Are mega-hurricanes the new normal?

It's hurricane season - and this one is a rough one.

Less than two weeks after Hurricane Harvey left a swath of deadly destruction in Louisiana and Texas, causing massive flooding in Houston, Hurricane Irma is ripping through the French Carribean.

Irma has already killed at least 10 people on various islands, hitting the dual-island nation of Antigua and Barbuda especially hard.

And already, another tropical storm has formed in the Atlantic behind Irma: Jose, which might hit Puerto Rico and other Caribbean islands on Friday and Saturday.

Meanwhile, Hurricane Katia in the Gulf of Mexico has started making its way toward the coast of Mexico, and is forecast to hit the state of Veracruz by early Saturday.

Experts have been warning that such disastrous storms will become more frequent in a warming world. Are Harvey and Irma a sign of what's to come?

'Active' hurricane season

Tropical storms are categorized from 1 to 5 according to their intensity - including their wind speed, the amount of rain they bring, and the extent of damage they cause, with category 5 storms being the most powerful possible.

While Harvey at its peak was a category 4 hurricane, Irma already is a category 5 monster, with winds of 180 miles per hour (285 kilometers per hour) - one of the strongest Atlantic hurricanes ever recorded. Irma is reportedly so strong, it's registering on earthquake detectors.

Jose and Katia are currently assessed as category 1.

While people in the US and the Caribbean were quite lucky the last years, "this year's hurricane season is very active," Andreas Friedrich at the German Weather Service in Frankfurt tells DW.

"It's partly due to the El Niño event. After a strong El Niño year - which we had last year - more intense hurricanes tend to form over the Atlantic."

El Niño is a weather phenomenon that is more or less normal.

But rising sea temperatures have indeed made things worse.

More fuel for hurricanes

"Currently, the Caribbean is seeing warmer sea surface temperatures - around 30 degree Celsius (86 Fahrenheit)," Friedrich says. "And that's another reason for this strong hurricane season."

The magic temperature for formation of hurricanes and other tropical storms is at least 26 degrees Celsius.

The warmer the water is, the more water vaporizes and gets sucked into the storm, fueling the storm and making it more intense.

"More energy in the atmosphere means more potential for extreme weather," Friedrich says.

With rising sea temperatures, the probability that the water surfaces reaches 26 degrees increases, making tropical storms more likely to occur.

Intensity but not frequency

Although there is evidence tropical storms are becoming more intense, scientists are reluctant to say that they are becoming more frequent with ongoing global warming.

"To be able to say so, we would need data showing a verifiable increase of storms in the last 30 years," meteorologist Friedrich says. "But that's simply not the case."

Instead, there is a lot of fluctuation - there are worse years and better years.

"Back in the 70s we also had periods with more and more intense hurricanes," Friedrich points out. "There is simply no detectable trend that the frequency of hurricanes has risen."

With tropical storms being quite rare, it is indeed difficult to find statistical proof for their increase over time, Mojib Latif of Geomar Helmholtz Center for Ocean Research Kiel tells DW.

Latif compares the effect that climate has on such extreme weather events to a loaded dice.

"You have to roll the dice very often to proof that the dice is loaded."

But he points out that only because you cannot prove a fact yet doesn't necessarily mean it is not true.

Record-breaking storms

Latif says there is a detectable trend, namely relating to how intense those tropical storms are.

"Strong tropical storms [of higher categories] appear to be becoming more frequent."

This is true not only for hurricanes in the Caribbean, but also for cyclones and typhoons in other parts of the world.

In 2008, researchers at Florida State University in Tallahassee found that Atlantic tropical cyclones are getting stronger on average, with their wind speed rising significantly.

"Our results are qualitatively consistent with the hypothesis that as the seas warm, the ocean has more energy to convert to tropical cyclone wind," they wrote in the journal "Nature."

Latif agrees. "The destructiveness of tropical storms in general has increased - just look at Harvey, with its enormous amounts of rain. And now Irma is record-breaking - again - we've never seen such high wind speeds over such a big area."

So is it climate change?

But blaming global warming for single events like Irma or Harvey is difficult, Latif points out.

"You can never say that the occurrence of any particular storm can be attributed to climate change - but we have to prepare ourselves that such storms will intensify."

Climate models forecast that hurricanes will become more intense - and more frequent - in the future.

Terry Dinan of the United States Congressional Budget Office in Washington assessed the potential increase in wind and storm surge damage caused by hurricanes making landfall in the US between now and 2075- He found that "climate change and coastal development will cause hurricane damage to increase faster than the US economy is expected to grow."

Moreover, the number of people facing substantial damage will increase more than eight-fold over the next 60 years, he wrote in the journal "Ecological Economics."

"What we are now experiencing is a foretaste of what climate models let us expect for the future," meteorologist Friedrich says.

So maybe it's time to stop quibbling over models - and start preparing for future mega-storms.