Study says global warming not worsening hurricanes

Seth Borenstein, Associated Press Yahoo News 19 May 08;

Global warming isn't to blame for the recent jump in hurricanes in the Atlantic, concludes a study by a prominent federal scientist whose position has shifted on the subject.

Not only that, warmer temperatures will actually reduce the number of hurricanes in the Atlantic and those making landfall, research meteorologist Tom Knutson reported in a study released Sunday.

In the past, Knutson has raised concerns about the effects of climate change on storms. His new paper has the potential to heat up a simmering debate among meteorologists about current and future effects of global warming in the Atlantic.

Ever since Hurricane Katrina in 2005, hurricanes have often been seen as a symbol of global warming's wrath. Many climate change experts have tied the rise of hurricanes in recent years to global warming and hotter waters that fuel them.

Another group of experts, those who study hurricanes and who are more often skeptical about global warming, say there is no link. They attribute the recent increase to a natural multi-decade cycle.

What makes this study different is Knutson, a meteorologist with the National Oceanic and Atmospheric Administration's fluid dynamics lab in Princeton, N.J.

He has warned about the harmful effects of climate change and has even complained in the past about being censored by the Bush administration on past studies on the dangers of global warming.

He said his new study, based on a computer model, argues "against the notion that we've already seen a really dramatic increase in Atlantic hurricane activity resulting from greenhouse warming."

The study, published online Sunday in the journal Nature Geoscience, predicts that by the end of the century the number of hurricanes in the Atlantic will fall by 18 percent.

The number of hurricanes making landfall in the United States and its neighbors — anywhere west of Puerto Rico — will drop by 30 percent because of wind factors.

The biggest storms — those with winds of more than 110 mph — would only decrease in frequency by 8 percent. Tropical storms, those with winds between 39 and 73 mph, would decrease by 27 percent.

It's not all good news from Knutson's study, however. His computer model also forecasts that hurricanes and tropical storms will be wetter and fiercer. Rainfall within 30 miles of a hurricane should jump by 37 percent and wind strength should increase by about 2 percent, Knutson's study says.

And Knutson said this study significantly underestimates the increase in wind strength. Some other scientists criticized his computer model.

MIT hurricane meteorologist Kerry Emanuel, while praising Knutson as a scientist, called his conclusion "demonstrably wrong" based on a computer model that doesn't look properly at storms.

Kevin Trenberth, a climate scientist, said Knutson's computer model is poor at assessing tropical weather and "fail to replicate storms with any kind of fidelity."

Trenberth, climate analysis chief at the National Center for Atmospheric Research in Boulder, Colo., said it is not just the number of hurricanes "that matter, it is also the intensity, duration and size, and this study falls short on these issues."

Knutson acknowledges weaknesses in his computer model and said it primarily gives a coarse overview, not an accurate picture on individual storms and storm strength. He said the latest model doesn't produce storms surpassing 112 mph.

But NOAA hurricane meteorologist Chris Landsea, who wasn't part of this study, praised Knutson's work as "very consistent with what's being said all along."

"I think global warming is a big concern, but when it comes to hurricanes the evidence for changes is pretty darn tiny," Landsea said.

Hurricane season starts June 1 in the Atlantic and a Colorado State University forecast predicts about a 50 percent more active than normal storm season this year. NOAA puts out its own seasonal forecast on May 22.

In a normal year about 10 named storms form. Six become hurricanes and two become major hurricanes. On average, about five hurricanes hit the United States every three years.

'Fewer hurricanes' as world warms
Mark Kinver, BBC News 18 May 08;

Hurricanes and tropical storms will become less frequent by the end of the century as a result of climate change, US researchers have suggested.

But the scientists added their data also showed that there would be a "modest increase" in the intensity of these extreme weather events.

The findings are at odds with some other studies, which forecast a greater number of hurricanes in a warmer world.

The researchers' results appear in the journal Nature Geoscience.

The team from the National Oceanic and Atmospheric Administration's (Noaa) Geophysical Fluid Dynamics Laboratory (GFDL) said its findings did not support the notion that human-induced climate change was causing an increase in the number of hurricanes and tropical storms.

"There have been some studies published that have suggested that this is the case, but this modelling study does not support that idea," observed lead author Tom Knutson.

"Rather, we actually simulate a reduction in hurricane frequency in the Atlantic."

Eye of the storm

Although the study projected that there would be fewer extreme weather events in the future, Dr Knutson said that these storms were likely to be more powerful.

"The model is simulating increased intensity of the hurricanes that do occur, and also increased rainfall rates.

"This is something that has been seen in previous studies, and the IPCC use this [scenario] as a likely projection for future climate warming.

"These changes in intensity are still fairly modest in size."

A previous study by Noaa scientists showed a 4% increase in storm intensity for every 1C (1.8F) increase in sea surface temperature. Yet, he explained, this study suggested only a 1-2% increase.

A sea surface temperature (SST) above 26.5C (79.7F) is one of the key factors in the formation and feeding of a hurricane.

Over recent decades, the surfaces of most tropical oceans have warmed by up to 0.5C (0.9F), which the Intergovernmental Panel on Climate Change (IPCC) believes has been caused by an increase in greenhouse gas concentrations in the atmosphere.

In November 2006, the global community of tropical cyclone researchers gathered at a workshop organised by the World Meteorological Organization to consider the impact of human activity on the frequency and intensity of cyclones.

In a concluding statement, the researchers said that although there was evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record, no firm conclusion could be made.

One reason for the uncertainty is the changes in observation methods used to record Atlantic hurricanes - a record that dates back to 1850.

From 1944, air reconnaissance flights were used to monitor tropical storms and hurricanes. This development allowed researchers to monitor a much greater area and not rely on ships' logs and storms reaching land.

And from the late 1960s, satellite technology has been used to monitor and track hurricanes.

Therefore, a reliable record of past hurricane activity only stretches back about 35 years.

Natural variations that affect SSTs - such as El Nino and La Nina episodes and the Atlantic Multi-decadal Oscillation - add to the difficulty of identifying the influence of human-induced climate change on the frequency and intensity of hurricanes.

Model mechanics

Dr Knutson's colleague and co-author, Isaac Held, said the team's model used a different approach to previous efforts, which gave them a high degree of confidence in their results.

"Most of the literature to date on hurricanes and climate change has used statistical techniques," he said.

"You've had time series of hurricane activity and time series of sea surface temperatures, and people correlate them."

Because there was a high degree of confidence that the sea surface temperature trend was going to continue to rise, Dr Held explained, people had "tried to conclude that hurricane activity will increase rather dramatically in the future".

"We tried to simulate the fundamental fluid dynamics and thermodynamics that control hurricane genesis in the Atlantic in a numerical model to a very high resolution."

He added that the team ran data from the past 25 years through the model, and it returned results closely correlated to what actually occurred.

"It is interesting and important to understand why it is that this model is capable of simulating an increase in hurricane activity that we have seen in recent decades, yet it predicts a decrease in the future.

"This implies that we cannot simply extrapolate the past 25 years into the future."

Dr Knutson said that he did not expect the study's findings to end the scientific debate surrounding the impact of human-induced climate change on tropical storms.

"We do not regard this study as the last word on this topic," he told reporters.

"The main point that we want to emphasise is that there is no evidence in this study that we are seeing large greenhouse-gas-driven increases in Atlantic hurricane or tropical storm frequencies."