'Floods of the century' will now occur every year by end of century

Most coastal communities on the planet will experience flooding every year that previously occurred once every 100 years, according to a recent scientific study. This will happen by the end of the century, even in a moderate scenario where carbon dioxide emissions peak in 2040. However, by as early as 2050, regions around the world could face major floods every nine to fifteen years.

The recently published study reveals that from now on, the historical average will no longer be the same, redefining "a 'flood of the century'" as an extreme water level that has only a 1% chance of exceeding in a given year based on historical data. Despite the name, these megafloods can affect the same area several years in a row or not occur at all in a century, due to the variability of the average.

Historical flood definition changing: extreme water

Water pushed inland by storms, tides, and waves can cause extreme flooding on the coast. However, this study focuses on sea level rise as a component that contributes to flooding on a much longer time scale. Coastal infrastructure will be closer to the water as the sea level rises, which will increase the likelihood of storms, tides, and storm surges affecting population centers.

The researchers performed trend analyzes and estimated future extreme sea levels under two carbon emissions scenarios outlined by the International Panel on Climate Change (IPCC) by using data from more than 300 tide gauges around the world.

One of the scenarios is based on carbon dioxide emissions continuing to increase until the end of the century. In the other scenario, emissions peak in 2040 and then decline. In both scenarios, the scientists observed that sea level rise will lead to an increase in major flooding in most of the places they studied.

Hamed Moftakhari, a civil engineer and professor at the University of Alabama who supervised the project, said that a warmer climate would exceed the threshold, which we expect to be exceeded once every hundred years on average, much more frequently until they are no longer considered 100-year events.

The study was published in Earth’s Future, AGU’s journal for interdisciplinary research on the past, present and future of our planet and its inhabitants.

Hamed Moftakhari added that communities could reduce the effects of such floods and avoid disasters by implementing a better strategy in terms of spatial planning, urban development, and coastal protection measures.

Building a safer future

The concept known as 'stationarity' is relied upon by engineers designing structures such as sea walls, levees, and breakwaters to protect communities from these extreme floods in order to predict future water levels.

"In 'stationarity,' we assume that the patterns we have observed in the past will remain unchanged in the future, but there are many factors stemming from climate change that are changing these patterns," Moftakhari said. "We can no longer assume the stationarity of coastal flooding."

This study used non-stationary methods and found that extreme sea levels will not change uniformly across many sites, unlike previous studies that relied on stationary estimates of extreme sea levels to predict 100-year floods.

As the climate changes, sea levels are rising due to warmer ocean temperatures and glacial meltwater, which is increasing the frequency and severity of coastal flooding. Consequently, engineers require accurate estimates of future flood risk that do not assume that our changing future will mirror historical coastal patterns.

Situation is no longer stationary

Moftakhari said, "Most tools, design guidelines, practice manuals, and so on are based on the assumption of stationarity, which makes it so uncertain. But we need to update them to allow us to observe the pace of change."

According to another study, more than 600 million people live in low-lying coastal regions and experts expect that number to increase. Well-designed coastal defense structures actively contribute to the ability of coastal communities to withstand major flooding.

Sea level rise will not be the same everywhere, but heavy ice sheets melting and the land beneath rising may cause a drop in sea level in higher latitudes. However, regions like the Gulf of Mexico are experiencing faster rates of sea level rise than the global average because the land is gradually sinking. Moftakhari states that coastal communities will need differentiated solutions based on the situation in each location.

"We know that the average sea level is rising, the question is: How are we going to deal with it?" Moftakhari said. "We have already seen that many parts of the coast are permanently flooded and losing land, and many cities and coastal islands are experiencing flooding much more frequently than in the past; it is time to learn how to deal with non-state problems.

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