THE FATE OF HABITABLE WORLDS IS INEXTRICABLY TIED TO ABRUPT CLIMATE TRANSITIONS. THE CONVENTIONAL HABITABLE ZONE (HZ) IS BOUNDED BY UNCONTROLLED ICE ALBEDO AND WATER VAPOR GREENHOUSE FEEDBACKS. IN THESE INSTANCES A SMALL CHANGE IN RADIATIVE FORCING BEGETS STRONG POSITIVE FEEDBACKS ON CLIMATE THAT CAN RESULT IN RUNAWAY GREENHOUSE AND RUNAWAY GLACIATION RESPECTIVELY. WHILE THESE REPRESENT THE END-MEMBER STATES OF THE HZ OTHER INTERMEDIARY CLIMATE TRANSITIONS CAN OCCUR AS WELL BETWEEN WATER-BELT TEMPERATE AND MOIST GREENHOUSE STATES ALL WITH IMPORTANT CONSEQUENCES FOR PLANETARY HABITABILITY. TO UNDERSTAND THE FUNDAMENTAL EVOLUTION OF TERRESTRIAL PLANETARY ATMOSPHERES ONE MUST UNDERSTAND THE PROCESSES THAT LEAD TO DRAMATIC TIPPING POINTS IN CLIMATE. HERE I PROPOSE TO USE ROCKE-3D AND SOCRATES TO SIMULATE ABRUPT CLIMATE TRANSITIONS THAT OCCUR AT THE THRESHOLDS OF HABITABILITY. THIS WILL INCLUDE STUDIES OF MOIST AND RUNAWAY GREENHOUSE ATMOSPHERES NEAR THE INNER EDGE OF THE HZ AND COLD ICY PLANETS WITH THICK ATMOSPHERES NEAR THE OUTER EDGE OF THE HZ.