The Science Fair Problem

by Michael Tobis     Department: Earth

At a fourth of July party last year I encountered an MD who had some doubts about the greenhouse effect. He had some of the usual skeptics' questions ("how could such a small amount of CO2 matter?" My answer: "The predominant gases don't have much affect on the radiation. You may or may not believe me but consider this: Could a small amount of arsenic affect my health? Absolute quantities don't settle the case")

Then he asked me a tougher one. He had helped his daughter pull together a science fair project on the greenhouse effect. They had opened a can of Dr Pepper and put in back in the fridge. They had set up two identical glass jars large enough to hold a soda can and a thermometer. They placed an open can in one and a closed can in the other and put them in sunlight to watch the temperature equilibrate. There was no detectable difference in temperature.

I muttered something about inadequate optical depth and grinned foolishly, thinking this was the last I would hear about it. But a couple of weeks ago I realized this wasn't anything like the case. I was called upon to judge a science fair, and sure enough one 5th grader did a very similar experiment. She in fact convinced herself that she had discovered a warming effect, but it was, alas, measurement noise.

Now I am alarmed. I imagine there are hundreds or thousands of similar science fair experiments going on out there. They've all seen the usual greenhouse effect cartoon, but it's really a very incomplete explanation of the situation. Neither the students nor their teachers nor their parents can be expected to pull together a useful quantitative hypothesis from this level of understanding.



It turns out that in addition to the optical depth of CO2 that I mentioned under pressure, there is the fact that CO2 at multiple temperatures must be involved to make the temperature effect measurable. So not only do you have to increase the amount of CO2 at your thermometer, you have to increase it enough so that the upper layers of the atmosphere interact with it significantly.

Is there a way to do a small, cheap, greenhouse effect demonstration? Perhaps out of reach of grade school kids but accessible to college kids or adults with a modest budget? I think there might be but it will take some thought.

Of course, there's no doubt the effect exists; comparison of the surface temperature of the earth and the moon suffices. Venus is much hotter than a similar planet at that orbit without an atmosphere would be. Greenhouse effect calculations, while a bit complex in practice, are in principle the most precise and well-understood parts of the climate system. But people can't touch Venus.

Is there a reasonable do-it-yourself project that can demonstrate the greenhouse effect? I think there is one. I'm working out the details. If anyone is interested, let me know.

Tags: do-it-yourself, experiment