Researchers from the Georgia Institute of Technology and the National Center for Atmospheric Research say the number of category 4 and 5 hurricanes (called typhoons or cyclones in other parts of the world) worldwide has nearly doubled over the past 35 years. The North Atlantic appears to be the area worst affected. Category 4 and 5 hurricanes have increased from 16 in the period of 1975-1989, to 25 in the period of 1990-2004, a rise of 56 percent. As well as being more numerous, the hurricanes also appear to be longer lasting, say the researchers. But although there has been a rise in category 4 and 5 storms, the total number of storms worldwide actually fell. The only exception to this overall decrease is the North Atlantic, where they have become more numerous and longer-lasting.