Over the past 30 years, severe storms that tended to hit the tropics are now moving farther toward the North and South Poles.

According to BBC News, the National Oceanic and Atmospheric Administration (NOAA) has led a study, published in the journal Nature, analyzing this trend. Each decade of the analysis, storms were reaching their peak intensity about 33 miles farther north and 38 miles farther south on average.

The observation has not yet been pinned down to one leading factor, but the researchers say climate change, ozone regression and aerosols are playing a part.

"By the time a serious storm reaches maximum intensity, someone has picked it up and is monitoring it, so we can be pretty confident of this data," study lead author James Kossin, NOAA scientists, told BBC News. "What we can't be sure of yet is exactly what's causing the trend. There is compelling evidence that the expansion of the tropics is attributable to a combination of human activities, but we don't know which is the primary factor.

"If ozone depletion is mainly to blame, then the situation is likely to stabilize by the middle of the century after ozone-depleting chemicals are phased out. But if climate change is the main factor then there's no end in sight to this phenomenon."

The researchers are correlating their findings with other studies that confirm the tropics are actually expanding. Since 1980, the same period of the NOAA study, tropic expansion has been linked to the same manmade causes, leading researchers believe the two are linked.

"The rate at which tropical cyclones are moving toward the poles is consistent with the observed rates of tropical expansion," Kossin said in an NOAA news release. "The expansion of the tropics appears to be influencing the environmental factors that control tropical cyclone formation and intensification, which is apparently driving their migration toward the poles."

Kerry Emmanuel, a professor at the Massachusetts Institute of Technology and a study co-author, said areas that have never experienced cyclones could be exposed to such extreme weather events.

"The absolute value of the latitudes at which these storms reach their maximum intensity seems to be increasing over time, in most places," Emanuel said in an MIT news release. "The trend is statistically significant at a pretty high level.

"It may mean the thermodynamically favorable conditions for these storms are migrating poleward."