Baltic Sea


The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by the countries of Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden, and the North and Central European Plain regions. It is the world's largest brackish water basin.
The sea stretches from 53°N to 66°N latitude and from 10°E to 30°E longitude. It is a shelf sea and marginal sea of the Atlantic with limited water exchange between the two, making it an inland sea. The Baltic Sea drains through the Danish straits into the Kattegat by way of the Øresund, Great Belt and Little Belt. It includes the Gulf of Bothnia, the Gulf of Finland, the Gulf of Riga and the Bay of Gdańsk.
The "Baltic Proper" is bordered on its northern edge, at latitude 60°N, by Åland and the Gulf of Bothnia, on its northeastern edge by the Gulf of Finland, on its eastern edge by the Gulf of Riga, and in the west by the Swedish part of the southern Scandinavian Peninsula.
The Baltic Sea is connected by artificial waterways to the White Sea via the White Sea–Baltic Canal and to the German Bight of the North Sea via the Kiel Canal.

Definitions

Administration

The Convention on the Protection of the Marine Environment of the Baltic Sea Area">Convention on the Protection of the Marine Environment of the Baltic Sea Area">Convention on the Protection of the Marine Environment of the Baltic Sea Area includes the Baltic Sea and the Kattegat, without calling Kattegat a part of the Baltic Sea, "For the purposes of this Convention the 'Baltic Sea Area' shall be the Baltic Sea and the Entrance to the Baltic Sea, bounded by the parallel of the Skaw in the Skagerrak at 57°44.43'N."

Traffic history

Historically, the Kingdom of Denmark collected Sound Dues from ships at the border between the ocean and the land-locked Baltic Sea, in tandem: in the Øresund at Kronborg castle near Helsingør; in the Great Belt at Nyborg; and in the Little Belt at its narrowest part then Fredericia, after that stronghold was built. The narrowest part of Little Belt is the "Middelfart Sund" near Middelfart.

Oceanography

Geographers widely agree that the preferred physical border between the Baltic and North Seas is the Langelandsbælt and the Drogden-Sill strait. The Drogden Sill is situated north of Køge Bugt and connects Dragør in the south of Copenhagen to Malmö; it is used by the Øresund Bridge, including the Drogden Tunnel. By this definition, the Danish straits is part of the entrance, but the Bay of Mecklenburg and the Bay of Kiel are parts of the Baltic Sea.
Another usual border is the line between Falsterbo, Sweden, and Stevns Klint, Denmark, as this is the southern border of Øresund. It is also the border between the shallow southern Øresund and notably deeper water.

Hydrography and biology

Sill sets a limit to Øresund and Darss Sill, and a limit to the Belt Sea. The shallow sills are obstacles to the flow of heavy salt water from the Kattegat into the basins around Bornholm and Gotland.
The Kattegat and the southwestern Baltic Sea are well oxygenated and have a rich biology. The remainder of the Sea is brackish, poor in oxygen, and in species. Thus, statistically, the more of the entrance that is included in its definition, the healthier the Baltic appears; conversely, the more narrowly it is defined, the more endangered its biology appears.

Etymology and nomenclature

The precise origin of the name is unknown. Tacitus called it the Suebic Sea, Latin: Mare Suebicum after the Germanic people of the Suebi, and Ptolemy Sarmatian Ocean after the Sarmatians, but the first to name it the Baltic Sea was the eleventh-century German chronicler Adam of Bremen. It might be connected to the Germanic word belt, a name used for two of the Danish straits, the Belts, while others claim it to be directly derived from the source of the Germanic word, Latin balteus "belt". Adam of Bremen himself compared the sea with a belt, stating that it is so named because it stretches through the land as a belt.
He might also have been influenced by the name of a legendary island mentioned in the Natural History of Pliny the Elder. Pliny mentions an island named Baltia with reference to accounts of Pytheas and Xenophon. It is possible that Pliny refers to an island named Basilia in On the Ocean by Pytheas. Baltia also might be derived from "belt", and therein mean "near belt of sea, strait".
Others have suggested that the name of the island originates from the Proto-Indo-European root *bʰel meaning "white, fair", which may echo the naming of seas after colours relating to the cardinal points. This bʰel root and basic meaning were retained in Lithuanian, Latvian and Slavic. On this basis, a related hypothesis holds that the name originated from this Indo-European root via a Baltic language such as Lithuanian. Another explanation is that, while derived from the aforementioned root, the name of the sea is related to names for various forms of water and related substances in several European languages, that might have been originally associated with colors found in swamps. Yet another explanation is that the name originally meant "enclosed sea, bay" as opposed to open sea.
In the Middle Ages the sea was known by a variety of names. The name Baltic Sea became dominant after 1600. Usage of Baltic and similar terms to denote the region east of the sea started only in the 19th century.

Name in other languages

The Baltic Sea was known in ancient Latin language sources as Mare Suebicum or even Mare Germanicum. Older native names in languages that used to be spoken on the shores of the sea or near it usually indicate the geographical location of the sea, or its size in relation to smaller gulfs, or tribes associated with it. In modern languages, it is known by the equivalents of "East Sea", "West Sea", or "Baltic Sea" in different languages:

Classical world

At the time of the Roman Empire, the Baltic Sea was known as the Mare Suebicum or Mare Sarmaticum. Tacitus in his AD 98 Agricola and Germania described the Mare Suebicum, named for the Suebi tribe, during the spring months, as a brackish sea where the ice broke apart and chunks floated about. The Suebi eventually migrated southwest to temporarily reside in the Rhineland area of modern Germany, where their name survives in the historic region known as Swabia. Jordanes called it the Germanic Sea in his work, the Getica.

