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

Extreme Water Loss and Abiotic O$_2$ Buildup On Planets Throughout the Habitable Zones of M Dwarfs