How can I show that methane and CO2 are greenhouse gases?

Factual Questions
1

I am a teacher and to try and demonstrate that methane and CO2 are greenhouse gases I got three flasks. One contained air, the second methane (from the gas jet) and a third CO2. I fixed three lamps so that each flask received the same amount of light. A few minutes later, all had gained equally in temperature.
How so? Are the changes too small to measure or is my apparatus fouled up? Any suggestions?

2

Well they are called greenhouse gases because they make the air work like the glass in a greenhouse. I assume you are using glass flasks, so that is part of the trouble. It would take many feet of the gases to make a measurable percentage of the greenhouse effect that the glass wall of the flask is giving you.

There are other issues with your setup. What the glass/gas does is block long wave (infrared) heat radiation. So:

A) You need something in the flask that would radiate heat. Most any solid object would do, best if it is black. Maybe a chunk of charcoal?

B) You need to heat the object with somethin other than infrared. If your light has lots of infrared, then as much is blocked getting in as getting out…so it is a wash. Maybe heat them in a microwave oven, and see which cools faster.

C) Your object needs someplace to radiate to. A clear sky on a low humidity clear night would be the most realistic, but a cool plate nearby might work.

3

Have you considered trying this experiment?

http://www.srh.noaa.gov/srh/jetstream/atmos/ll_gas.htm

Note they work on a scale of an hour, not a few minutes. You may need to let it sit longer, as a small lamp may put out only a rather modest amount of energy.

4

Hmm…I decided to try Google, and came up with this site, where they seem to say the experiment may not work so well.

5

One point to keep in mind is that greenhouse gases in the atmosphere have many miles of thickness to work with. Granted, CO[sub]2[/sub] is only a fraction of a percent, but you’d still need a greater thickness than you could easily get in a high-school lab to get the same effect.

6

Yes, that whatsupwiththat website seemed just fine until it mentioned the OMG only .00000338% CO2! It’s a pretty standard tactic of climate change deniers to mention that number without explaining the atmosphere is several miles thick.

I think this would be a very difficult thing to demonstrate in a high school lab. I agree that the experiment run in the BBC thing is likely flawed.

7

I’m not sure if you can easily and reliably demonstrate the effect of greenhouse gasses in a simple classroom experiment, but here are some things to keep in mind:

  1. The mass of the gas inside a flask is tiny compared to that of the flask.
  2. Greenhouse gasses absorb and re-radiate IR radiation

To overcome #1 I suggest using a very large yet low-mass container - perhaps a large balloon?
To measure the effect of #2 you could place the balloons between an IR source (incandescent lightbulb?) and an IR detector. The CH4 and CO2 balloons should shade the detector more effectively. I have no idea if this would actually work IRL, whatever you decide on you should definitely try it out a few times beforehand to make sure you get consistent results.

8

This guy demonstrates CO2 infrared absorption in a lab

Looks like Mythbusters did it too, way back when (Oh, Scottie, why did you leave us?)

9

I asked a 10-year old kid to set up the following (this was on a sunny day in summer):

In the morning, take two glass tumblers, fill each half way with water at room temp, where one has tap water, the other has carbonated water. Stick a standard body-temp measuring, mercury thermometer in each glass. Cover both with clingfilm (the thermos were small enough that their tops were under the rim of the glass). Tape the clingfilm round the rim of each glass so it’s airtight. Put both glasses by the window (it’s a sunny day) so the sunlight will shine on them for several hours. Measure the temps at the start (say 9am) and make sure it’s the same for both glasses. Come back to record the temps every 30 or 45 mins.

Over the course of 7 hours, both glasses obviously warmed up, but the carbonated water ended up 4 degrees warmer.

This seemed pretty conclusive to me, but does anyone know if I might have been measuring some other effect?

10

experiments should be repeated. a result to show something true has to be repeatable. a single trial of an experiment could be a fluke.

once an experiment is designed well and you want a conclusion then doing it at least three times, with consistent results, gives a number result that you could find useful.