Middle Ages

In the early Middle Ages, Norse merchants built a trade empire all around the Baltic. Later, the Norse fought for control of the Baltic against Wendish tribes dwelling on the southern shore. The Norse also used the rivers of Russia for trade routes, finding their way eventually to the Black Sea and southern Russia. This Norse-dominated period is referred to as the Viking Age.
Since the Viking Age, the Scandinavians have referred to the Baltic Sea as Austmarr. "Eastern Sea", appears in the Heimskringla and Eystra salt appears in Sörla þáttr. Saxo Grammaticus recorded in Gesta Danorum an older name, Gandvik, -vik being Old Norse for "bay", which implies that the Vikings correctly regarded it as an inlet of the sea. Another form of the name, "Grandvik", attested in at least one English translation of Gesta Danorum, is likely to be a misspelling.
In addition to fish the sea also provides amber, especially from its southern shores within today's borders of Poland, Russia and Lithuania. First mentions of amber deposits on the South Coast of the Baltic Sea date back to the 12th century. The bordering countries have also traditionally exported lumber, wood tar, flax, hemp and furs by ship across the Baltic. Sweden had from early medieval times exported iron and silver mined there, while Poland had and still has extensive salt mines. Thus, the Baltic Sea has long been crossed by much merchant shipping.
The lands on the Baltic's eastern shore were among the last in Europe to be converted to Christianity. This finally happened during the Northern Crusades: Finland in the twelfth century by Swedes, and what are now Estonia and Latvia in the early thirteenth century by Danes and Germans. The Teutonic Order gained control over parts of the southern and eastern shore of the Baltic Sea, where they set up their monastic state. Lithuania was the last European state to convert to Christianity.

An arena of conflict

In the period between the 8th and 14th centuries, there was much piracy in the Baltic from the coasts of Pomerania and Prussia, and the Victual Brothers held Gotland.
Starting in the 11th century, the southern and eastern shores of the Baltic were settled by migrants mainly from Germany, a movement called the Ostsiedlung. Other settlers were from the Netherlands, Denmark, and Scotland. The Polabian Slavs were gradually assimilated by the Germans. Denmark gradually gained control over most of the Baltic coast, until she lost much of her possessions after being defeated in the 1227 Battle of Bornhöved.
File:Eerste fase van de Zeeslag in de Sont - First phase of the Battle of the Sound - November 8 1658.jpg|thumb|The naval Battle of the Sound took place on 8 November 1658 during the Dano-Swedish War.
Between the 13th and 17th centuries the Baltic Sea was, depending on the period, dominated by the Netherlands, Germany, Sweden and Russia. In the 13th to 16th centuries, the strongest economic force in Northern Europe was the Hanseatic League, a federation of merchant cities around the Baltic Sea and the North Sea. In the sixteenth and early seventeenth centuries, Poland, Denmark, and Sweden fought wars for Dominium maris baltici. Eventually, it was Sweden that virtually encompassed the Baltic Sea. In Sweden, the sea was then referred to as Mare Nostrum Balticum. The goal of Swedish warfare during the 17th century was to make the Baltic Sea an all-Swedish sea, something that was accomplished except the part between Riga in Latvia and Stettin in Pomerania. However, the Dutch dominated the Baltic trade in the seventeenth century.
In the eighteenth century, Russia and Prussia became the leading powers over the sea. Sweden's defeat in the Great Northern War brought Russia to the eastern coast. Russia became and remained a dominating power in the Baltic. Russia's Peter the Great saw the strategic importance of the Baltic and decided to found his new capital, Saint Petersburg, at the mouth of the Neva river at the east end of the Gulf of Finland. There was much trading not just within the Baltic region but also with the North Sea region, especially eastern England and the Netherlands: their fleets needed the Baltic timber, tar, flax, and hemp.
During the Crimean War, a joint British and French fleet attacked the Russian fortresses in the Baltic; the case is also known as the Åland War. They bombarded Sveaborg, which guards Helsinki; and Kronstadt, which guards Saint Petersburg; and they destroyed Bomarsund in Åland. After the unification of Germany in 1871, the whole southern coast became German. World War I was partly fought in the Baltic Sea. After 1920 Poland was granted access to the Baltic Sea at the expense of Germany by the Polish Corridor and enlarged the port of Gdynia in rivalry with the port of the Free City of Danzig.
After the Nazis' rise to power, Germany reclaimed the Memelland and after the outbreak of the Eastern Front occupied the Baltic states. In 1945, the Baltic Sea became a mass grave for retreating soldiers and refugees on torpedoed troop transports. The sinking of the Wilhelm Gustloff remains the worst maritime disaster in history, killing 9,000 people. In 2005, a Russian group of scientists found over five thousand airplane wrecks, sunken warships, and other material, mainly from World War II, on the bottom of the sea.

Since World War II

Ammunition dumping

Since the end of World War II, various nations, including the Soviet Union, the United Kingdom and the United States have disposed of chemical weapons in the Baltic Sea, raising concerns of environmental contamination. Today, fishermen occasionally find some of these materials: the most recent available report from the Helsinki Commission notes that four small scale catches of chemical munitions representing approximately of material were reported in 2005. This is a reduction from the 25 incidents representing of material in 2003. Until now, the U.S. Government refuses to disclose the exact coordinates of the wreck sites. Deteriorating bottles leak mustard gas and other substances, thus slowly poisoning a substantial part of the Baltic Sea.
In addition to chemicals, tons of German ammunition were dumped into the Baltic after the war at the behest of the Allies, who wanted to ensure that they would not start another war. By 2025, these were leaking contaminates into the water and the German government was piloting solutions.

