Direct Demonstration of the Greenhouse Effect

Consider these three "theories:" climate change, evolution, and gravity. Why are two of them hotly debated by non-scientists, but not gravity? In part, the answer is that climate change and evolution are more complex processes and not readily observable over short time scales to most people. In contrast, the "theory of gravity" is tested every day by billions of people world-wide and is therefore not challenged. While there are numerous "demonstrations" of the greenhouse effect available online, unfortunately, many of them are based on poor understanding of the physical principles involved. For this reason, we sought to develop simple and direct experiments that would demonstrate aspects of the greenhouse effect that would be suitable for museums, K-12, and/or college classrooms. We will describe two experiments. In the first, we use a simple plexiglass tube, approximately 12 cm long, with IR transparent windows. The tube is first filled with dry nitrogen and exposed to an IR heat lamp. Following this, the tube is filled with pure, dry CO2. Both tubes warm up, but the tube filled with CO2 ends up about 0.7 degrees C warmer. It is useful to compare this 12 cm column of CO2 to the column in the earth's atmosphere, which is equivalent to approximately 2.7 meters of pure CO2. This demonstration would be suitable for museum exhibits to demonstrate the physical basis of CO2 heating in the atmosphere. In the second experiment, we use FTIR spectroscopy to quantify the CO2 content of ambient air and indoor/classroom air. For this experiment, we use a commercial standard of 350 ppm CO2 to calibrate the absorption features. Once the CO2 content of ambient air is found, it is useful for students to compare their observed value to background data (e.g. NOAA site in Hawaii) and/or the "Keeling Curve". This leads into a discussion on causes for local variations and the long-term trends. This experiment is currently used in our general chemistry class but could be used in many other science classes. Both of the above experiments should lead to a greater understanding of the scientific basis for the greenhouse effect.