16 September 2005

Warming Oceans Behind Stronger Hurricanes?

by Kate Melville

Researchers from the Georgia Institute of Technology and the National Center for Atmospheric Research (NCAR) 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%. As well as being more numerous, they also appear to be longer lasting, say the researchers. "What we found was rather astonishing. 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," said Peter Webster from Georgia Tech.

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. The study, appearing in Science, looked at data on storms that occurred worldwide from 1970 to 2004.

What's interesting is that the increase in the number of severe storms seems to coincide with an observed rise in global sea surface temperatures that occurred over the same period. "Our work is consistent with the concept that there is a relationship between increasing sea surface temperature and hurricane intensity," said Webster. "However, it's not a simple relationship."

To shed some light on what, if any, connection there is between climate change and hurricane formation, NCAR is planning a series of detailed computer simulations. "NCAR is now embarking on a focused series of computer experiments capable of resolving thunderstorms and the details of tropical cyclones," said Greg Holland of NCAR. "The results will help explain the observed intensity changes and extend them to realistic climate change scenarios."

But Webster says that more data is needed to fully explore the possible relationship between human-induced global warming and increasingly destructive hurricanes. "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. We don't know a lot about how evaporation from the ocean surface works when the winds get up to around 100 miles per hour, as they do in hurricanes," said Webster. He believes that determining the basic role of hurricanes in the climate of the planet is crucial to connecting trends in hurricane intensity to overall climate change.

"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," Webster concluded.