Territorial changes

After 1945, the German population was expelled from all areas east of the Oder-Neisse line, making room for new Polish and Russian settlement. Poland gained most of the southern shore. The Soviet Union gained another access to the Baltic with the Kaliningrad Oblast, that had been part of German-settled East Prussia. The Baltic states on the eastern shore were annexed by the Soviet Union. The Baltic then separated opposing military blocs: NATO and the Warsaw Pact. During the Cold War the Baltic Sea remained on the periphery of the main axis of confrontation between these two blocs. Neutral Sweden developed incident weapons to defend its territorial waters after the Swedish submarine incidents. This border status restricted trade and travel. It ended only after the collapse of the Communist regimes in Central and Eastern Europe in the late 1980s.
File:MM10249.jpg|thumb|right|In 1981, the Soviet submarine S-363 ran on ground deep inside Swedish territorial waters, an incident which illustrated the impact of the Cold War on the Baltic Sea.
Finland and Sweden joined NATO in 2023 and 2024, respectively, making the Baltic Sea almost entirely surrounded by the alliance's members, leading some commentators to label the sea a "NATO lake". However, the legal status of the sea has not changed and it is still open to all nations. Such an arrangement has also existed for the European Union since May 2004 following the accession of the Baltic states and Poland. The remaining non-NATO and non-EU shore areas are Russian: the Saint Petersburg area and the Kaliningrad Oblast exclave.

Infrastructure

The Baltic Sea today is of significant economic and security importance due to its dense network of submarine cables, energy pipelines, ports and offshore energy platforms. In recent years, there have been a number of incidents of sabotage in the Baltic Sea, resulting in damage to critical infrastructures. The most notable incidents include the Nord Stream pipelines sabotage in 2022, where a series of underwater explosions destroyed both Nord Stream 1 and 2. In 2023, there was another incident involving the Balticconnector gas pipeline and a nearby data cable, which were damaged by the Hong Kong-flagged container ship NewNew Polar Bear.
Other significant incidents include the recent damage to several undersea communication cables. The most recent incident of relevance was the Estlink 2 incident|rupture of the Estlink 2 cable] in late 2024. It is suspected that the oil tanker Eagle S, believed to be part of a Russian shadow fleet, is responsible.
These events followed a series of responses from both NATO and the EU. In response, NATO Baltic Sea states have increased their naval presence in the Baltic Sea, and the NATO operation Baltic Sentry was established. Simultaneously, the EU has implemented a series of measures designed to enhance the protection of critical maritime infrastructure. The EU has also underscored the commitment to strengthening cooperation with NATO.

Storms and storm floods

Winter storms begin arriving in the region during October. These have caused numerous shipwrecks, and contributed to the extreme difficulties of rescuing passengers of the ferry MS Estonia en route from Tallinn, Estonia, to Stockholm, Sweden, in September 1994, which claimed the lives of 852 people. Older, wood-based shipwrecks such as the Vasa tend to remain well-preserved, as the Baltic's cold and brackish water does not suit the shipworm.
Storm surge floods are generally taken to occur when the water level is more than one metre above normal. In Warnemünde about 110 floods occurred from 1950 to 2000, an average of just over two per year.
Historic flood events were the All Saints' Flood of 1304 and other floods in the years 1320, 1449, 1625, 1694, 1784 and 1825. Little is known of their extent. From 1872, there exist regular and reliable records of water levels in the Baltic Sea. The highest was the flood of 1872 when the water was an average of above sea level at Warnemünde and a maximum of above sea level in Warnemünde. In the last very heavy floods the average water levels reached above sea level in 1904, in 1913, in January 1954, on 2–4 November 1995 and on 21 February 2002.

Geography

Geophysical data

An arm of the North Atlantic Ocean, the Baltic Sea is enclosed by Sweden and Denmark to the west, Finland to the northeast, and the Baltic countries to the southeast.
It is about long, an average of wide, and an average of deep. The maximum depth is which is on the Swedish side of the center. The surface area is about and the volume is about. The periphery amounts to about of coastline.
The Baltic Sea is one of the largest brackish inland seas by area, and occupies a basin formed by glacial erosion during the last few ice ages.

Extent

The International Hydrographic Organization defines the limits of the Baltic Sea as follows:

Subdivisions

The northern part of the Baltic Sea is known as the Gulf of Bothnia, of which the northernmost part is the Bay of Bothnia or Bothnian Bay. The more rounded southern basin of the gulf is called Bothnian Sea and immediately to the south of it lies the Sea of Åland. The Gulf of Finland connects the Baltic Sea with Saint Petersburg. The Gulf of Riga lies between the Latvian capital city of Riga and the Estonian island of Saaremaa.
The Northern Baltic Sea lies between the Stockholm area, southwestern Finland, and Estonia. The Western and Eastern Gotland basins form the major parts of the Central Baltic Sea or Baltic proper. The Bornholm Basin is the area east of Bornholm, and the shallower Arkona Basin extends from Bornholm to the Danish isles of Falster and Zealand.
In the south, the Bay of Gdańsk lies east of the Hel Peninsula on the Polish coast and west of the Sambia Peninsula in Kaliningrad Oblast. The Bay of Pomerania lies north of the islands of Usedom/Uznam and Wolin, east of Rügen. Between Falster and the German coast lie the Bay of Mecklenburg and Bay of Lübeck. The westernmost part of the Baltic Sea is the Bay of Kiel. The three Danish straits, the Great Belt, the Little Belt and The Sound, connect the Baltic Sea with the Kattegat and Skagerrak strait in the North Sea.

