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Insight and Environment

There's little doubt we're to blame for hurricanes getting worse

Climate scientists are still scrapping over the details, but the increased ferocity, unpredictability and spread of tropical storms is in line with predictions

By Michael Le Page

31 October 2018

hurricane damage

Mexico Beach, Florida, was largely wiped out by Hurricane Michael

Joe Raedle/Getty

HURRICANE Michael was a big one. Intensifying faster than expected, its 250-kilometres-per-hour winds were just short of Category 5 strength, making it the fourth most powerful hurricane to hit the US mainland since records began.

The building codes in the Florida panhandle, where Michael made landfall on 10 October, are designed to protect only against winds of 180 kilometres an hour. Most houses were built long before even these codes were introduced. In seaside towns such as Mexico Beach, below, the winds and storm surge left few buildings standing.

Michael is just the latest in a series of extraordinary tropical storms around the world (see “Trails of destruction”). “It’s been pretty shocking,” says hurricane expert Jeff Masters of online information service Weather Underground.

These storms are not just stronger, as climate scientists have long predicted for a warming world. They are also forming and moving over regions far beyond their normal range, as well as producing more rainfall and higher surges, and strengthening more rapidly, giving us less warning of their arrival.

“There are many reasons to be concerned,” says atmospheric scientist Adam Sobel at Columbia University, New York. “We are not as prepared as we should be.”

Tropical storms are fuelled by the evaporation of warm water from the ocean surface. Hurricane season peaks in the northern hemisphere from August through to October, when the oceans are at their warmest.

There are relatively few hurricanes each year, a lot of variability from year to year and problems with our records – for instance, some storms may have been missed before satellite records began in the 1970s. For those reasons, there is still some debate about whether ocean warming through climate change is creating clear trends.

But the evidence is growing. The number of storms with wind speeds of 200 km/h or more has doubled since 1980 and those with winds of 250 km/h and up have tripled. “Modelling and theory says we should expect to see the strongest hurricanes get stronger,” says Masters. “By gosh, we certainly have seen some very strong hurricanes the last two years.”

Worrying trends

Modelling suggests these maximum wind speeds will rise still further as the world warms. If we don’t limit carbon emissions, by 2100 we could be seeing storms with speeds topping 370 km/h, says Timothy Hall at the NASA Goddard Institute for Space Studies in New York. That’s worse even than it sounds because a wind’s destructive power is proportional to its speed cubed.

Flooding is often a storm’s most severe impact. A warmer atmosphere contains more moisture, meaning more rain – or snow – can fall when conditions are right. Each degree of warming could result in 5 to 2o per cent more rainfall, says Sobel, resulting in a greater risk of inland flooding when storms make landfall. A trend yet to be confirmed is that hurricanes also seem to be moving more slowly as the world warms, dumping more rain in one place. Hurricane Florence this year and Hurricane Harvey last year may be examples.

Storm surges are certainly getting higher because of global warming: average sea level has already risen around 0.2 metres, and could rise another 3 metres by 2100. A storm surge’s size also depends on wind strength, the area of water it blows over and how long it blows for. Bigger, stronger and slower-moving storms might pile up higher surges on top of rising seas, although this trend remains uncertain, says Sobel.

The warming of the oceans also means that the region where storms can reach peak strength is expanding out from the tropics. In the northern hemisphere, it is migrating 50 kilometres northwards each decade. In the southern hemisphere, it is moving south.

“The region where hurricanes reach their peak strength is expanding out from the tropics”

This raises the risk of places at higher latitudes, such as Europe, being hit by major storms. By the time storms get that far north or south, they may technically be extratropical storms, powered by the temperature difference between warm and cold air masses. “But it doesn’t really matter much how it’s classified if it does a lot of damage,” says Masters. “This has major implications for the design of infrastructure.”

Perhaps the most disturbing feature of Hurricane Michael was how it intensified rapidly just before landfall. Such storms are likely to become increasingly common as the planet warms, according to modelling done in 2016 by Kerry Emanuel at the Massachusetts Institute of Technology. The rate at which tropical storms can intensify rises exponentially as the maximum potential strength, which depends on ocean heat, goes up.

“That’s very concerning because it doesn’t give people time to prepare,” says Masters. Forecasters won’t be able to help much: although weather models are now great at predicting the paths storms will take, they are much less adept at predicting changes in intensity. These are thought to depend on small-scale features of storms far beyond the power of models to capture.

We are also unsure of the effects a warming world might have on typical storm tracks, or how many storms will form. Earlier models had suggested there would be fewer storms overall, with a greater number of strong ones, but some more recent models have started showing an overall rise in numbers.

Either way, we need to adapt. The main reason why the costs of tropical storms have soared in recent decades is that ever more people are living in the danger zones. Countries should be discouraging development in places at risk, says Sobel.

And if people insist on living in harm’s way, they should prepare for the worst. The photographs of places like Mexico Beach show both the immense destructive power of hurricanes, and that some buildings, like the now-iconic Sand Palace, can withstand it if they are designed to.

While the devil is still in the detail, there seems to be no doubt that tropical storms are set to become ever more dangerous as the world warms – and besides limiting further warming, now is the time to prepare. “Is this the future?” says Masters. “I think it very well could be.”

Trails of destruction

Tropical storms have set a series of disturbing records over recent years – in wind speed, intensity and geographical range.

October 2018: Hurricane Leslie

Kept its hurricane status until it was within 300 kilometres of Portugal. No hurricane had been recorded in this region of the Atlantic since modern weather records began.

September 2018: Typhoon Jebi

Its storm surge flooded Kansai airport, Japan’s third biggest, shutting it down for 17 days. The airport was supposedly built to withstand surges.

September 2018: Hurricane Florence

Dumped a record amount of rain in the Carolinas and caused extensive flooding, just a month before Hurricane Michael hit Florida (see main story).

September 2018: Storm Zorba

Caused extensive flooding and wind damage in Greece and Turkey. This was a peculiarly intense “medicane”, a sort of hybrid storm, with a hurricane-like eye, that forms when extratropical storms push cold air over the Mediterranean and the system starts deriving energy from the warmer waters below it. Medicanes remain rare, but a study last year concluded that the risk will double if warming continues unabated.

October 2017: Hurricane Ophelia

Was literally off the charts used by the US National Hurricane Center, forming further to the north-east than any on record. With peak winds of more than 180 km/h, it weakened into an extratropical storm before striking Ireland, where it caused extensive damage.

September 2017: Hurricane Irma

Maintained wind speeds of more than 300 km/h for 37 hours, longer than any other storm, and left a trail of destruction across the Caribbean.

August 2017: Hurricane Harvey

Vying with Hurricane Katrina, which hit New Orleans in 2005, for the title of costliest hurricane on record, it dumped more rain on the US than any other hurricane, causing extensive flooding around Houston, Texas.

February 2016: Cyclone Winston

The strongest tropical storm on record in the southern hemisphere, with winds of over 280 km/h. It killed 44 people in Fiji.

October 2015: Hurricane Patricia

A Pacific hurricane, it became the strongest tropical storm ever recorded in terms of wind speeds, increasing from 135 km/h to a peak of 345 km/h in just 24 hours. It was small in extent, however, weakening before hitting a sparsely populated part of Mexico.

This article appeared in print under the headline “Kicking up storms”

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