The “Greenhouse Gases”-Carbon dioxide, Methane and Water vapour act like the glass in a greenhouse. They allow the full spectrum of radiation and in particular from the cold ultra violet  to the warm infra red rays to pass through in one direction.

However light reflected back from the earth in the infra-red or warm part of the spectrum is impeded. In consequence  this warmth is “trapped” in the atmosphere and the world in turn becomes warmer,

This effect is important for life on earth; when interfered with by serious pollution e.g. the Eruption of Krakatau the world can have long periods of cold, crops fail, people starve.The spectre of nuclear war where huge amounts of dust would be blasted into the atmosphere included a nuclear winter as the sun was excludedfrom the earth

If too much warming occurs the rain forests become dryer and the tundras melt, their capacity to recycle carbon dioxide is diminished and more carbon dioxide is released. More carbon dioxide means a yet greater greenhouse effect, more warming…thus the idea of a “tipping point” where earth warming goes out of control