Temperature and ice

The water temperature of the Baltic Sea varies significantly depending on exact location, season and depth. At the Bornholm Basin, which is located directly east of the island of the same name, the surface temperature typically falls to during the peak of the winter and rises to during the peak of the summer, with an annual average of around. A similar pattern can be seen in the Gotland Basin, which is located between the island of Gotland and Latvia. In the deep of these basins the temperature variations are smaller. At the bottom of the Bornholm Basin, deeper than, the temperature typically is, and at the bottom of the Gotland Basin, at depths greater than, the temperature typically is. Generally, offshore locations, lower latitudes and islands maintain maritime climates, but adjacent to the water continental climates are common, especially on the Gulf of Finland. In the northern tributaries the climates transition from moderate continental to subarctic on the northernmost coastlines.
On the long-term average, the Baltic Sea is ice-covered at the annual maximum for about 45% of its surface area. The ice-covered area during such a typical winter includes the Gulf of Bothnia, the Gulf of Finland, the Gulf of Riga, the archipelago west of Estonia, the Stockholm archipelago, and the Archipelago Sea southwest of Finland. The remainder of the Baltic does not freeze during a normal winter, except sheltered bays and shallow lagoons such as the Curonian Lagoon. The ice reaches its maximum extent in February or March; typical ice thickness in the northernmost areas in the Bothnian Bay, the northern basin of the Gulf of Bothnia, is about for landfast sea ice. The thickness decreases farther south.
Freezing begins in the northern extremities of the Gulf of Bothnia typically in the middle of November, reaching the open waters of the Bothnian Bay in early January. The Bothnian Sea, the basin south of Kvarken, freezes on average in late February. The Gulf of Finland and the Gulf of Riga freeze typically in late January. In 2011, the Gulf of Finland was completely frozen on 15 February.
The ice extent depends on whether the winter is mild, moderate, or severe. In severe winters ice can form around southern Sweden and even in the Danish straits. According to the 18th-century natural historian William Derham, during the severe winters of 1703 and 1708, the ice cover reached as far as the Danish straits. Frequently, parts of the Gulf of Bothnia and the Gulf of Finland are frozen, in addition to coastal fringes in more southerly locations such as the Gulf of Riga. This description meant that the whole of the Baltic Sea was covered with ice.
Since 1720, the Baltic Sea has frozen over entirely 20 times, most recently in early 1987, which was the most severe winter in Scandinavia since 1720. The ice then covered. During the winter of 2010–11, which was quite severe compared to those of the last decades, the maximum ice cover was, which was reached on 25 February 2011. The ice then extended from the north down to the northern tip of Gotland, with small ice-free areas on either side, and the east coast of the Baltic Sea was covered by an ice sheet about wide all the way to Gdańsk. This was brought about by a stagnant high-pressure area that lingered over central and northern Scandinavia from around 10 to 24 February. After this, strong southern winds pushed the ice further into the north, and much of the waters north of Gotland were again free of ice, which had then packed against the shores of southern Finland. The effects of the aforementioned high-pressure area did not reach the southern parts of the Baltic Sea, and thus the entire sea did not freeze over. However, floating ice was additionally observed near Świnoujście harbor in January 2010.
In recent years before 2011, the Bothnian Bay and the Bothnian Sea were frozen with solid ice near the Baltic coast and dense floating ice far from it. In 2008, almost no ice formed except for a short period in March.
During winter, fast ice, which is attached to the shoreline, develops first, rendering ports unusable without the services of icebreakers. Level ice, ice sludge, pancake ice, and rafter ice form in the more open regions. The gleaming expanse of ice is similar to the Arctic, with wind-driven pack ice and ridges up to. Offshore of the landfast ice, the ice remains very dynamic all year, and it is relatively easily moved around by winds and therefore forms pack ice, made up of large piles and ridges pushed against the landfast ice and shores.
In spring, the Gulf of Finland and the Gulf of Bothnia normally thaw in late April, with some ice ridges persisting until May in the eastern extremities of the Gulf of Finland. In the northernmost reaches of the Bothnian Bay, ice usually stays until late May; by early June it is practically always gone. However, in the famine year of 1867 remnants of ice were observed as late as 17 July near Uddskär. Even as far south as Øresund, remnants of ice have been observed in May on several occasions; near Taarbaek on 15 May 1942 and near Copenhagen on 11 May 1771. Drift ice was also observed on 11 May 1799.
The ice cover is the main habitat for two large mammals, the grey seal and the Baltic ringed seal, both of which feed underneath the ice and breed on its surface. Of these two seals, only the Baltic ringed seal suffers when there is not adequate ice in the Baltic Sea, as it feeds its young only while on ice. The grey seal is adapted to reproducing also with no ice in the sea. The sea ice also harbors several species of algae that live in the bottom and inside unfrozen brine pockets in the ice.
Due to the often fluctuating winter temperatures between above and below freezing, the saltwater ice of the Baltic Sea can be treacherous and hazardous to walk on, in particular in comparison to the more stable fresh water-ice sheets in the interior lakes.

