Land uplift mitigates sea level rise on the Finnish coast

Sea level rise is distributed unevenly around the world

Sea level rise is one of the most significant consequences of climate change. The amount of water in the oceans will increase because the climate warming melts glaciers. Also, thermal expansion increases the volume of sea water. When considering all the world’s oceans, sea level rose at an average rate of about 3.7 mm per year over the period 2006–2018 [1].

However, sea level does not rise at the same rate everywhere. Different areas in the ocean warm up differently, so thermal expansion raises the water level more strongly in some areas than in others. [1] Also, the meltwater from glaciers and ice sheets does not distribute evenly among the world's oceans [2]. There are also big uncertainties in sea-level rise forecasts, especially related to the melting of the glaciers of Greenland and Antarctica [1], [3].

Melting of Antarctica's glacier effects to the sea level rise of the Baltic Sea

Sea level rise in the Baltic Sea and Finnish coasts is not affected much by the melting of the nearby Greenland glacier but by the melting of the Antarctic glacier far away. Water that melts from Greenland runs south. However, the melting of the Antarctic glaciers has a full effect in the north. [3]

The gravitational pull of a large ice mass attracts seawater towards it, but when the mass melts away, the gravitational force weakens. As a result, water no longer accumulates as much near the glacier. In other words, the melting of the glaciers adds water to the seas which raises the sea level. On the other hand, when glaciers get smaller, their gravitational pull is weaker, and less water accumulates in the vicinity of the glacier and sea level decreases in those areas. [2]

Land is rising in Finland

On the Finnish coast, a significant factor mitigating sea level rise is land uplift. Land is rising because of the slow rebound of the Earth’s crust from the pressure exerted by the ice sheet of the last Ice Age [4]. The land uplift is strongest in the Quark area, approximately 90 cm per century, and weakest on the southern coast of Finland. For instance, in Helsinki, the land is rising approximately 40 cm per century. [5] The race between land uplift and sea level rise determines the direction in which the coastline shifts over time.

Land uplift has been stronger than sea level rise along the entire Finnish coast over the past century. According to the measurements, the mean sea level has decreased on average by 1–7 mm per year, depending on the location, from the beginning of the 20th century until 2018. In Finland, sea level has been measured for over a hundred years. The oldest tide gauge, that is a sea level measurement station, has been operating at Hanko since 1887, and nowadays there are 14 stations along the Finnish coast. [3].

However, the global sea level rise has already altered the situation. In the 2020´s the sea level decline was as biggest 5 mm per year. On the southern coast of Finland, the sea level doesn´t decrease but turns to rise. So according to the most probable scenario, sea level rise will surpass land uplift on the southern coast of Finland, so that land will be submerged by seawater. However, in the northern parts of the Gulf of Bothnia, the land continues to rise more than the sea level, revealing new land. [3]

The future sea level depends on greenhouse gas emissions

The projections for sea level on the Finnish coast are based on international research and climate model results. They have been calculated up to the year 2100. The projections also take into account specific characteristics of the Baltic Sea region, such as how changes in the wind climate affect the water exchange in the Danish straits between the Baltic Sea and the North Sea. If westerly winds strengthen as a result of climate change, it may slightly contribute to sea level rise in Finland. [3]

The projections have been calculated for three alternative emission scenarios, describing how anthropogenic greenhouse gas emissions might develop in the future. Sea level rise is smallest under the low-emission scenario (SSP1-2.6) and largest under the high-emission scenario (SSP5-8.5). [3]

Uncertainty in the projections is related not only to emission development but also to the fact that the behavior of ice sheets in the warming climate is not yet well understood. There are significant uncertainties, especially regarding the melting rate of the West Antarctic ice sheet, as its base is below sea level. Such an ice sheet may shrink more quickly due to the warming of the ocean and atmosphere compared to an ice sheet located on land. [3]

In the Gulf of Finland, sea level will rise

According to the projections, the mean sea level on the southern coast of Finland is expected to rise with every scenario. In case of the low-emission scenario, the rise would be approximately 5–15 cm by the year 2100. Under the medium-emission scenario, the rise would most likely be around 20–30 cm, and in case of high emissions, it would be 50–60 cm [3].

For example, in Helsinki, the realization of the medium-emission scenario would mean that the sea level, in the N2000 height system, would be on average +45 cm in the year 2100 [3], compared to approximately +20 cm in the 2010s (figure 1, Helsinki).

In the Gulf of Bothnia, the land uplift mitigates sea level rise

In the Gulf of Bothnia, land uplift may continue to exceed sea level rise in the future (figure 1, Vaasa). New land would still emerge from the sea, albeit at a slower pace than before. Only under the high-emission scenario land uplift would be less than sea level rise across the entire Gulf of Bothnia. [3]

In the Bay of Bothnia, under the medium-emission scenario, sea level will most likely decrease by 20–30 cm by 2100. Under low emissions, sea level would decrease by about 40 cm, while high emissions would cause a slight increase of a few centimeters. [3]

In the Sea of Bothnia, sea level rise and land uplift are expected to roughly balance each other out during this century. Under low emissions, sea level would fall by 20–40 cm by the year 2100, and about 5–20 cm under medium emissions. With the realization of the high-emission scenario, sea level would rise by about 5–20 cm. [3]

All the numbers above are based on the most likely projections within a specific emission scenario. Due to uncertainties associated with the projections, even higher sea level rise is possible. [3]

Coastal floods may intensify in Finland

On the Finnish coast, the momentary water level depends primarily on winds, atmospheric pressure and the amount of water in the Baltic Sea. The Baltic Sea is connected to the oceans only through the narrow and shallow Danish straits, and the variations of the total amount of water in the Baltic Sea are mainly determined by the flow of water in and out through the straits. Currents, in turn, depend on factors such as wind and atmospheric pressure conditions in the straits area. If the total water volume is low in the Baltic Sea, even a storm cannot raise the water as high as it would when the total water volume is large.

Sea level rise projections are calculated for mean sea level. Sea level is constantly fluctuating and sometimes it reaches exceptional heights, which causes floods on the coast. When the mean sea level rises, also the floods will be higher in the future. [6]

In addition, climate change affects wind conditions and storms and reduces ice cover in wintertime. These factors contribute to short-term fluctuations in sea level. Potentially they increase the number of floods and cause water to reach even higher levels during such events. Both the maximum sea levels and short-term fluctuations in the Baltic Sea have increased over the past century. The changes in wind conditions seem to be at least partly responsible for this phenomenon. [7]