What are rogue waves and what causes them?

For centuries, seafaring folks' reports of terrifyingly huge waves that appeared out of nowhere were dismissed as fantasy by those on dry land.   

Now, the existence of these "real sea monsters" that have "swallowed up dozens of ships" is widely accepted, said Metro, and experts are working to understand what causes them and whether climate change is making them more common. 

What are rogue waves?

Rogue waves are "giant colossi of the sea", wrote Professor in Ocean Engineering at the University of Melbourne, Alessandro Toffoli, on The Conversation. The "notorious phenomenon" sees "unimaginable mountains of water as tall as 10-storey buildings" appear "seemingly out of nowhere".

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They should not be confused with tsunamis, which are large waves (or series of waves) that crash onto a coastline, usually after an earthquake or volcanic eruption under the water. 

In contrast, rogue waves have "much steeper slopes", said BBC Science Focus, and they don't just break on shorelines – they can break in the open ocean. Many accounts describe them as "walls of water" because of their "unusually deep troughs", said Indy100.

A study published in Science Direct, based on media reports, recorded just 210 rogue waves worldwide between 2011 and 2018, but the "real number is thought to be much higher", said BBC Science Focus.

On New Year's Day 1995, a 26m high surge was recorded at a gas platform in the North Sea, and scientists started taking reports of rogue waves seriously. In 2018, eight crew members had to be rescued when massive waves sank a fishing boat off Hawaii, and, in 2022, a suspected rogue wave smashed into a cruise ship sailing from Antarctica to Argentina, killing one passenger.

Rogue waves can also form in large bodies of fresh water, said National Geographic. One of the most famous US shipwrecks of the 20th century, the Edmund Fitzgerald, was probably caused by at least one rogue wave on Lake Superior, one of the Great Lakes of North America.

What causes rogue waves?

Some scientific studies have found that wave heights could increase as a result of climate change, so experts are scrambling to understand them better.

Exactly what triggers rogue waves is "up for debate", said BBC Science Focus, and it's thought that there are "several different factors at play". The overlap of multiple waves at the same location and time can cause concentrated energy, leading to tall waves. In other words, "wave-wave" interactions, where waves travelling in different directions cross each other, can create even bigger waves. 

Strong winds may also play their part, because they "push harder" on some already tall wave forms. Rogue waves have been more common when waves were travelling in one direction, rather than spreading out. "It's understandable," University of South Florida researcher Laura Azevedo told the magazine, because "when you have all the energy of the sea creating that one wave, that's a big wave."

A study on phys.org found that extreme wave heights in many places are likely to grow by between 5% and 8% by 2100, depending on how much carbon dioxide humans "pump into the atmosphere over the coming decades".

Although it's "difficult" to predict when rogue waves will strike, as "understanding grows", scientists could use modelling techniques to make "more accurate long-term predictions". 

All of this research and progress is belated vindication for the 19th-century French explorer and naval officer, Jules Dumont d’Urville, who gained "fame and prestige" for his sailing expeditions. When he reported seeing rogue waves of over 100 feet high on his voyages his claims were dismissed and he was "publicly ridiculed" by then French prime minister Franҫois Arago, noted the American Physical Society.