Study: Tropical cyclones migrating out of tropics

WASHINGTON— Tropical cyclones worldwide are moving out of the tropics and toward areas with larger populations, likely because of global warming, a surprising new study finds. But Atlantic hurricanes — which occur in the region including the United States — are not following the trend.

While other studies have looked at the strength and frequency of the storms, which are called hurricanes in North America and typhoons or cyclones elsewhere, this is the first study that looks at where they are geographically when their strength peaks. It found in the last 30 years, tropical cyclones, regardless of their size, are peaking an average of 33 miles farther north each decade in the Northern Hemisphere and 38 miles farther south each decade in the Southern Hemi- sphere.

That adds up to a movement of about 100 miles toward the more populous mid-latitudes since 1982, the starting date for the study released Wednesday by the journal Nature.

“The storms en masse are migrating out of the tropics,” said study lead author James Kossin of the National Climatic Data Center and the University of Wisconsin. Kossin used historical tracks of storms in the Western Pacific, Eastern Pacific, North Indian Ocean, South Indian Ocean, South Pacific and the Atlantic.

That result is that more people are at risk, especially in the Northern Hemisphere, because “you’re going to hit more population areas,” said Yale University historian and cartographer Bill Rankin, who wasn’t part of the study.

In the region where Japan tracks cyclones, they are peaking 42 miles far- ther north each decade. That means cyclones that used to be at their strongest around the same latitude as the northern Philippines are now peaking closer to Hong Kong, Taiwan, Shanghai, Japan and South Korea, Kossin said. There are about 60 million people in Taiwan, Hong Kong, Shanghai and To- kyo alone.

The trend, however, is not holding in the Atlantic basin. The study has seen a northward drift of only 4 miles a decade, which could be merely random variation. Kossin said the Atlantic region is different because of changes in pollution over the United States and other factors.