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Our best guess about global warming may be wrong

Scientists wonder whether rising CO2 may trigger something else that further warms the climate

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Mary Knox Merrill/Staff

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Fifty-five million years ago, the world was a much warmer place. The poles were ice-free year-round. Palm trees grew in Alaska. Forests stretched right into the Arctic Circle.

There, swamps like those in today’s southeastern United States hosted alligators, snakes, and giant tortoises.

Scientists call this time in Earth’s history the Eocene, the dawn of the age of mammals. And climatologists have naturally taken a keen interest in how it began.

They know that a dramatic spike in carbon dioxide associated with rapid climate change kicked off the epoch – called the “Paleocene-Eocene Thermal Maximum” (PETM). But what scientists don’t understand about the PETM may hold the most relevant lessons for where the world’s climate is headed today.

So far, scientists have been unable to reproduce the PETM in a climate model. In order to get the climate they suspect existed, they have to crank up carbon dioxide far beyond what they think was actually the case.

They’re missing something – and that something may be key to understanding what happens after atmospheric CO2 increases beyond an unknown threshold. At some point, rising CO2 may trigger something else that further warms the climate. In other words, we may have significantly underestimated the effects of the CO2 now being released into the atmosphere. If the Eocene is any indication, the world is probably in for more warming than suspected.

A new study in the journal Nature highlights the mystery. Just before the PETM, CO2 levels were already gradually rising. Then, in a geological instant – a few thousand years – average global temperatures rose about 7 degrees C (13 degrees F.).

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