Habitable Zones & Global Climate: June 2013

For most terrestrial life on Earth, oxygen is necessary for survival. But the planet's atmosphere did not always contain this life-sustaining substance, and one of science's greatest mysteries is how and when oxygenic photosynthesis--the process responsible for producing oxygen on Earth through the splitting of water molecules--first began.

Recent calculations suggest that the inner edge of the habitable zone around the Sun could be as far out as 0.99 astronomical units (AU)- much closer to the orbit of Earth than had been thought. This reopens the question of whether future increases in atmospheric CO2 might trigger a runaway or moist greenhouse. A runaway greenhouse implies complete ocean vaporization; a moist greenhouse implies that the stratosphere becomes wet, leading to ocean loss via hydrogen escape to space.