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Venus has a thick atmosphere that is 96.5 percent carbon dioxide. The planet's water boiled off to space long ago. Could that really happen on Earth? The key to the argument is a well-documented positive feedback loop. As carbon dioxide warms the planet through the greenhouse effect, more water evaporates from the ocean-which amplifies the warming, because water vapor is a greenhouse gas too. That positive feedback is happening now. Some scientist argue fossil-fuel burning could cause the process to run out of control, vaporizing the entire ocean and sterilizing the planet.
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