What do we mean by a runaway greenhouse effect?
A runaway greenhouse effect occurs when a net positive feedback between surface temperature and atmospheric opacity increases the greenhouse effect on a planet until it's oceans boil away.
A runaway greenhouse effect involving carbon dioxide and water vapour may have occurred on Venus. In this scenario early Venus may have had a global ocean. With an increase in heat and brightness of the sun, the water vapour in the atmosphere also increased. This increased the temperature leading to a situation in which the oceans boiled and all the water vapour entered the atmosphere.
Earth's climate has swung repeatedly between warm periods and ice ages during its history. Some climate scientists suggest that as the Earth is too far from the Sun, it's current luminosity may not cause a runaway greenhouse condition on Earth. Others believe that burning coal and mining shale oil may result in runaway greenhouse on Earth.
Most scientists however predict that a runaway greenhouse effect is inevitable in the long run as the Sun gradually becomes bigger and hotter as it ages. This will potentially spell the end of all life on Earth. The loss of oceans will turn the Earth into a primarily desert planet with the only water left being a few evaporating ponds scattered near the poles and huge salt flats around what was once the ocean floor.