In his book Storms of my Grandchildren, noted climate scientist James Hansen issued the following warning: "If we burn all reserves of oil, gas, and coal, there is a substantial chance we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty." Venus has a thick atmosphere that is 96.5 percent carbon dioxide, which keeps its surface at nearly 900°F (482°C). The planet's water boiled off to space long ago. Could that really happen on Earth, which is farther from the sun, and where the CO2 level is just now rising past 400 parts per million? 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. Hansen argues that fossil-fuel burning could cause the process to run out of control, vaporizing the entire ocean and sterilizing the planet. Read more http://news.nationalgeographic.com/...e-global-warming-venus-ocean-climate-science/
Mars' atmosphere is 95.32% carbon dioxide. No run away greenhouse there. Maybe it has more to do with atmospheric pressure and not so much the CO2. Venus' pressure is 92x that of the earth at the surface.
