Rising sea levels could cause coastal flooding, including in California

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Floods, droughts, famine – the record-smashing El Niño that is growing ever stronger is already starting to have massive impacts in many regions around the world. Dramatic sea level rises too have already happened in the eastern Pacific and are set to continue in the coming months.

“Presently, the east Pacific is an astounding 40 centimetres above normal, and a strong westerly wind event will drive those sea levels even higher over the next few weeks,” says Paul Roundy at the University at Albany in New York state.

This could lead to coastal flooding and erosion in many places along the eastern Pacific, from Alaska to Chile, with especially high tides making things even worse. “Frequent coastal inundation will likely develop in many areas and may become severe or record breaking, causing significant disruption and possibly also some significant damage,” says Daniel Swain at the California Institute for Water Resources.

There are several factors combining to produce record-high sea levels in the eastern Pacific in the coming months. The most important is the seesaw-like shift in sea level across the equatorial Pacific caused by El Niño, with levels falling in the west and rising in the east. In simple terms, huge quantities of warm water are sloshing eastwards across the Pacific, driven by bursts of wind blowing in the opposite direction to the usual trade winds.

More technically, these winds trigger what are known as Kelvin waves, says Thomas Monahan at the University of Oxford. These are very different to normal waves, he says. “A Kelvin wave is more similar to raising the water level in a bathtub.”

These waves take around 60 days to cross the Pacific and then travel along coasts. The end result is a sustained rise in sea level that will persist for months. “It’ll peak with the highest sea surface temperatures, which right now are projected for around late November or December,” says Monahan.

Because this El Niño is so exceptional, the changes in sea level will also be exceptional. In fact, a study by Monahan and his colleagues has found that changes in sea level are amplified by strong El Niño events, rather than being linearly related. “So, if you were to just extrapolate from smaller events, you would miss what is happening in the stronger events,” he says.

The changes will be greatest nearest the equator. Based on analysis of past events, Monahan’s team calculates that sea level could rise between 24 and 40 cm in the Galapagos islands, depending on how strong this El Niño becomes. On the mainland, the effects are set to be a bit smaller. Peru and Ecuador could see rises of between 19 and 33 cm.

These rises are occurring on top of the 20-cm rise in global mean sea level since 1900 due to global warming. In other words, even if this El Niño isn’t as strong as the 1877/78 one, the sea level rises will still be bigger than they were then.

In places such as California, further from the equator, the team estimates that sea level will rise between 9 and 13 cm. However, unlike in South America, several extra factors are at play along the west coast of the US and Canada.

One is extra high tides because of long-term lunar cycles. There is the 18.6-year lunar nodal cycle, due to the moon’s orbit being inclined relative to Earth’s equator, and the 8.85-year perigean cycle, due to the rotation of the moon’s elliptical orbit. The highest tides due to these cycles will coincide with this El Niño, effectively adding around 5 cm to sea level along California on top of the El Niño effect.

“We’re already seeing this start to play out in California, where we’ve seen some major flooding occurring in the aftermath of tropical storm Rachel,” says Monahan. “Rather problematically, the biggest peaks in the tides are expected to occur during Thanksgiving week and Christmas week, among the highest in terms of travel and people being around.”

What’s more, El Niños deepen an area of low pressure near the Aleutian islands off Alaska, increasing storm activity, he says. That means storm surges can get bigger during El Niños.

The final factor is that these storms whip up bigger waves. During the 2015/16 El Niño, waves over 15 metres hit Oregon – the largest ever recorded. So, the danger for the west coast is that massive waves and big storm surges could coincide with extra high tides at a time when sea level is much higher than normal, massively increasing the risk of coastal flooding and erosion.

“We may see coastal flooding and erosion in some areas that rivals or exceeds historical records – perhaps by a significant margin,” says Swain.

This is part of the reason why California declared a state of emergency in September. Another major concern is extreme rainfall due to El Niño.

“Rainfall with this El Niño event is the biggest concern for California. In the comparable event of 1877, the Central Valley profoundly flooded,” says Roundy. “There is risk of something similar this year, and coastal flooding at the same time is a genuine risk.”