Answer questions like ...

1. Burning fossil fuels is the major source of human-produced carbon dioxide. But what is the second leading source?
2. In the 10,000 years before the Industrial Revolution in 1751, carbon dioxide levels rose less than 6 percent. Since then, they've risen by:
3. We produce more than 36 billion tons of carbon dioxide per year. Where does the majority of it end up?

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