Molecules have discrete resonant frequencies too (this is the basis of quantum theory). When microwaves are ‘fired’ at water molecules (microwave radiation, not the ovens themselves!), they are at a specific frequency that excites water molecules only, causing them to vibrate. Since the temperature of a substance is simply the measure of how much the molecules of that substance are vibrating, any increase in vibration is an increase in temperature! As most foods are comprised largely of water molecules it is therefore possible to cook them with microwaves!
But what is the link to global warming? Well, it is well known that infrared or IR radiation (which is the ‘heat’ you feel from fires, light bulbs, etc.), along with increasing quantities of greenhouse gases in the atmosphere, is responsible for global warming; and here’s why:
Greenhouse gases (such as carbon dioxide and methane) also have discrete resonant frequencies, but rather than being activated by microwave radiation, they are ‘activated’ by IR radiation, and (like the water molecules in food) the greenhouse gases begin to resonate. This ‘activation’ only occurs during the day while the sun is out, during the night the IR radiation is released (just as heat is released as the food from the microwave cools once the oven has gone ‘ping’) and about half is absorbed by the Earth’s surface (the other half is emitted into space). This process reached an equilibrium millions of years ago and is responsible for life as we know it.
Global warming, climate change, whatever you like to call it, is occurring because more and more greenhouse gases are present in the atmosphere, thus absorbing more and more IR radiation (that is vibrating more) and eventually causing the surface temperature to creep up! As sure as microwave ovens keep cooking food, greenhouse gases will keep absorbing and emitting IR radiation – that is a fact. What is not understood, is what the consequences are of the inevitable temperature increase – this is enough for me to believe that we should start reducing carbon emissions!