The green house effect

Yasmin Jayathirtha

All students study environmental science from a young age and the newspapers carry a lot of stories on environmental issues. Global warming is hot news these days (joke intended). However, the stories while giving data and details are not very informative about the science behind the predictions. Much of the forecasts and speculations are based upon models of how this very complex system operates and what perturbs it. We build up a model using a few parameters and see whether the model agrees with the reality. If it does not we will have to consider other factors and add them in. This uses knowledge of how simple systems behave but in modelling we learn how the various systems interact with each other. The science is robust and the computers have become powerful enough to deal with large and varied data. However, we (the lay people) pretty much have to take it on trust and if there is any query or dissent, we do not know whether it is a real failure of the model or a denial of an ‘inconvenient truth’. While the subject is called EVS or environmental science, much of the classroom work for school students is theory based.

To explain the greenhouse effect and global warming, we use examples of the greenhouse itself, how a car with the windows closed heats up, how the atmospheres of Venus, Earth, and Mars contribute to their temperatures and so on. But it is good to see if we can model some of the effects. The experiments that follow also illustrate a very important principle; that when we set up a model, we pick and study one factor at a time, so controls are very important. We may be leaving out factors that do make a difference and so we find that a strong effect does not show up in the real world. Our models will also exaggerate the conditions so that the effects will be seen clearly. This may sound a little irrelevant, but will make sense when one considers the experiments.

Theory
The earth’s atmosphere acts like glass in a greenhouse. When visible light falls on to a piece of glass, the light passes through and is absorbed by whatever it falls on. Those surfaces will radiate energy to the surroundings but as heat (infrared) energy not light. Glass does not let infrared rays pass through so the energy remains trapped inside, making the greenhouse warmer than the surroundings. Radiation is also more effectively absorbed by dark surfaces than light coloured ones. They also radiate better.

Apparatus
Aluminium plates – 2
Thermometers (0 –100 °C) – 3
1 dm3 bottles (plastic/glass) with stoppers – 2
Pieces of lead foil (2cm x 3cm) – 3
Black paint
Candle

Experiment 1
Cover one of the plates with soot from a candle or a luminous Bunsen flame. Put a small drop of wax on both the plates and expose to sunlight. After a while you will see that the wax on the sooty plate has begun to melt. The plates can also be put on either side of a lamp or Bunsen flame. By bringing one’s hands close to the plates, we can compare the heat being radiated by the plates.

Experiment 2
Check that the thermometers show the same reading in air. Paint half of one of the bottles black. Cover the bulbs of the thermometers with lead foil. This acts as the surface to absorb light and re-radiate it. Fix two of the thermometers into the bottle stoppers and put them in the bottles. Stand the bottles and the ‘bare’ thermometer on a windowsill or table so that sunshine falls on the lead foil. Read the temperatures at the start and every minute for about 15 minutes. By this time the temperature of each thermometer will rise and then level off. But each thermometer will level off at different temperatures, with the bare thermometer showing the least and the black-walled bottle, the most rise in temperatures.

The lead foil acts as the surface absorbing energy. The bottles act as the atmosphere, trapping the heat inside and so containing the energy. The black walled bottle shows the effect of dark surfaces in efficiently absorbing and radiating heat (infrared energy).

This very simple model only shows the static effect of heating. On earth, the heating of the atmosphere gives rise to convection currents and set up global wind patterns. This, in turn, has an effect on the temperature of the ocean currents and of rainfall patterns. So, we can discuss the limitations of modelling the world in a bottle. But one can change conditions, compare different plastics and compare plastics and glass. Next month, we can consider how to demonstrate the effect of ‘greenhouse’ gases.

References
• Classic Chemistry Demonstrations Compiled by Ted Lister Royal Society of Chemistry
• School Experiments for Greenhouse Effect Internet search

The author works with Centre for Learning, Bengaluru. She can be reached at yasmin.cfl@gmail.com.