text: Every object (that has a temperature above absolute zero) emits radiation.  What form this radiation takes depends on the temperature of the object - the sun is hot and emits visible light, the earth is cooler and emits heat.  The heat that is emitted from earth would be radiated out into the space if it wasn't for the greenhouse gasses in our atmosphere.  They absorb the heat, and emit it later, so that the heat stays in our atmosphere, where we can benefit from it.\n\nGreenhouse gasses are not harmful to the earth.  Examples of greenhouse gasses are water vapor and carbon dioxide, and they keep up the temperature of earth.  With greenhouse gasses, the average temperature on earth is 15 degrees Celsius above zero (60 degrees Farenheit above zero).  If there were no greenhouse gasses, the average temperature would be 18 degrees Celsius below zero (zero degrees Farenheit).  Quite a difference!\n\nWhen people speak of greenhouse gasses being harmful to the earth, they (most likely) mean that an increase of greenhouse gasses could be harmful, because that would probably mean increasing temperatures, which could lead to a  change in the climate.