Hurricane science: Will they grow in the greenhouse?

Is global warming causing more frequent, more powerful hurricanes? Analysis of weather patterns and damage losses over the last 100 years suggests that hurricanes are getting worse - and that rising temperatures are to blame.

But Dr Chris Landsea, a scientist with the National Hurricane Centre in Florida, believes that the case is far from proven. He argues that hurricane activity may not be significantly different compared to that of a century ago. What is changing is our ability to identify and measure hurricanes - and the amount of material wealth that we put in their way.

Speaking at a Lloyd’s Market Academy masterclass entitled “Hurricanes and Global Warming: Expectations Versus Observations”, Dr Landsea said that global warming is real and that temperatures have risen by 1 deg C over the last 100 years.

But while a rise in sea surface temperature and more moist air does make hurricanes stronger and more numerous, he said, the greenhouse effect is not pronounced.

Scientists’ models agree that the predicted change in global temperature will increase hurricanes’ strength by only 1% per degree of warming and that at the current rate of global warming windstorms will be just 3% stronger by the year 2100.

However, the models also suggest that a more turbulent atmosphere makes it more difficult for hurricanes to form – so there will be 40% fewer, albeit slightly stronger, hurricanes by the end of the century, Dr Landsea said.

Dr Landsea also put a question mark over the use of damage losses to correlate global warming with hurricane intensity. He said that with total losses running at an average of $20 billion per year in the period 1996 – 2005, it certainly was the most damaging decade in history.

“But it is to do with people having more stuff, bigger houses and multiple cars,” he said. “Per capita wealth has increased over the last century, especially on the coast… in 1900 there were only about eight people living in Palm Beach, compared to 1.1 million now.”

To reinforce his point Dr Landsea looked at the big loss making hurricanes in US history and adjusted them to today’s socio-economic landscape. He found that in terms of total losses Hurricane Katrina would be in third place behind the 1900 Galveston hurricane and the 1926 Miami windstorm, the latter creating around $150 billion in total losses.  These numbers will come as no surprise to reinsurers who have been using catastrophe models to assess US Hurricane risk for nearly 20 years. “The decade 1996–2005 was very damaging – but it isn’t an outlier,” he said, “It is similar to what happened in the early 20th century.”

Dr Landsea further explained that finding climate change signals in the hurricane database is not clear cut either: “There has been a doubling of recorded hurricanes since 1871, but is something missing?” His theory is that the methods at scientists’ disposal to locate and measure hurricanes has improved hugely thanks to today’s array of coastal stations, buoys, aircraft, satellites and vessels at sea. “The hurricane database reflects all that increased sophistication,” he said.

Summarising his views on hurricane science Dr Landsea said that he expects fewer hurricanes over the next 90 years but with a small increase in intensity due to global warming. Storm surge will increase in line with slightly more intense storms and also the rise in sea levels; there will be a 10% increase in rainfall accompanying hurricanes. But there is no clear indication of changes in the genesis or track of hurricanes.

“Global warming isn’t having much effect on hurricane activity, which has been swinging back and forth for centuries,” Dr Landsea told his audience in Lloyd’s Old Library. “The disturbing news for all of you is that these busy periods can last for 40 years - and we are in the middle of such a period right now.”

Lloyd’s Head of Exposure Management, Paul Nunn said “Dr Landsea is a hurricane scientist, along with Knutson, Holland, Emanuel, Elsner, whose work is highly valued and closely followed by the cat modeling and reinsurance industry.

Clearly the impact of climate change on hurricanes is complex and not yet clear cut; insurers must ensure their premiums cover the observed increase in risk, whatever the cause.