Worldwide since the 1970s, there has been a near-doubling in the number of Category 4 and 5 storms %u2013 the strength that saw Hurricane Katrina do such damage to the US Gulf coastline late in August 2005.
Peter Webster of the Georgia Institute of Technology, Atlanta, says the trend is global, has lasted over several decades and is connected to a steady worldwide increase in tropical sea temperatures. Because of all these factors, it is unlikely to be due to any known natural fluctuations in climate such as El Niño, the North Atlantic Oscillation or the Pacific Decadal Oscillation.
"We can say with confidence that the trends in sea surface temperatures and hurricane intensity are connected to climate change," says Webster's co-author Judy Curry, also of the Georgia Institute of Technology. The team looked at the incidence of intense tropical storms and the study results are the strongest affirmation yet that Katrina-level hurricanes are becoming more frequent in a warmer world.
The study finds there has been no general increase in the total number of hurricanes, which are called cyclones when they appear outside the Atlantic. Nor is there any evidence of the formation of the oft-predicted "super-hurricanes". The worst hurricane in any year is usually no stronger than in previous years during the study period.
But the proportion of hurricanes reaching categories 4 or 5 - with wind speeds above 56 metres per second - has risen from 20% in the 1970s to 35% in the past decade.
"This trend has lasted for more than 30 years now. So the chances of it being natural are fairly remote," says Greg Holland of the National Center for Atmospheric Research (NCAR) at Boulder, Colorado.
Moreover, says Webster, natural fluctuations tend to be localised. "When the east Pacific warms, the west Pacific cools, for instance. But sea surface temperatures are rising throughout the tropics today." The surface waters in the tropical oceans are now around 0.5C warmer during hurricane seasons than 35 years ago.
The International Panel on Climate Change and Australia's own Greenhouse Office both make certain predictions about what the greenhouse world will be like.
More super-hurricanes? Check. More bushfires? Check. More droughts? Check. Greater intensity in all three? Check. Urgent government action to minimise the harm greenhouse will do the planet? Um...