IB Physics 2015 Higher level Paper 2 Question 6

Question: Students are expected to provide a definition of the enhanced greenhouse effect. 

Mark Scheme: addition of greenhouse gases / named greenhouse gas to the atmosphere; increasing the temperature of Earth’s surface / global warming. 

Comments: 

In Physics for the IB diploma, the enhanced greenhouse effect is defined as “the additional warming of the caused by increased quantities of greenhouses gases. The increase in the greenhouse gas concentrations is mainly due to human activity (Tsokos, 2008, p. 824).” In a sense, the mark scheme is better than the textbook definition because it specifies the effect more clearly. That is, the mark scheme includes “increasing the temperature of the Earth’s surface,” whereas the textbook definition only mentions “additional warming.” On the other hand, the textbook definition states that “the increase in the greenhouse gas concentrations is mainly due to human activity.” However, this fact is not found in the mark scheme.

Importantly, the mark scheme and textbook definition of enhanced greenhouse effect can still be improved. For example, they could specify greenhouse gases such as carbon dioxide and methane. Furthermore, the mark scheme and textbook definition could clarify that the increase in the greenhouse gas concentrations can be due to human activities such as burning fossil fuels and agricultural farming. Conversely, Freeman Dyson, for example, opines that good scientists should be skeptical of the global warming. However, the majority of climate scientists believe that the global warming is “very likely” caused by human greenhouse gas emissions. Currently, there are different physical models on the enhanced greenhouse effect.

Feynman insights?:

One simple model to understand the distribution of various gases on the Earth is to assume we have a column of gas extending to a great height at thermal equilibrium, as well as without winds and other kinds of disturbance. Based on this model, Feynman explains that “if we have different kinds of molecules with different masses, they go down with different exponentials. The ones which were heavier would decrease with altitude faster than the light ones. Therefore, we would expect that because oxygen is heavier than nitrogen, as we go higher and higher in an atmosphere with nitrogen and oxygen the proportion of nitrogen would increase. This does not really happen in our own atmosphere, at least at reasonable heights, because there is so much agitation which mixes the gases back together again. It is not an isothermal atmosphere. Nevertheless, there is a tendency for lighter materials, like hydrogen, to dominate at very great heights in the atmosphere, because the lowest masses continue to exist, while the other exponentials have all died out (Feynman et al., 1963, section 40–1 The exponential atmosphere).” Essentially, the densities of various gases in earth’s atmosphere decrease exponentially with height based on the assumption of constant temperature and constant gravitational field.

Furthermore, Feynman elaborates how water vapors radiate heat to the sky. In his own word, “when you go up in altitude the air is colder. The ground is heated by the sun, and the re-radiation of heat to the sky comes from water vapor high in the atmosphere; so at high altitudes, the air is cold — very cold — whereas lower down it is warm. You may say, “Then it’s very simple. Warm air is lighter than cold; therefore the combination is mechanically unstable and the warm air rises.” Of course, if the temperature is different at different heights, the air is unstable thermodynamically. Left to itself infinitely long, the air would all come to the same temperature. But it is not left to itself; the sun is always shining (during the day). So the problem is indeed not one of thermodynamic equilibrium, but of mechanical equilibrium (Feynman et al., 1964, section 9–4 Thunderstorms).” That is, a realistic model of earth’s atmosphere for greenhouse effect cannot be based on thermodynamic equilibrium.

Note:

Physicists prefer to define heat as a process rather than a noun (Romer, 2001; Wong, Chu, & Yap, 2014). Thus, it is good to avoid phrases such as “heat-trapping gases” or “too much heat is trapped on Earth.”

References:

1. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley. 

2. Feynman, R. P., Leighton, R. B., & Sands, M. (1964). The Feynman Lectures on Physics, Vol II: Mainly electromagnetism and matter. Reading, MA: Addison-Wesley. 

3. Romer, R. H. (2001). Heat is not a noun. American Journal of Physics, 69(2), 107-109. 

4. Tsokos, K. A. (2008). Physics for the IB diploma (5th ed.). Cambridge: Cambridge University Press. 

5. Wong, C. L., Chu, H. E., & Yap, K. C. (2014). Developing a Framework for analyzing definitions: A Study of The Feynman Lecture. International Journal of Science Education, 36(15), 2481-2513.