The Earth's climate is warming. No one knows exactly what effects this gradual change will ultimately have on our planet, but that does not mean we have no way of finding out. In fact, there are several logical places to look for what the end result of global warming, and the future of our planet, might be. We need search no further than the other planets and moons in our solar system.
Much of our fundamental knowledge about climate and its potential for change or stability comes from comparative planetology. Astrobiologists are constantly thinking about what it takes for a planet to maintain a livable climate like our own over the long haul, which for us means billions of years. Spacecraft studies of Earth's neighbors in the solar system have revealed that both of our near-neighbor planets, Venus and Mars, once enjoyed Earthlike environments at some time in their past, until each underwent a change of climate that would have been fatal for any surface life. What we have learned from our celestial investigations is that there are many processes that can doom a once mild planet to an eternity of fire or ice. As we come to know more about these planetary histories, and as we now incorporate new information about recent space probes, Earth's relative climate stability over billions of years stands out as an anomaly.
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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.
When we compare Earth with other planets, we see how easy it is for a planet to permanently lose a pleasant climate. Venus and Mars each started out with warm oceans and volcanoes, and the same carbonate thermostat that keeps Earth temperate was once also operating on our planetary siblings. Yet on each of these other planets, the feedback loop broke down, and the climate veered off toward an uninhabitable state. For each, the reasons behind the collapse are different yet illustrate two destinies that could have been ordained for our planet.
Venus shows us what happens if an Earth is made too close to its star. As the brightening sun warmed Venus's oceans, more water evaporated. Water vapor is a powerful greenhouse gas, causing still more water to evaporate by raising temperatures. This powerful positive feedback leads to a runaway greenhouse effect. As all the water boiled off Venus, its atmosphere's hydrogen seeped into space. With the water gone, carbonate rocks could no longer form. Nothing could remove the CO2 from the atmosphere, yet the volcanoes kept pumping it out. This is why Venus today has an almost pure CO2 atmosphere 100 times as thick as Earth's, and a much higher temperature. It's now hot enough there to fry our spacecraft and destroy all forms of earthly life.