I recently spent an afternoon at Science World in Vancouver, escaping a cold rainy day. In one corner was a display on how global warming works. I thought it was worth repeating here, paraphrased and expanded upon. The explanation went something like this:
Carbon dioxide (Co2) is what is put into the atmosphere when fossil fuels are burned. Humans, mostly in Europe and North America, have been burning more and more fossil fuels (coal, natural gas, and oil are the 3 major culprits) at an increasing rate since the Industrial Revolution started several hundred years ago. At the same time we have been cutting down more and more trees. Trees clean the air by taking in carbon dioxide and putting out oxygen. Carbon dioxide molecules in the atmosphere trap heat. Therefore:
More Carbon Dioxide + Less Trees to Absorb Carbon Dioxide =
More Carbon Dioxide than ever before in the atmosphere, trapping heat in a way the planet has never experienced before
Now I am not a scientist, and admittedly there are disagreements even among scientists about the rate of heat trapping and its effects on the global climate. There are also other gases such as methane and nitrous oxide that are also affecting our environment. But this was such a straightforward explanation, one that a child could easily understand.