While 400,000 years sounds like a long time, the temperature change is so drastic that the impact was striking in terms of animal extinctions, said one of the study's authors, paleontologist Bruce MacFadden of the University of Florida.

For comparison, a report last week by the Intergovermental Panel on Climate Change said dramatic consequences -- rising sea levels, changing ocean currents and more intense storms -- could result from an estimated temperature increase of 3 to 7 degrees F (1.8 to 4 degrees C) this century.

"If we're talking about major changes at 2 to 4 degrees C, if we go to 8 degrees C, the latitudinal seasonal temperature belts would shift, the tropics would contract, there would be huge differences," MacFadden said in a telephone interview.

"It would be incredible, profound, whatever superlative you'd like to use," he said.

TEETH AS TIME CAPSULES

Paleontologists have known for more than a century that many species went extinct at the transition between the Eocene and Oligocene, based on the fossil record before and after the shift. However, they lacked the analytic tools to determine how much temperature change there was over how long a period, and exactly when it occurred.

Fossil research showed that creatures that might be at home in the tropics, like warm-loving crocodilians, roamed what is now Nebraska on the American Great Plains before the temperature transition. After the transition, they were gone.

Scientists believe changes in ocean currents were to blame for the shift.

To figure out the details of what French researchers dubbed Grande Coupure -- "big cut" -- MacFadden and his co-authors examined the preserved teeth and bones of fossil horses and another cloven-hoofed mammal called an oreodont.

They analyzed oxygen and carbon isotopes contained in the samples; oxygen isotopes told the scientists at what temperature the teeth were formed and carbon isotopes revealed the relative humidity, the University of Florida said in a statement.

Using isotope data and precise locations and dates for the fossil teeth, the researchers were able to pinpoint the details of the transition, the biggest climate change since the end of the dinosaurs about 65 million years ago.

MacFadden likened the ancient horses' teeth to "little time capsules that allow us to analyze chemicals captured millions of years ago within the animals' skeletons."