Hydrography

The Baltic Sea flows out through the Danish straits; however, the flow is complex. A surface layer of brackish water discharges per year into the North Sea. Due to the difference in salinity, by salinity permeation principle, a sub-surface layer of more saline water moving in the opposite direction brings in per year. It mixes very slowly with the upper waters, resulting in a salinity gradient from top to bottom, with most of the saltwater remaining below deep. The general circulation is anti-clockwise: northwards along its eastern boundary, and south along with the western one.
The difference between the outflow and the inflow comes entirely from fresh water. More than 250 streams drain a basin of about, contributing a volume of per year to the Baltic. They include the major rivers of north Europe, such as the Oder, the Vistula, the Neman, the Daugava and the Neva. Additional fresh water comes from the difference of precipitation less evaporation, which is positive.
An important source of salty water is infrequent inflows of North Sea water into the Baltic. Such inflows, important to the Baltic ecosystem because of the oxygen they transport into the Baltic deeps, happen on average once per year, but large pulses that can replace the anoxic deep water in the Gotland Deep occur about once in ten years. Previously, it was believed that the frequency of MBIs had declined since 1980, but recent studies have challenged this view and no longer display a clear change in the frequency or intensity of saline inflows. Instead, a decadal variability in the intensities of MBIs is observed with a main period of approximately 30 years.
The water level is generally far more dependent on the regional wind situation than on tidal effects. However, tidal currents occur in narrow passages in the western parts of the Baltic Sea. Tides can reach in the Gulf of Finland.
The significant wave height is generally much lower than that of the North Sea. Quite violent, sudden storms sweep the surface ten or more times a year, due to large transient temperature differences and a long reach of the wind. Seasonal winds also cause small changes in sea level, of the order of . According to the media, during a storm in January 2017, an extreme wave above has been measured and significant wave height of around has been measured by the FMI. A numerical study has shown the presence of events with significant wave heights. Those extreme waves events can play an important role in the coastal zone on erosion and sea dynamics.

Salinity

The Baltic Sea is the world's largest brackish sea. Only two other brackish waters are larger according to some measurements: The Black Sea is larger in both surface area and water volume, but most of it is located outside the continental shelf. The Caspian Sea is larger in water volume, but—despite its name—it is a lake rather than a sea.
The Baltic Sea's salinity is much lower than that of ocean water, as a result of abundant freshwater runoff from the surrounding land, combined with the shallowness of the sea itself; runoff contributes roughly one-fortieth its total volume per year, as the volume of the basin is about and yearly runoff is about.
The open surface waters of the Baltic Sea "proper" generally have a salinity of 0.3 to 0.9%, which is border-line freshwater. The flow of freshwater into the sea from approximately two hundred rivers and the introduction of salt from the southwest builds up a gradient of salinity in the Baltic Sea. The highest surface salinities, generally 0.7–0.9%, are in the southwestern most part of the Baltic, in the Arkona and Bornholm basins. It gradually falls further east and north, reaching the lowest in the Bothnian Bay at around 0.3%. Drinking the surface water of the Baltic as a means of survival would actually hydrate the body instead of dehydrating, as is the case with ocean water.
As saltwater is denser than freshwater, the bottom of the Baltic Sea is saltier than the surface. This creates a vertical stratification of the water column, a halocline, that represents a barrier to the exchange of oxygen and nutrients, and fosters completely separate maritime environments. The difference between the bottom and surface salinities varies depending on location. Overall it follows the same southwest to east and north pattern as the surface. At the bottom of the Arkona Basin and Bornholm Basin it is typically 1.4–1.8%. Further east and north the salinity at the bottom is consistently lower, being the lowest in Bothnian Bay where it is slightly below 0.4%, or only marginally higher than the surface in the same region.
In contrast, the salinity of the Danish straits, which connect the Baltic Sea and Kattegat, tends to be significantly higher, but with major variations from year to year. For example, the surface and bottom salinity in the Great Belt is typically around 2.0% and 2.8% respectively, which is only somewhat below that of the Kattegat. The water surplus caused by the continuous inflow of rivers and streams to the Baltic Sea means that there generally is a flow of brackish water out through the Danish straits to the Kattegat. Significant flows in the opposite direction, salt water from the Kattegat through the Danish straits to the Baltic Sea, are less regular and are known as major Baltic inflows .

Major tributaries

The rating of mean discharges differs from the ranking of hydrological lengths and the rating of the nominal lengths. Göta älv, a tributary of the Kattegat, is not listed, as due to the northward upper low-salinity-flow in the sea, its water hardly reaches the Baltic proper:

Islands and archipelagoes

Countries that border the sea: Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden.
Countries lands in the outer drainage basin: Belarus, Czech Republic, Norway, Slovakia, Ukraine.
The Baltic Sea drainage basin is roughly four times the surface area of the sea itself. About 48% of the region is forested, with Sweden and Finland containing the majority of the forest, especially around the Gulfs of Bothnia and Finland.
About 20% of the land is used for agriculture and pasture, mainly in Poland and around the edge of the Baltic Proper, in Germany, Denmark, and Sweden. About 17% of the basin is unused open land with another 8% of wetlands. Most of the latter are in the Gulfs of Bothnia and Finland.
The rest of the land is heavily populated. About 85 million people live in the Baltic drainage basin, 15 million within of the coast and 29 million within of the coast. Around 22 million live in population centers of over 250,000. 90% of these are concentrated in the band around the coast. Of the nations containing all or part of the basin, Poland includes 45% of the 85 million, Russia 12%, Sweden 10% and the others less than 6% each.

