| http://www.w3.org/ns/prov#value | - But scientists warn that today's global warming means that it could be happening again.The ancient hot spell, which lasted 50,000 to 100,000 years, goes by the unwieldy name of Paleocene-Eocene Thermal Maximum, or PETM. It was caused by a sudden -- in geological terms -- doubling or tripling of carbon dioxide and other greenhouse gases in the atmosphere.Climate scientists say the result was a mass
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