Hot, hotter, hot: climate seesawed during dinosaur age.

Dinosaurs, too, endured climate change. Although scientists had speculated that the world some 120 million years ago was unvaryingly hot, climate fluctuated dramatically, a new report argues. Twice during a 250,000-year period in the reign of the dinosaurs, tropical sea-surface temperatures varied by 6[degrees]C, the findings suggest. That's twice as much variability as is known to have occurred at any other time in history.

"Since animals evolved on Earth, this range of temperature shift is unprecedented," says Simon C. Brassell of Indiana University in Bloomington, an author of the report, which appears in the October Geology. To determine the ancient temperatures. Brassell and his colleagues used a new method that measures the composition of the cell membranes of a marine microbe that accumulated on the ocean floor.

The researchers analyzed those membranes in sediment cores from the Pacific Ocean floor 1,000 miles east of Japan. During the Cretaceous period, long before modern tectonic-plate movement, the site would have been near the equator in the middle of the Pacific. The researchers calculated that, on two occasions, the ocean-surface temperature dropped from around 35[degrees]C to less than 30[degrees]C in just a few thousand years. Each cooler period lasted between 20,000 and 35,000 years.

Because temperatures vary more slowly at sea than on land, temperature changes elsewhere on Earth were likely to have been even more dramatic, Brassell says. Furthermore, the Pacific Ocean was so large at the time that temperatures would have been stabler there than in smaller oceans. Finally, at more-extreme latitudes, changing ocean-circulation patterns or ice sheets--phenomena that don't occur in the tropics--might have further affected temperatures.

During the early Aptian epoch of the Cretaceous period, huge quantities of organic material fell to the ocean floor, depleting the oxygen in the ocean and forming a black goo that didn't rot. The large quantities of carbon locked up on the bottom of the ocean reduced atmospheric carbon and so would have tended to cool the climate. But the researchers are unsure whether this depletion triggered the drops, and, if so, why they occurred so suddenly.

The newly discovered changes in climate wouldn't have brought about the extinction of the dinosaurs, Brassell notes. That extinction occurred 55 million years later.

Brassell adds that the suddenness of the temperature variations suggests that gradual changes to the atmosphere can have abrupt impacts on climate. "It's like driving an automatic car," he says. "As you push down on the accelerator, all of a sudden you find that you're in a different gear."

Scientists suspect that a rise in atmospheric methane, an event analogous to the current rise in carbon dioxide, might have kicked off the warm Aptian period, says Paul Wilson of the National Oceanography Center in Southampton, England. "What we see here is that the Earth's system has a kind of innate capability to get itself out of jail," Wilson says. "You don't get into a runaway greenhouse scenario"

However, he cautions that the timescale for such adjustments is long from a human perspective. Furthermore, the atmospheric changes 120 million years ago were small relative to the current increase in carbon dioxide.