What is a "runaway greenhouse effect" for a planet? when a planet's temperature gets hot enough that water (from the surface such as in oceans, etc.) evaporates into the atmosphere. in the atmosphere, the water acts to further trap radiation from the earth's surface, enhancing the greenhouse effect. it's a necesary part of a planet's evolution from the cold early stages of solar system formation to the warmer stages of the planet now. the planet becomes so hot that greenhouse gasses are released from the rocks on the surface of the planet and are dissolved into the oceans, thereby causing unusually high levels of chemical heating in the oceans on a planet. the regular greenhouse effect is amplified by additional volcanic activity early the planet's history to cause a "runaway" greenhouse effect?