Cities

The biggest coastal cities :
Other important ports:
The Baltic Sea somewhat resembles a riverbed, with two tributaries, the Gulf of Finland and Gulf of Bothnia. Geological surveys show that before the Pleistocene, instead of the Baltic Sea, there was a wide plain around a great river that paleontologists call the Eridanos. Several Pleistocene glacial episodes scooped out the river bed into the sea basin. By the time of the last, or Eemian Stage, the Eemian Sea was in place. Sometimes the Baltic Sea is considered a very large estuary, with freshwater outflow from numerous rivers.
From that time the waters underwent a geologic history summarized under the names listed below. Many of the stages are named after marine animals that are clear markers of changing water temperatures and salinity.
The factors that determined the sea's characteristics were the submergence or emergence of the region due to the weight of ice and subsequent isostatic readjustment, and the connecting channels it found to the North Sea-Atlantic, either through the straits of Denmark or at what are now the large lakes of Sweden, and the White Sea-Arctic Sea. There are a number of named and dated stages in the evolution of the Baltic Sea:
The land is still emerging isostatically from its depressed state, which was caused by the weight of ice during the last glaciation. The phenomenon is known as post-glacial rebound. Consequently, the surface area and the depth of the sea are diminishing. The uplift is about eight millimeters per year on the Finnish coast of the northernmost Gulf of Bothnia. In the area, the former seabed is only gently sloping, leading to large areas of land being reclaimed in what are, geologically speaking, relatively short periods.

Biology

Fauna and flora

The fauna of the Baltic Sea is a mixture of marine and freshwater species. Among marine fishes are Atlantic cod, Atlantic herring, European hake, European plaice, European flounder, shorthorn sculpin and turbot, and examples of freshwater species include European perch, northern pike, whitefish and common roach. Freshwater species may occur at outflows of rivers or streams in all coastal sections of the Baltic Sea. Otherwise, marine species dominate in most sections of the Baltic, at least as far north as Gävle, where less than one-tenth are freshwater species. Further north the pattern is inverted. In the Bothnian Bay, roughly two-thirds of the species are freshwater. In the far north of this bay, saltwater species are almost entirely absent. For example, the common starfish and shore crab, two species that are very widespread along European coasts, are both unable to cope with the significantly lower salinity. Their range limit is west of Bornholm, meaning that they are absent from the vast majority of the Baltic Sea. Some marine species, like the Atlantic cod and European flounder, can survive at relatively low salinities but need higher salinities to breed, which therefore occurs in deeper parts of the Baltic Sea. The common blue mussel is the dominating animal species, and makes up more than 90% of the total animal biomass in the sea.
There is a decrease in species richness from the Danish belts to the Gulf of Bothnia. The decreasing salinity along this path causes restrictions in both physiology and habitats. At more than 600 species of invertebrates, fish, aquatic mammals, aquatic birds and macrophytes, the Arkona Basin is far richer than other more eastern and northern basins in the Baltic Sea, which all have less than 400 species from these groups, with the exception of the Gulf of Finland with more than 750 species. However, even the most diverse sections of the Baltic Sea have far fewer species than the almost-full saltwater Kattegat, which is home to more than 1600 species from these groups. The lack of tides has affected the marine species as compared with the Atlantic.
Since the Baltic Sea is so young there are only two or three known endemic species: the brown alga Fucus radicans and the flounder Platichthys solemdali. Both appear to have evolved in the Baltic basin and were only recognized as species in 2005 and 2018 respectively, having formerly been confused with more widespread relatives. The tiny Copenhagen cockle, a rare mussel, is sometimes considered endemic, but has now been recorded in the Mediterranean. However, some consider non-Baltic records to be misidentifications of juvenile lagoon cockles. Several widespread marine species have distinctive subpopulations in the Baltic Sea adapted to the low salinity, such as the Baltic Sea forms of the Atlantic herring and lumpsucker, which are smaller than the widespread forms in the North Atlantic.
A peculiar feature of the fauna is that it contains a number of glacial relict species, isolated populations of arctic species which have remained in the Baltic Sea since the last glaciation, such as the large isopod Saduria entomon, the Baltic subspecies of ringed seal, and the fourhorn sculpin. Some of these relicts are derived from glacial lakes, such as Monoporeia affinis, which is a main element in the benthic fauna of the low-salinity Bothnian Bay.
Cetaceans in the Baltic Sea are monitored by the countries bordering the sea and data compiled by various intergovernmental bodies, such as ASCOBANS. A critically endangered population of harbor porpoise inhabit the Baltic proper, whereas the species is abundant in the outer Baltic and occasionally oceanic and out-of-range species such as minke whales, bottlenose dolphins, beluga whales, orcas, and beaked whales visit the waters. In recent years, very small, but with increasing rates, fin whales and humpback whales migrate into Baltic sea including mother and calf pair. Now extinct Atlantic grey whales and eastern population of North Atlantic right whales that is facing functional extinction once migrated into Baltic Sea.
Other notable megafauna include the basking sharks.

Environmental status

Satellite images taken in July 2010 revealed a massive algal bloom covering in the Baltic Sea. The area of the bloom extended from Germany and Poland to Finland. Researchers of the phenomenon have found that algal blooms have occurred every summer for decades. Fertilizer runoff from surrounding agricultural land has exacerbated the problem and led to increased eutrophication.
Approximately of the Baltic's seafloor is a variable dead zone. The more saline water remains on the bottom, isolating it from surface waters and the atmosphere. This leads to decreased oxygen concentrations within the zone. It is mainly bacteria that grow in it, digesting organic material and releasing hydrogen sulfide. Because of this large anaerobic zone, the seafloor ecology differs from that of the neighboring Atlantic.
Plans to artificially oxygenate areas of the Baltic that have experienced eutrophication have been proposed by the University of Gothenburg and Inocean AB. The proposal intends to use wind-driven pumps to pump oxygen-rich surface water to a depth of around 130 m.
After World War II, Germany had to be disarmed, and large quantities of ammunition stockpiles were disposed directly into the Baltic Sea and the North Sea. Environmental experts and marine biologists warn that these ammunition dumps pose an environmental threat, with potentially life-threatening consequences to the health and safety of humans on the coastlines of these seas.

