  We show that terrestrial planets in the habitable zones of M dwarfs older
than $\sim$ 1 Gyr could have been in runaway greenhouses for several hundred
Myr following their formation due to the star's extended pre-main sequence
phase, provided they form with abundant surface water. Such prolonged runaway
greenhouses can lead to planetary evolution divergent from that of Earth.
During this early runaway phase, photolysis of water vapor and hydrogen/oxygen
escape to space can lead to the loss of several Earth oceans of water from
planets throughout the habitable zone, regardless of whether the escape is
energy-limited or diffusion-limited. We find that the amount of water lost
scales with the planet mass, since the diffusion-limited hydrogen escape flux
is proportional to the planet surface gravity. In addition to undergoing
potential desiccation, planets with inefficient oxygen sinks at the surface may
build up hundreds to thousands of bars of abiotically produced O$_2$, resulting
in potential false positives for life. The amount of O$_2$ that builds up also
scales with the planet mass; we find that O$_2$ builds up at a constant rate
that is controlled by diffusion: $\sim$ 5 bars/Myr on Earth-mass planets and up
to $\sim$ 25 bars/Myr on super-Earths. As a result, some recently discovered
super-Earths in the habitable zone such as GJ 667Cc could have built up as many
as 2000 bars of O$_2$ due to the loss of up to 10 Earth oceans of water. The
fate of a given planet strongly depends on the extreme ultraviolet flux, the
duration of the runaway regime, the initial water content, and the rate at
which oxygen is absorbed by the surface. In general, we find that the initial
phase of high luminosity may compromise the habitability of many terrestrial
planets orbiting low mass stars.
