The Baltic Sea is, for the most part, a friendly sea. After all, it once allowed Vikings to roam and Hanseatic merchants to trade, giving rise to coastal towns along its shores. Still, even our sea occasionally causes trouble. When it's limited to dangerous waves, it's relatively easy these days to take shelter in a harbor — forecasts can now predict wave height and location at least two days in advance. Moreover, over the past 40 years, wave heights haven't increased significantly, write TalTech professor Tarmo Soomere, researcher Katri Viigand and senior researcher Maris Eelsalu.

Flooding a more complicated story

Flooding, however, is a more complicated matter — though scientists can also predict it at least two days in advance. But you can't pick up a harbor, house or highway and move it inland to safety. A port must be able to withstand the sea's fury and continue offering safe shelter for ships. Houses and roads simply need to be built far enough from the water.

Determining what counts as "far enough" is a multimillion-dollar question. More than that, the answer changes over time, even for the same location. In this article, researchers from TalTech writing in the journal Ocean Engineering analyze just how much risk extremely high sea levels may pose in the near future and how the threat has shifted across various locations over the past few decades.

Extremely high water levels are rare visitors, but they tend to follow a certain logic. They're relatively easy to forecast and to provide guidance for coastal users — as long as those rules remain stable over time. In such cases, experts say that the "climate" of extreme sea levels is stationary.

Sometimes the sea spills over into the shore — it's simply what seas do. It's like a deep breath taken by the ocean. If we know how far the water is likely to reach once every 10, 50 or 100 years, builders can properly assess the risks and make informed decisions about where to place structures, how high they need to be or how tall breakwaters must be built. This has been the standard approach for decades.

Baltic Sea peculiarity

Surprise, surprise — our sea doesn't work that way. The "climate" of extreme sea levels in the Baltic is radically different. In fact, the pattern of extremely high water levels is in constant flux across nearly the entire sea. Technically speaking, both the potential height of extreme levels and their likelihood of occurring have changed significantly.

Whether this is a result of climate change or simply the Baltic's inherently unpredictable nature remains unclear. What is clear, however, is that in several areas, the risks associated with extremely high water levels have grown considerably over the past few decades. Fortunately, Estonia has largely avoided this trend — but not Lithuania, for example, which has traditionally been considered relatively safe. In contrast, these risks have decreased in the western and northern parts of the sea.

What matters more is the clear evidence that the sea's behavior can change significantly within just a few decades — and that such change is an inherent part of how the Baltic functions. This isn't just an observation.

The risks associated with extreme sea levels and how they evolve can, in fact, be forecast. Not with the standard ocean models used for day-to-day wave height or water level monitoring, but through more complex and powerful mathematical tools, namely those provided by generalized extreme value theory.

When applied smartly, these tools yield a single metric for each stretch of coastline, one that shows whether the risk is increasing or not. And the results are not just useful or interesting — they're simply beautiful.

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