For years, scientists have puzzled over an odd gap in the fossil record for early dinosaurs: Even as dinosaurs spread through most of the world 200 million years ago, some yet-unknown force kept them from colonizing the tropics.

Now researchers may have found the explanation: Climate change. Exceedingly high levels of greenhouse gases appear to have rendered the Earth’s equatorial zones all but uninhabitable for the planet’s largest terrestrial beasts.

That’s the core finding of a study that compares the geographic distribution of Triassic-era dinosaurs with new data that details the environmental conditions of the time. The report, published this week in the Proceedings of the National Academy of Sciences, seeks to account for the pronounced absence of large dinosaur remains in Triassic rocks from tropical regions. During that early dinosaur epoch, massive plant-eating reptiles roamed middle and upper latitudes, but only a handful of small, meat-eating dinosaurs colonized the tropics.

The reason appears to be related to harsh climactic conditions that made it difficult for many species to survive, according to Britain’s University of Southampton and eight other institutions. Clues gleaned from fossils point to dramatic shifts in climate, particularly in the tropics.

“Dinosaurs were fast-growing and they required a lot of resources,” said co-author Randall Irmis of the University of Utah. “With unpredictable, hot and dry conditions, plant populations were changing all the time, and you might not have the dependable food supply you need.”

Among the impacts were frequent wildfires that cut into food supplies, he said.

The weather extremes resulted in part from high levels of greenhouse gases – caused mostly by volcanic activity – which at that time were more than four times the historically high levels seen in this century.

Earth was a “greenhouse world,” Irmis said, with higher sea levels and no polar ice caps.

The study’s authors said their findings carry warnings for man, as rising greenhouse gas levels raise questions about the future habitability of parts of the planet.

“Our data reflect that there are possibly substantial hurdles to human sustainability in the future if we undergo the high CO2 levels predicted to occur in the coming 100 to 200 years,” said lead author Jessica Whiteside of Southampton University.