The number of the strongest hurricanes worldwide has nearly doubled over the past 35 years, even though the total number of hurricanes has dropped since the 1990s, a study released on Thursday said.

This shift occurred as global sea surface temperatures have increased over the same period, said researchers from US National Center for Atmospheric Research (NCAR) and the Georgia Institute of Technology. Their paper was published in the Sept.16 issue of Science.

"What we found was rather astonishing," said Peter Webster, a professor at Georgia Tech who led the study. The researchers surveyed the number, duration, and intensity of hurricanes that occurred worldwide from 1970 to 2004.

In the 1970s, there was an average of about 10 Category 4 and 5 hurricanes per year globally. Since 1990, the number of Category 4 and 5 hurricanes has almost doubled, averaging 18 per year globally, Webster said.

Strongest storms are also making up a larger share of the total number of hurricanes, the researchers said. Category 4 and 5 hurricanes made up about 20% of all hurricanes in the 1970s, but over the last decade they accounted for about 35% of these storms.

Category 4 hurricanes have sustained winds from 200 to 250 kilometers per hour; Category 5 systems, such as the Hurricane Katrina that hit New Orleans on August 29, feature winds over 250 kilometers per hour. Both are strongest in the hurricane category.

The largest increases in the number of intense hurricanes occurred in the North Pacific, Southwest Pacific, and the North and South Indian Oceans, with slightly smaller increases in the North Atlantic Ocean.

The North Atlantic is the only region experiencing more hurricanes and tropical cyclones overall since 1995, according to the researchers. The region has averaged eight to nine hurricanes per year in the last decade, compared to six to seven per year earlier.

Category 4 and 5 hurricanes in the North Atlantic have increased at an even faster clip, from 16 in the period of 1975-89 to 25 in the period of 1990-2004, a rise of 56%.

All this is happening as sea surface temperatures have risen across the globe between 0.3 and 0.6 degree in different regions for hurricane seasons since the 1970s, the researchers found.

"Our work is consistent with the concept that there is a relationship between increasing sea surface temperature and hurricane intensity," Webster said.

"However, it's not a simple relationship. In fact, it's difficult to explain why the total number of hurricanes and their longevity has decreased during the last decade, when sea surface temperatures have risen the most."

Whether all of this is due to human-induced global warming is still uncertain, while the link between hurricane intensity and overall climate change is still unclear, the researchers noted.

"We need a longer data record of hurricane statistics, and we need to understand more about the role hurricanes play in regulating the heat balance and circulation in the atmosphere and oceans," Webster said in a statement.

"If we can understand why the world sees about 85 named storms a year and not, for example, 200 or 25, then we might be able to say that what we're seeing is consistent with what we'd expect in a global warming scenario. Without this understanding, a forecast of the number and intensity of tropical storms in a future warmer world would be merely statistical extrapolation."