Early warning signals of the global warming that is predicted by many scientists could be found among musk oxen and caribou in Greenland, according to U.S. and Danish researchers.

Though separated by 600 miles and a continental ice sheet, the populations of the two wild animals fluctuated in similar ways in response to natural changes in long-term weather patterns in the North Atlantic, according to an article in this week's Nature.

Researchers from Penn State University and the University of Copenhagen say Greenland's herds of caribou and musk oxen increased and dropped based on weather conditions created by the North Atlantic Oscillation (NAO), an annual see-sawing in atmospheric pressure in the Northern Hemisphere.

The phenomenon affects the circulation of the seas and is considered a main driver of winter weather, bringing warmer and wetter seasons to some areas and colder and drier conditions to others.

Biologists Eric Post of Penn State University and Mads C. Forchhammer of the University of Copenhagen compared population figures for caribou on Greenland's west coast with musk oxen on the eastern coast from 1961 to 1981.

The two animals have different diets and live in different climates, with the caribou in the lower Arctic region and musk oxen in the high Arctic.

But the study found that one factor - their exposure to the oscillation - influenced their populations in similar ways.

The researchers say the study shows that musk oxen and caribou could be an early warning signal for global warming.

Post said the study also shows that warming might affect a variety of animals with different diets and habitats and "drive all of them down together."

"From a conservation biology perspective, that's the last thing you want to see," Post said. "Anytime multiple populations go up and down in sync, there's an increased risk that all of them will go extinct."

Musk oxen eat grasses and are indigenous to northeastern Greenland; caribou graze on willows, birch leaves and a variety of small plants in western Greenland - a continent three times the size of Texas with a human population of 56,000.

Musk oxen were counted by Danish military patrols using dog sleds. Caribou counts were based on estimates by wildlife officials, the researchers said.

Forchhammer, an associate professor of biology who has lived in Greenland and is an expert on the musk ox, said the findings about the animals surprised him and his colleague.

"We didn't expect it because they live in such different areas, but it shows how important a system like the [oscillation] can be," Forchhammer said.