A rising tide

In 1962, a storm surge surprised the port of Hamburg, leveling buildings, killing more than 300 people, and reminding residents of a natural hazard they had all but forgotten.

Hamburg has since fortified its defenses against the sea, but storm surges still threaten coastlines around the world, sometimes claiming thousands of lives.

This month (13-17 September, 2010), an international group of scientists met at the University of Hamburg to discuss the worldwide risk of storm surges and what can be done about them.

"Man-made climate change will in many cases exaggerate the risk," as will development along the coast, said Hans von Storch, director of the GKSS Research Center in Geesthacht, Germany.

Storch initiated the one-off, interdisciplinary conference because he thought scientists should help prepare for that increased risk now.

Storm surges happen when storm winds cause sea levels to rise well above normal.

If there are no protections such as dykes, or if the dykes are breached, the sea can flood vast stretches of land.

Indeed, storm surges can be responsible for most of the damage from major storms, Storch said, such as Hurricane Katrina in the United States in 2005 or the tropical storm Nargis, which killed about 140,000 people in Burma in 2008.

"We need to be able to design scenarios of possible futures, including all factors: global and local, man-made and natural," Storch said.

Past lessons

Looking to the past can provide us with some useful clues for what we need to look out for.

The recorded history of storm surges in the North Sea region around Hamburg goes back to the middle ages, said geo-archaeologist Dirk Meier, who works on the North Sea coast.

His research shows that there were several large storm surges in the 14th century, including one in 1362, which destroyed much of the outer salt marshes of North Frisia, and possibly killed around 25,000 people, he said.

There were also major losses of land and life due to storm surges in 1634, 1717, and 1825, but things started to change, Meier said, as professional dyke builders arrived on the scene.

When the storm surge hit Hamburg in 1962, more than a century had passed since the last major surge event. The city had let the dykes deteriorate, and people didn't understand the need to respond to warnings.

Today, regular media coverage keeps the memory of the 1962 storm surge alive, said human geographer Beate Ratter at the University of Hamburg and the GKSS Research Center.

Yet thanks to effective new dykes, "the people in Hamburg nowadays feel safe," and in a recent survey nobody listed storm surges as one of the city's top five problems, Ratter said.

Global change

One possible reason that the 14th century brought so many storm surges to the North Sea is that it came during a transition from a cold period to a warm period. It was "a time of changing weather conditions," Meier said.

In the coming century, scientists aren't sure whether the number of storm surges will increase due to climate change, said oceanographer Kevin Horsburgh of the UK's National Oceanography Centre.

So far, there doesn't seem to be an increase, and climate models' predictions aren't clear when it comes to storm numbers or the strength and direction of the storm winds that drive surges.

In fact, the effect of climate change on storms will likely vary by region, said climate modeler Kevin Hodges of the UK's National Centre for Earth Observation. For example, models predict that there will be fewer storms in the Mediterranean Sea, but more in other areas, as storm tracks change.

On the other hand, "we know the sea level is rising," and could rise about a half meter to a meter in the next century, Horsburgh said.

What's more, the world's deltas are sinking, said oceanographer James Syvitski of the Community Surface Dynamics Modeling System at the University of Colorado at Boulder.

That makes the half-billion people who live along them – including in mega-cities – much more vulnerable to flooding, Syvitski said. "Most of this flooding isn't occurring in the rich countries. It's occurring in the poor countries."

Forecasting and mitigation

Several new projects aim to improve storm-surge forecasting and warning systems, which are already widespread.

In November, the European Space Agency plans to launch a million-Euro Storm Surge Project.

"We have abundance in information from satellites," said Boram Lee of the Intergovernmental Oceanographic Commission of UNESCO in Paris, which is supporting the project.

The project's goal is to feed that satellite information more effectively to forecasters, modellers, and other storm-surge researchers.

In the United States, the National Oceanic and Atmospheric Association is in the early stages of its 10-year Storm Surge Roadmap program, which aims to better communicate the risk of storm surges to the public and decision-makers, said the agency's Jesse Feyen.

For example, new warnings tell people how much water to expect above ground in their area, instead of the less intuitive number that scientists use, Feyen said.

The most important initiatives may be the ones that simply raise awareness, as the deadly 1962 Hamburg surge shows.

"We must be conscious, open, and honest" about the risks that people face if they live near the coast, said engineer Wolfgang Kron of the Geo Risks Research Department at Munich Re, an insurance company.

"If you have a dyke, people behind the dyke are protected against frequent losses, and they just forget about the risk."

Author: Chelsea Wald
Editor: Nathan Witkop