Over much of Earth history the geomagnetic field has provided a unique protective boundary by balancing the erosive solar wind well above the top of the atmosphere.  On the contrary, the lack of evidence for a large scale magnetic field at Venus has been cited as the reason why the planet lost much of its water.  Similarly, plate tectonics on Earth have maintained a stable carbon cycle and moderate climate, while Venus’ climate is in a runaway state with all surface volatiles in the atmosphere.  Active surface tectonics is the critical difference that both stabilizes Earth climate and cools the interior fast enough to maintain dynamo action in the core.  In this talk I develop a numerical “box” model that couples volatile exchange between the mantle, atmosphere, ocean, and crust.  The model is applied to the atmospheric evolution for the two planets assuming similar initial volatile abundances, and constant magnetic field strengths and plate speeds.  The Earth model maintains a moderate surface greenhouse temperature by reaching a steady state carbon cycle, with most CO2 buried in the crust.  At Venus’ orbit a runaway greenhouse is difficult to avoid because ground temperature does not reach the water liquid-vapor phase equilibrium, precluding precipitation and leaving all CO2 in the atmosphere.  Escape of hydrogen to space is magnetic field limited above a critical mixing ratio and displays a minimum in escape flux for Earth-like magnetic moments.    The model demonstrates how the dynamics of a nascent planetary interior can stabilize surface volatile cycling and atmospheric evolution.