The Baltic Sea differs from other seas in several ways, most notably due to its enclosed nature and limited water exchange.

Katri Viigand, a researcher at the Tallinn University of Technology's Institute of Cybernetics, explained that as a result, the water level in the Baltic Sea changes in a unique way.

The sea has no significant tides, and water exchange with the global ocean occurs only through the narrow Danish straits. "This is actually one of the key factors that greatly affects the Baltic Sea's water level," she told radio show "Labor."

From time to time, enough water flows through the Danish straits into the Baltic Sea to raise the water level across the entire region by up to 80 centimeters. "This elevated water level can last for a couple of weeks, or in some cases even months. Occasionally, a storm will form against the backdrop of a high water level," Viigand added.

When this combines with a storm surge caused by wind, severe flooding can occur — such as in Germany in 2023, when the coast of Schleswig-Holstein was hit by one of the strongest storms in recorded history.

In addition to storm surge and the background water level, another important factor in flooding is the Baltic Sea's seiche — slow oscillations of water masses from one coast to another.

In a recent study, Viigand and her German colleagues investigated the relative significance of these factors in flooding across different parts of the Baltic Sea. The study found that different components dominate depending on the region.

"The research showed that in the eastern part of the Baltic Sea, the background water level has the greatest impact. Without it, we generally would not see extreme water levels—or levels high enough to cause flooding," Viigand said.

The same is true for Sweden's coast, which is relatively well protected from prevailing storm winds.

In the southwest, such as along Germany's coast, the impact of storm surge is greater. There, extreme water levels can occur even without a significant background level, driven by strong storms alone.

Viigand added that in some cases, moderate background water levels and storm surges combined can still be enough to cause flooding.

Waves also play a major role in influencing sea level in coastal areas. In such cases, storm waves approach the shore perpendicularly, allowing them to penetrate further inland and raise the water level even more. These formations can cause significant damage, including coastal erosion.

"Our coastline bends a lot, and the shoreline orientation changes quite a bit," Viigand added. As a result, there are always places where waves hit the shore head-on, increasing the risk of flooding. At the same time, she pointed out that waves more often reach the shore at a steep angle, which promotes sediment transport, the movement of sand and soil along the coast.

The researchers used statistical models, including generalized extreme value distributions, to create forecasts. Viigand said the analysis showed that increases in extreme sea level fluctuations in the Baltic Sea will not occur uniformly. In some regions, the frequency of extreme water levels may not increase at all, while in others, growth could be moderate or even intense.

"Certain areas along the eastern coast of the Baltic Sea — particularly Estonia, Latvia, and Lithuania — will be significantly more at risk from increases in extreme water levels in the future," Viigand noted.

Among other things, the analysis confirmed that one of the most threatened areas looking ahead is Pärnu. The researcher said this likely comes as no surprise to many, as Pärnu Bay is situated in a corner of the Gulf of Riga, where wind directions can push additional water masses into the bay. Over several days, this can cause the water level to rise by up to one meter.

To illustrate this, she cited the infamous January 2005 storm. At that time, the water level in Pärnu was measured at 2.75 meters. In the current height system, that corresponds to roughly three meters.

Viigand added that the maximum effects of key components — background water level, local storm surge, and favorable wind direction — have never been observed to coincide fully.

In the longer term, two geophysical processes will influence the water level of the Baltic Sea. Global sea levels are rising, and their effects are gradually reaching the Baltic. Additionally, post-glacial rebound is occurring.

The northern part of the Baltic, such as areas in Finland and Sweden, is still rising, while the southern part, including Germany's and Poland's coastlines, is already subsiding. "Estonia's coast lies on that boundary. Slowly, in some areas, we're already beginning to feel the effects of global sea level rise," Viigand said.

Although future sea level projections are based on existing statistics and the assumption that systems remain stable, Viigand warned that climate change may alter the mechanisms that affect sea levels.

For instance, regions that currently don't appear highly vulnerable could become much more so in the future. Changes in wind systems have also been predicted, which could significantly impact coastal processes going forward.

Viigand's research is published in the journal Natural Hazards.

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