What is a runaway greenhouse effect?
A runaway greenhouse effect (sometimes called an
abrupt greenhouse effect) happened on Venus, where
positive feedbacks caused the atmosphere to grow warmer and
warmer until the oceans boiled away. All the oceans boiled into
steam in the atmosphere and the water vapor (H2O) broke into
hydrogen (H) and oxygen O2). The hydrogen escaped into space and
the oxygen recombined, usually forming carbon dioxide. Venus'
atmosphere is now 96% carbon dioxide and the temperature at its
surface is 467°C (872°F).
A positive feedback occurs on earth with the ice caps.
White ice caps reflect the sun's heat, so the earth stays a bit
cooler. This is called the albedo effect. But if other things
happen and the ice melts, then the land underneath is dark and does
NOT reflect the sun's heat. Instead it absorbs it,
and the earth gets hotter. (The earth getting hotter makes the
earth get hotter still!)
Another positive feedback on earth is the frozen tundra.
As the earth warms, the frozen tundra thaws. As the tundra thaws it
releases methane gas, a powerful greenhouse gas. The methane gas
adds to the greenhouse gases and makes the earth warmer. (The earth
getting hotter releases methane making the earth hotter still!)
Earth does not have a runaway greenhouse effect. Earth
has an enhanced, or accelerated greenhouse effect, but scientists
are fairly confident that our positive feedbacks will never be
strong enough to cause a runaway greenhouse effect.