Every year, the Arctic tern migrates from pole to pole, flying thousands of miles in a matter of weeks. Species as small as dragonflies and as big as gray whales swim and fly from continent to continent in just a few months.

According to a new study, it took the world's largest herbivorous dinosaurs, Brontosaurus and Brachiosaurus, some 15 million years to trek the length of the supercontinent Pangea, from present day South America to what's now Greenland.

The new research, published Monday in the journal PNAS, suggests climate-related barriers were responsible for the sluggish pace.

It was only with the assistance of a 2 million-year-long dip in atmospheric CO2 that the dinosaurs were able to complete their journey, researchers found.

Scientists knew Brontosaurus and Brachiosaurus enjoyed an expansive range, but until now, the timing of their proliferation across Pangea wasn't clear.

By comparing the magnetic signatures trapped in the rocks of fossil sites in Argentina, Brazil, Britain, Canada and elsewhere, researchers were able to constrain the timing of their south-to-north migration.

Researchers were surprised by how long it took the dinosaurs to get from South America to Greenland.

"In principle, the dinosaurs could have walked from almost one pole to the other," lead study author Dennis Kent said in a news release.

"There was no ocean in between. There were no big mountains. And yet it took 15 million years. It's as if snails could have done it faster," said Kent, an adjunct research scientist at Columbia University's Lamont-Doherty Earth Observator.

Had these plant-eating dinosaurs covered just a mile a day, the journey would have taken less than 20 years -- instead, it took 15 million.

When researchers compared the timeline of the slow-going journey with ancient climate records, they realized the timing of the migration corresponded with a dramatic dip in atmospheric CO2.

Scientists estimate the drop in CO2 yielded friendlier climate conditions, removing climate barriers that made long-distance travel near-impossible.

At the outset of the Triassic, CO2 levels broached 4,000 parts per million, ten times today's concentrations. As a result, Pangea likely featured large, extremely dry regions, boxing in Brontosaurus and Brachiosaurus.

Bands of tropical ecosystems, soaked by monsoons, may have also blocked travel, compelling the dinosaurs to remain within their more temperate environs in what is now Argentina and Brazil.

"We know that with higher CO2, the dry gets drier and the wet gets wetter," said Kent.

Researchers estimate that as CO2 levels declined, tropical environs dried out a bit and arid regions got a little wetter. As a result, Brontosaurus and Brachiosaurus may have felt compelled to move -- their path no longer blocked by climate barriers.

It's possible the path from South America to Greenland was marked by a series of long rivers and strings of lakes -- a superhighway of temperature climate conditions paved by a dip in CO2 levels -- the researchers said.

"Once they arrived in Greenland, it looked like they settled in,'" said Kent. "They hung around as a long fossil record after that."

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