Future change

, and pollution from agriculture and forestry, impose such strong effects on the ecosystems of the Baltic sea, that there is a concern the sea will turn from a carbon sink to a source of carbon dioxide and methane. Modelling climate change and the impact of well characterised factors such as post-glacial rebound before the year 2050, is complicated by the unique properties of the Baltic Sea area compared to say the adjacent North Sea and controversy as to the relative contributions of socio-economic factors such as land use to any warming component. These include its current brackish water, the southern subbasin tendency to have a vertical stratification of the halocline, and the northern subbasin seasonal sea ice cover. High confidence future projections include: air temperature warming, more heavy precipitation episodes, less snow with less perifrost and glacial ice mass in northern catchment areas, more mild winters, raised mean water temperature with more marine heatwaves, intensified seasonal thermoclines without change in the thermohaline circulation, and sea level rise. There are many more projections but these have lower confidence.

Economy

Construction of the Great Belt Bridge in Denmark and the Øresund Bridge-Tunnel, linking Denmark with Sweden, provided a highway and railroad connection between Sweden and the Danish mainland. The undersea tunnel of the Øresund Bridge-Tunnel provides for navigation of large ships into and out of the Baltic Sea. The Baltic Sea is the main trade route for the export of Russian petroleum. Countries neighboring the Baltic Sea have expressed concerns about this since a major oil leak in a seagoing tanker would be especially disastrous for the Baltic given the slow exchange of water in the ecosystem. The tourism industry surrounding the Baltic Sea is naturally concerned about oil pollution.
Much shipbuilding is carried out in the shipyards around the Baltic Sea. The largest shipyards are at Gdańsk, Gdynia, and Szczecin, Poland; Kiel, Germany; Karlskrona and Malmö, Sweden; Rauma, Turku, and Helsinki, Finland; Riga, Ventspils, and Liepāja, Latvia; Klaipėda, Lithuania; and Saint Petersburg, Russia.
Construction of the Fehmarn Belt Fixed Link between Denmark and Germany is due to finish in 2029. It will be a three-bore tunnel carrying four motorway lanes and two rail tracks.
Through the development of offshore wind power the Baltic Sea is expected to become a major source of energy for countries in the region. According to the Marienborg Declaration, signed in 2022, all EU Baltic Sea states have announced their intentions to have 19.6 gigawatts of offshore wind in operation by 2030.

Ferries

There are several cargo and passenger ferries that operate on the Baltic Sea, such as
Piers
Resort towns
Critical maritime infrastructure include pipelines, ports, undersea cables and energy installations. Following a series of incidents between 2022 and 2025, critical infrastructure in the Baltic Sea has drawn growing political attention. In September 2022, both Nord Stream I and Nord Stream II were damaged by explosives close to Bornholm in Denmark. In October 2023, the Balticconnector gas pipeline was damaged by the anchor of the Chinese container vessel Newnew Polar Bear. In November 2024, telecoms cables were damaged, another case of suspected sabotage from a Chinese bulk carrier, departing from a Russian port. In December 2024, a ship registered in the Cook Islands which was thought to be part of a Russian shadow fleet, the Estlink 2, was suspected to have damaged internet cables. These incidents have led to responses from NATO, the European Union and national governments. NATO has increased its air and naval presence, as well as agreed to establish the Maritime Centre for the Security of Critical Undersea Infrastructure within NATO's Allied Maritime Command, among other cooperation efforts. The EU has updated its Maritime Security Strategy, launched an action plan and a coordination group for infrastructure protection, while national governments have strengthened surveillance, legal tools, and seabed defence capabilities. Aside from technical standards, political decisions influence what is deemed as "critical" infrastructure. Critical maritime infrastructure is deemed as critical since economies in today's society are dependent on this infrastructure. Therefore, they require an extra layer of protection, whether by security policies or military protection.

Hybrid warfare

In the current geopolitical climate, CMI is facing challenges posed by hybrid warfare threats. Hybrid threats in the Baltic Sea are often associated with Russian actions and operate below the official threshold of war, which poses a political challenge. The challenge posed by hybrid threats is that they operate in a grey area between peace and violence. For example, the Nord Stream pipeline sabotage demonstrated this problem by exploitation of legal ambiguities, the complexity of attribution, and the disruption of alliance cohesion. The incident highlights the vulnerability of critical infrastructure and the absence of coherent political responses. Responding to hybrid threats requires sustained and coordinated efforts between civilian and military actors. However, the maritime domain presents unique difficulties, including jurisdictional overlaps, fragmented responsibilities, and the challenge of adapting land-based security frameworks to the sea. Hybrid tactics, such as uncrewed aerial vehicle surveillance, covert sabotage, and information manipulation, aim not only to damage infrastructure but also to undermine public trust and create strategic instability in the region.

