For Earth to maintain a steady average temperature and a stable climate, the amount of energy radiated from our planet must balance the energy it receives from the Sun. If more energy is received than is lost, the planet gets hotter; if less, then we get colder, until balance is restored. Put simply, energy in = energy out!
Now, some of the incoming energy from the Sun gets filtered out before it ever reaches the Earth’s surface: in particular, harmful X-rays and most ultra-violet (UV) radiation. This happens because these forms of radiation are absorbed by molecules of nitrogen, oxygen and ozone in the upper atmosphere, and converted to thermal (heat) energy.
(Note: you might wish to research the story of the “ozone hole” discovered in the 1970s).
About 30% of solar radiation reaching the Earth actually gets reflected straight back out into space, mainly by the white snow & ice of the polar regions, and by the tops of clouds. You can imagine that if the polar ice caps melted, then less energy would be reflected (and vice-versa). But how does the remaining energy (not reflected or absorbed in the upper atmosphere) interact with the Earth’s land surface, oceans and atmosphere?
It turns out that most of the electromagnetic radiation arriving at the Earth’s surface is absorbed by the surface waters of the oceans, and by rocks, soils and plants on land, making Earth’s surface warmer. The warm surface then re-radiates thermal energy back out towards space as infra-red radiation.
This is where things gets a bit complicated! It turns out that outgoing infra-red waves interact with molecules in the atmosphere in a very important way. The bonds that hold atoms together in the molecules of “greenhouse gases” like H2O, CO2 and CH4 are flexible and, if ‘pushed’ at the right frequency, start to “wobble” (oscillate) in tune with the infra-red waves, absorbing some of their energy. This is called molecular resonance.
Molecular resonance occurs in response to particular frequencies of radiation, especially in the infra-red. Resonance is familiar to anyone who has heard windows rattle as a heavy lorry drives past, or seen a washing machine start to shudder as it winds down from a fast spin – it occurs when any system is subjected to an external oscillation at its natural vibration frequency.
Molecular resonance (of H2O) is also the means by which microwave ovens cook food!