Past greenhouse warming events provide clues to what the future may hold

If carbon dioxide emissions from the burning of fossil fuels continue on a "business-as-usual" trajectory, humans will have added about 5 trillion metric tons of carbon to the atmosphere by the year 2400. A similarly massive release of carbon accompanied an extreme period of global warming 55 million years ago known as the Paleocene-Eocene Thermal Maximum (PETM).

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