Since the start of the industrial revolution, humans have burned through enough fossil fuels—coal, oil, and natural gas—to add some 365 billion metric tons of carbon to the atmosphere. Deforestation has contributed another 180 billion tons. Each year, we throw up another nine billion tons or so, an amount that’s been increasing by as much as six percent annually. As a result of all this, the concentration of carbon dioxide in the air today—a little over four hundred parts per million—is higher than at any other point in the last eight hundred thousand years. Quite probably it is higher than at
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