Final answer:
The runaway greenhouse effect on Venus was initiated by increased surface temperatures vaporizing water, elevating CO2 and H2O levels that amplified heat retention. This positive feedback loop caused Venus to establish a significantly hotter equilibrium, transforming its climate from Earth-like conditions to the extreme environment we see today.
Step-by-step explanation:
The runaway greenhouse effect on Venus was caused by its high surface temperature which vaporized all surface water, leading to an increase in atmospheric CO2 and H2O. These gases intensified the greenhouse effect, trapping more heat and causing the surface temperature to rise further, perpetuating the cycle. Venus' atmosphere evolved from having a moderate greenhouse effect to a dominating one, establishing a new, much hotter equilibrium near its surface, a process significantly different from the typical large greenhouse effect.
Venus may once have resembled Earth, with moderate temperatures and water oceans, but additional heating, such as an increase in the energy output of the Sun, could have tipped the balance. Increased evaporation of water and gas release from surface rocks led to the runaway greenhouse effect, radically changing Venus' oceans and atmosphere. It is a stark reminder of the importance of maintaining a stable greenhouse effect on our own planet.