IMAGINE CROCODILES creeping under palm trees in Wyoming. Imagine a North Pole with no year-round ice, where forests of tall trees grow above the Arctic circle. Imagine tropical plants thriving outdoors in London and Midwestern winters no colder than comfortably cool.
      These conditions actually existed on Earth. Several million years after the mass extinction that claimed the dinosaurs, another curious worldwide phenomenon took place -- the late Paleocene and early Eocene epochs sustained temperatures as high as 8 degrees warmer than present-day conditions over a period of 5 million years. What's more, winters in continental interiors were surprisingly mild, even for a globally warm period.
      Paleontologists know about the warm winters of 55 million years ago because they have found fossil ancestors of crocodiles and palms -- which cannot tolerate cold weather -- in continental interiors at midlatitudes. Scott Wing, a paleobotanist at the Smithsonian Institution's National Museum of Natural History, has spent two decades unearthing plant fossils from this time period in the badlands of Wyoming. These findings raised a deep question about climate: how did the winters stay temperate so far from the oceans? In today's climate, the interiors of continents at midlatitudes get cold in the winter since they have short days and long nights and lie far from the warming ocean.
      "Lawrence, Kansas doesn't give a darn about what the oceans are doing," UC Santa Cruz paleoclimate modeler Lisa Sloan says. She wondered: did the ocean's stabilizing influence somehow reach farther inland in warmer times? Not likely, she answered in a 1990 paper.
      Sloan and colleague Eric Barron's computer model showed that even during globally warm periods, winters in continental interiors should have been cold, far colder than crocodiles and palm trees could stand. Either something was wrong with their model -- painstakingly constructed to simulate today's climate conditions -- or Wing and other paleontologists had somehow misread their fossils. In short, the notion of Wyoming's warm winters left Sloan cold.
      Her conclusion startled fossil hunters. Many insisted the computer model was simply wrong.

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