Global warming: can you demonstrate the greenhouse effect?

In BBC1 documentary The climate wars¹ broadcast last September, the presenter, Iain Stewart, demonstrated that CO₂ absorbs infrared (ir) energy and so showed how it can trap heat in the Earth’s atmosphere and contribute to the ‘greenhouse effect’.

The apparatus comprised a 1 m long, 20 cm diameter tube filled with CO₂ from a cylinder. A lighted candle was held at one end of the tube while a thermal imaging infrared camera viewed it from the other end. The bright false-colour image of the hot candle on the camera screen slowly disappears as CO₂ was introduced into the tube, showing that the gas absorbs in the infrared.

Since I built the apparatus for the programme let me share with you what I learnt about this experiment.