Ward and Brownlee predict that there will be two variations of this future warming feedback: the "moist greenhouse" where water vapor dominates the troposphere and starts to accumulate in the stratosphere, and the "runaway greenhouse" where water vapor becomes a dominant component of the atmosphere such that the Earth starts to undergo rapid warming that could send its surface temperature to over 900 °C , causing its entire surface to melt and killing all life, perhaps in about three billion years' time.