How has the Baltic Sea evolved into what it is today?

The Baltic Sea has formed in an ancient depression in the crystalline basement rock. The present Baltic Sea basin has evolved over millions of years as a result of geological processes. Some of the earliest sediment deposits have been eroded because of several ice ages.

During the Eemian warm period, the Baltic Sea was connected to the North Sea and the White Sea

Before the last Ice Age, there was a warmer interglacial period called the Eemian warm period, which occurred about 130,000 to 115,000 years ago.

During the Eemian warm period, the ocean level was elevated, making Fennoscandia an island. Wide straits connected the Baltic Sea basin to both the North- and White Seas. The Eemian Sea was more saline than the present Baltic Sea.

Although no Eemian deposits have been found in the present-day sea areas of Finland, they have been discovered in the southern Baltic Sea.

During the Ice Age, the Baltic Sea was covered by a continental ice sheet

The last Ice Age occurred from approximately 115,000 to 11,700 years ago. At that time, the entire Baltic Sea basin was occasionally covered by continental ice sheet. This latestglaciation period is known as the Weichselian glaciation and its peak phase occurred some 20,000 years ago. Even then, the entire Baltic Sea basin was covered by up to ~3 kilometres thick ice sheet.

Thus, in geological terms the present-day Baltic Sea is very young.

The continental ice sheet began to retreat from the southern Baltic Sea area about 18,000 years ago and had  retreated entirely from the Baltic Sea basin approximately 10,000 years before present. Thereafter, no continental ice sheet has occurred in the Baltic Sea basin.

After the Ice Age, the Baltic Sea was by turns a freshwater lake then a brackish basin

During and after the deglaciation, the Baltic Sea basin has alternately been a freshwater lake and a brackish water basin. 

About 18,000 years ago, the Baltic Ice Lake began to form on the edge of the retreating continental ice sheet as the climate warmed. The water level of this ice lake was well above ocean level.

Varved clays, i.e. clay sediments deposited repeatedly within a one-year period, indicate that the seasons varied greatly at this time. In summer, warmer temperatures caused rapid ice melt, which deposited a thick layer of fine material. Conversely, colder winter temperatures meant less melting and thus, less deposition.

The Baltic Ice Lake phase lasted over 6,000 years.

The first brackish water phase after the Ice Age began when the Baltic Sea connected with the North Sea through central Sweden, approximately 11,700 years ago. This phase ended approximately one thousand years later when the sea connection closed again.

The latest freshwater lake phase of the Baltic Sea is known as the Ancylus Lake, which began 10,700 years ago and lasted a little under 2,000 years.

Clayey sediments containing darker sulphide-rich layers, were formed during the Ancylus Lake period. 

The Littorina Sea was the last brackish water phase of the Baltic Sea

The latest brackish water phase of the Baltic Sea began when the Baltic was connected to the North Sea via the Danish straits, some 9,000 to 10,000 years ago. The actual brackish water phase began in the main basins of the Baltic Sea about 8,000 years ago. This phase, which began slightly earlier in the southern parts of the Baltic Sea compared to the northern reaches, is known as the Littorina Sea.

The aquatic ecosystems of the Baltic Sea experienced dramatic changes when the water mass changed both physically and chemically. Freshwater lake conditions changed to brackish water in a relatively short timeframe.

Although the Littorina Sea closely resembles the modern Baltic, it was much more saline than today.

The continental ice sheet covered most of Finland. The ice lake extended to the southern parts of the Baltic Sea basin The continental glacier ice sheet still covers Finland. As it retreats, the ice lake opens a strait across central Sweden. The land freed by the melting continental glacier ice sheet uplifted and closed the connection to the Atlantic. The ice sheet then only covered a small part of the northern Baltic Sea. The post-glacial rise of sea level reached the level of the Danish straits and gave rise to channels that allowed saltwater inflow into the previously freshwater lake.

The Baltic Sea of today has been like this for a few thousand years

The Baltic Sea has been a brackish water basin like today for a few thousand years. However, even during this time it has undergone changes. The land has risen, sea level has fallen in some places, and the connection with the North Sea has been weakened such that there has been a reduction in salinity.

In the future, if the land uplift continues and the sea level does not rise significantly, the Gulf of Bothnia will close in the Kvarken area. This will happen in approximately 2,000 years. At this time, the water connection between the Bothnian Sea and Bothnian Bay will close and the latter will become the largest inland lake in Europe, i.e. the Bothnian Lake.

 The Gulf of Bothnia, circa 2,000 years in the future.
The Gulf of Bothnia, circa 2,000 years in the future.

You can also read about the geological history of the Baltic Sea at