Venus, Mars and Titan: Predicting Earth’s Climate Future
David Grinspoon, a planetary scientist, author, and 2006 recipient of the Carl Sagan Medal has some very interesting insights into the world outside of Earth. In "Ghosts of Climates Past," an article published in SEED Magazine, Grinspoon presents a convincing theory on the ways we might be able to predict the environmental future of Earth by studying the environmental pasts of some of the other planets in our solar system.
Climate is hard to predict because planetary climate systems are full of nonlinear feedback effects that tend to dampen (negative feedback) or amplify (positive feedback) any possible changes. Even though the sun has grown steadily brighter since its birth 4.6 billion years ago, Earth’s climate has stayed within the range of liquid water and "life as we know it" due to a negative feedback loop that sucks carbon out of atmospheric CO2, producing carbonate rock, and volcanoes that pump CO2 back into the air. This process creates a natural thermostat. Regardless of climate, the Earth exhales CO2. But the rate of removal of atmospheric CO2 is highly sensitive to this climate, speeding up exponentially when it is hotter and grinding to a halt when the continents freeze over. A lengthy ice age will always cause a buildup of warming CO2, and a hot period will eventually lower the CO2, cooling our planet again. As the sun slowly brightens over billions of years (by 30 percent so far over its lifetime), this thermostat gradually lowers the overall CO2 content of the atmosphere. Given enough time to respond to any provocation, our climate is stable.