A future of more extreme floods, brought to you by climate change

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Extreme floods along the coastline may become much more common if sea levels continue to climb unchecked, new research says. Scientists estimate that as soon as 2030, a 4-inch sea level rise could double the frequency of severe flooding in many parts of the world, and increase it by as much as 25 times in the tropics. For the communities and ecosystems in the floodwaters’ path, the toll could be catastrophic.

Right now, the global sea level is slowly but surely creeping upwards a fraction of an inch each year (0.118 to 0.157 inches per year to be exact). That doesn’t seem like much, but we’re already feeling the consequences of rising waters and eroding coastlines. Tides high enough to flood homes and infrastructure have become more common in some parts of the US like Florida — “turning it from a rare event into a recurrent and disruptive problem,” as a report by the National Oceanic and Atmospheric Administration put it earlier in 2017. In Louisiana, an entire community was driven from their homes on Isle de Jean Charles by rising seas.

Sea water levels are climbing because carbon dioxide and other heat-trapping greenhouse gases are causing global temperatures to warm, which in turn melts the land-based ice. That extra water flows into the world’s oceans. As we keep pumping these gases into the air, oceans will rise even faster. By how much? Scientists are trying to figure that out. Based on some estimates, we can probably expect between 4 and 8 inches of sea level rise by 2050, and between 1 and 7 feet by 2100. The problem is that when sea levels go up, flooding gets worse. That’s why a team of researchers led by Sean Vitousek at the University of Illinois, Chicago, wanted to find out just how bad that flooding could get.

“It doesn’t take much sea level rise to dramatically increase the frequency of flooding,” says Vitousek, who grew up in Hawaii. “I worry that the coastal zone is going to be a very different place in 50 to 100 years.”

The floods that Vitousek set out to investigate weren’t the sunny-day soakings from high tides. Instead, he wanted to understand how the extreme floods that pummel coastlines once every 25 or 50 years would play out across a warmer, soggier globe. Using statistical modeling, the scientists discovered that if sea levels continue to climb as expected, these 50-year floods will strike many regions at least twice as frequently. The findings were published today in the journal Scientific Reports.

Extreme floods depend on the combination of three things. One of them is the tide, which rises and falls according to the gravitational effects of the Sun and Moon. Another are the waves, which can vary in size depending on where you are in the world and which atmospheric phenomena are occurring nearby. And then there are storm surges — where the low atmospheric pressures and powerful storm winds push waves to dangerous heights.


A diagram of coastal flooding.
Image from Scientific Reports

The researchers combined three models that track the tides, wave heights, and storm surges around the world to figure out where, and how often, they come together to cause extreme flood events. Then they added another variable to the equation: rising sea levels.

They discovered that in places where waves are already big, and floods are already severe as a result of natural disasters like hurricanes — in the Gulf of Mexico, for example — sea level rise only boosts the risk of extreme floods a little bit, if at all. But in areas where tides don’t significantly vary the water level, and wave heights stay relatively consistent, sea level rise will turn more typical floods into extreme ones.

The worst effects will be felt in the tropics — the warm, sunny region stretching across the Earth’s equator. Places like the Marshall Islands in the Pacific or the central African coast could be hit by a 25-fold increase in the frequency of extreme floods; today, these devastating events occur only once every 50 or so years. Even just four inches of sea level rise doubles the risk for extreme floods for places like Vancouver, Seattle, San Francisco, and Los Angeles, the study says.

The increase we’re already seeing in flooding today highlights the relevance of this study for the future, William Sweet wrote in an email to The Verge. Sweet is an expert on coastal flooding with NOAA and was not involved in the research.

Sweet notes, however, two caveats to keep in mind. The first is that only 21 years of data was used to calculate the probability of events that occur once every 50 or so years. The short time span means that it’s harder to get a true sense for which events are common, and which ones are rare. Vitousek acknowledges that this is a challenge with studying extreme events because fortunately, they don’t happen very often. “We’d love more data,” he says.

The second is that sea levels don’t actually rise evenly around the globe. So, it will take more number crunching to figure out exactly when specific regions will hit the sea levels that tip the balance towards more frequent extreme floods. Still, Sweet says, it’s a solid and important study.

The dire predictions the study makes aren’t necessarily set in stone, Vitousek says. If we start reducing carbon emissions, figure out a way to remove carbon from the atmosphere — or, preferably, both — we could keep extreme floods at bay. “It is a pretty sobering possibility, but there are things we can do to fix the problem,” he says. Still, Vitousek adds, “It’s going to be very difficult to make this happen without a lot of human intervention.”



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