Carl Sagan studied Venus while developing explanations for the runaway greenhouse effect. Venus became a crucial natural laboratory for understanding how a planet’s atmosphere can trap heat and transform its climate.
Sagan’s research helped explain why Venus, despite being similar to Earth in size and composition, has an extraordinarily hot surface. Its dense atmosphere is dominated by carbon dioxide, and its clouds and atmospheric structure produce an intense greenhouse effect. The planet’s surface temperature is roughly 465°C, hot enough to melt lead.
The term runaway greenhouse describes a feedback process in which warming causes more water to evaporate, water vapor traps additional heat, and the cycle prevents the planet from efficiently radiating energy into space. Venus’s present atmosphere is more complex than any single early model, but the planet remains the classic example of extreme greenhouse warming.
This work also gave Sagan a way to discuss Earth’s climate. Venus is not a direct forecast of Earth’s future, because the planets differ greatly in atmosphere, distance from the Sun, geology, and water history. Still, Venus showed how planetary climate can change dramatically.