Changes in the "polar vortex," the pattern of winds that encircles the pole, may well be a sign of more severe weather shifts to come, they said.

"The recent trend (of change) seems unprecedented in the historical model," David Thompson of the University of Washington told reporters at a meeting of the American Geophysical Union here.

Scientists presented reports showing that, on balance, the polar vortex has tightened since 1970 -- meaning that the upper-atmosphere winds are blowing in a smaller, stronger circle around the polar region itself.

As a result, frigid polar weather has not moved as far south during the Northern Hemisphere winter, leading to climate changes ranging from higher temperatures across Europe and Asia to a sharp decrease in Spanish rainfall.

Scientists also theorize the shifting polar vortex could be responsible for fiercer winter storms across western North American and western Europe as higher temperatures in the lower, more-populous latitudes clash with the very cold temperatures above the North Pole,  generating high-altitude winds that later pull heat and moisture from the Atlantic and Pacific oceans.

Natural phenomenon

While the effects of the change are becoming clear, scientists say they are still unsure if the shift is a natural phenomenon or part of a broader pattern of change blamed on accumulating greenhouse gasses.  "We can't be sure that what we're seeing is not natural,"  said John Wallace of the University of Washington.

Those who believe man may be to blame theorize that as greenhouse gases warm the lower atmosphere they are cooling the upper atmosphere, setting up a climate change that is slowly drawing the vortex winds closer to the pole.

Many weather observers hope the trend will become clearer over the next several years.

While the past couple of winters have seen a return to higher pressures over the Arctic and more wintry weather over parts of the Northern Hemisphere, they will be watching to see if this trend holds -- if it does, it could mean that the earlier change in polar winds was part of a natural cycle that is now returning to normal.

If the coming winters shift back toward warmer weather, however, "it would be enough to convince most scientists that the changes are human induced, that they're not going to go away, and that they may be an indicator of even bigger changes to come," Wallace and Thompson said in a release.