"The concentration of atmospheric carbon dioxide has changed greatly since fossilized life began on Earth nearly 600 million years ago. In fact, there is only 1/19 as much CO2 in the air today as there was 520 million years ago. That high CO2 was hardly the recipe for disaster."

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If it were only a few degrees, that would be serious, but we could adapt to it. But the danger is the warming process might be unstable and run away. We could end up like Venus, covered in clouds and with the surface temperature of 400 degrees. It could be too late if we wait until the bad effects of warming become obvious. We need action now to reduce emission of carbon dioxide.

— Stephen Hawking