxide concentration - 10ppm to 20ppm 100ppm to 200ppm 1000ppm to 2000ppm, etc. (As you can see, if you plot these numbers to Arrhenius’s greenhouse law, ln(C/Co) becomes ln(2), right? As an example, assume that the temperature increase between 10ppm to 20ppm CO2 change is 1C. Therefore, temperature will also increase by only 1C when CO2 concentration increases from 1000 to 2000ppm.)

What does that tell us? If you add more carbon dioxide into the atmosphere, the increasing rate of temperature will decrease (if we have much greater carbon dioxide concentration in the atmosphere, climate sensitivity will decrease).

Outgoing spectrum of the Earth With an atmosphere

The impact of carbon dioxide decreases when we have a higher concentration of carbon dioxide in the atmosphere because carbon dioxide will be more saturated.

What does this mean?

In this figure, you can clearly see a big dip around 667 (cycles/cm) showing that the carbon dioxide molecules are working very hard to absorb and re-radiate infrared energy at this specific wavelength range (creating a bending motion!). The dip is almost flat at the bottom, meaning that at this wavelength band (~667 cycles/cm) carbon dioxide is nearly saturated. Therefore, if we add more carbon dioxide into the atmosphere, the CO2 band will continue to widen, but the curve will not go any deeper.

What is the most concerning GHG?

With your knowledge and understanding of GHGs thus far, what greenhouse gas do you think will be most concerning in the future?

Many scientists consider water vapor as one of the most concerning GHG for the future climate, since the water molecule is currently under saturated in the back radiation spectrum and its concentration can increase significantly when temperature increases (due to increasing evaporation)! We will learn more about this in the following week.