Planetary atmospheres need to be in radiation balance; thermal radiation emitted by Earth must equal net absorption of stelar radiation. However, for a hot moist atmosphere, there is a limit on outgoing thermal radiation which is decoupled from surface temperature. If net absorbed stellar radiation exceeds this limit the planet will heat uncontrollably, the so- called “runaway greenhouse”. Here we show that a runaway greenhouse steam atmosphere may be a stable state for a planet receiving the same insolation as Earth, contrary to previous results. This is because the thermal radiation limit is much lower than previously thought and that steam atmosphere would absorb much more solar radiation than previously thought. Thus any sufficient heating of the planet, including greenhouse warming, could trigger a runaway greenhouse. At the present state of knowledge, causation by extreme anthropogenic global change on Earth cannot be totally excluded.