  Water photolysis and hydrogen loss from the upper atmospheres of terrestrial
planets is of fundamental importance to climate evolution but remains poorly
understood in general. Here we present a range of calculations we performed to
study the dependence of water loss rates from terrestrial planets on a range of
atmospheric and external parameters. We show that CO2 can only cause
significant water loss by increasing surface temperatures over a narrow range
of conditions, with cooling of the middle and upper atmosphere acting as a
bottleneck on escape in other circumstances. Around G-stars, efficient loss
only occurs on planets with intermediate CO2 atmospheric partial pressures (0.1
to 1 bar) that receive a net flux close to the critical runaway greenhouse
limit. Because G-star total luminosity increases with time but XUV/UV
luminosity decreases, this places strong limits on water loss for planets like
Earth. In contrast, for a CO2-rich early Venus, diffusion limits on water loss
are only important if clouds caused strong cooling, implying that scenarios
where the planet never had surface liquid water are indeed plausible. Around
M-stars, water loss is primarily a function of orbital distance, with planets
that absorb less flux than ~270 W/m^2 (global mean) unlikely to lose more than
one Earth ocean of H2O over their lifetimes unless they lose all their
atmospheric N2/CO2 early on. Because of the variability of H2O delivery during
accretion, our results suggest that many 'Earth-like' exoplanets in the
habitable zone may have ocean-covered surfaces, stable CO2/H2O-rich
atmospheres, and high mean surface temperatures.