Geopolitical Dimensions of Critical Maritime Infrastructure

The Baltic Sea region in the post-Cold War era has for a long time been regarded as an area with little geopolitical tensions. With the Soviet presence in the south, the American influence through the NATO members Denmark and Germany and the neutral states Sweden and Finland, an equilibrium existed, often referred to as the "Nordic balance". This balance also prolonged after the steady integration of the region into western institutions.
However, in recent years this geopolitical reality has increasingly been challenged by the neo-imperial ambitions of Russia, manifesting itself in aggression against Ukraine. Russia has also pursued a strategy of regional dominance in the Baltic Sea, designating the regional sea as a zone of strategic influence in its naval doctrine published in 2022. Yet, such a positioning in itself was significantly complicated by the accession of Sweden and Finland to NATO.
Against this geopolitical background, it becomes clear, why many of the recent infrastructure projects in the Baltic Sea were subject to big political debates. Projects such as the Balticconnector, which links the Finnish and Estonian gas markets and has been described by the European Commission as an expression of European solidarity, whilst the Gas Interconnection Poland–Lithuania which connects the Polish and Lithuanian gas networks, and the development of multiple LNG terminals, have all played a significant role in reducing European reliance on Russian energy supplies. These initiatives form part of broader efforts to enhance regional integration and bring the Baltic Sea states into closer alignment with the European Union. In contrast, the Nord Stream pipelines, particularly Nord Stream 2, became a source of political controversy. Critics argued that the project would increase European dependence on Russian gas, bypass transit countries such as Ukraine and Poland, and undermine EU energy solidarity by strengthening Russia's leverage over countries like Germany. Nord Stream 2 experienced prolonged delays and was ultimately suspended following the imposition of international sanctions against Russia after the invasion of Ukraine in 2022. In addition to political and economic controversies, the Nord Stream pipelines also became the subject of security-related concerns regarding their potential strategic implications in the Baltic Sea region. Prior to the 2022 sabotage of the Nord Stream pipelines, security experts and several Eastern European states had raised concerns that such infrastructure could be exploited by Russia for intelligence gathering and military purposes in the Baltic Sea. These concerns gained renewed attention following the sabotage incident, which highlighted challenges in the legal and regulatory frameworks governing the protection of critical infrastructure. The difficulty in conclusively attributing the attack also drew attention to the limitations of existing mechanisms for responding to hybrid threats in the maritime domain.

Helsinki Convention

1974 Convention

For the first time ever, all the sources of pollution around an entire sea were made subject to a single convention, signed in 1974 by the then seven Baltic coastal states. The 1974 Convention entered into force on 3 May 1980.

1992 Convention

In the light of political changes and developments in international environmental and maritime law, a new convention was signed in 1992 by all the states bordering on the Baltic Sea, and the European Community. After ratification, the Convention entered into force on 17 January 2000. The Convention covers the whole of the Baltic Sea area, including inland waters and the water of the sea itself, as well as the seabed. Measures are also taken in the whole catchment area of the Baltic Sea to reduce land-based pollution. The convention on the Protection of the Marine Environment of the Baltic Sea Area, 1992, entered into force on 17 January 2000.
The governing body of the convention is the Helsinki Commission, also known as HELCOM, or Baltic Marine Environment Protection Commission. The present contracting parties are Denmark, Estonia, the European Community, Finland, Germany, Latvia, Lithuania, Poland, Russia, and Sweden.
The ratification instruments were deposited by the European Community, Germany, Latvia and Sweden in 1994, by Estonia and Finland in 1995, by Denmark in 1996, by Lithuania in 1997, and by Poland and Russia in November 1999.

Coordination in the Baltic Sea region

European Union

The European Union is one core framework shaping regional security coordination in the Baltic Sea region. The EU has  recognised this area as one of thirteen designated zones for territorial cooperation. Following the accession of the Baltic States in 2004, the Baltic Sea is now considered an EU internal sea. The following initiatives form the basis of the EU's engagement in Maritime domain awarenes and Maritime Situational Awareness in the Baltic Sea:
  • 2006: Maritime Surveillance Network, a project aiming at facilitating communication between maritime information systems in Europe, that is undertaken by the European Defence Agency.
  • 2009: Sea Surveillance Co-Operation Baltic Sea, a Maritime Situational Awareness cooperation between Baltic Sea countries with the objective of sharing information effectively.
  • 2009: EU Strategy for the Baltic Sea Region, a macro-regional strategy that involves EU member states bordering the Baltic Sea and the EU Commission. The strategy is centred on four core pillars: the environment, prosperity, accessibility and maritime security.
  • 2021–2027: Interreg Baltic Sea Region, an EU co-funded transnational cooperation network.

    NATO

The North Atlantic Treaty Organisation is the primary provider of a collective defense system. Following the accession of Finland in 2023 and Sweden in 2024, the majority of the states bordering the Baltic Sea have become members of NATO, simplifying the organisational geography of the region.
The following NATO initiatives and bodies are particularly relevant for the Baltic Sea region:
  • 2023: Critical Undersea Infrastructure Coordination Cell, a centre aiming at connecting military and civilian stakeholders.
  • 2023: Maritime Centre for the Security of Critical Undersea Infrastructure, a centre aiming at protecting the allies' critical undersea infrastructure.
  • 2023: EU-NATO Task Force on the Resilience of Critical Infrastructure, a cooperation on increasing the resilience of critical infrastructure, supply chains and technology.
  • 2025: Baltic Sentry, a NATO military operation aiming at increasing the military presence in the Baltic Sea to improve the safety of critical infrastructure.
  • Baltic Operations, a multinational naval manoeuvre that is held annually in the Baltic Sea.

    Nordic Defence Cooperation

The Nordic Defence Cooperation is a military alliance comprising the Nordic countries of Denmark, Finland, Iceland, Norway and Sweden. It was established in 2009. The objectives of this cooperation structure include improving the national defense of each country, identifying shared strategic interests, and promoting the development of coordinated, effective responses. The strategy paper 'Vision 2025' outlines plans to enhance collaboration with the Baltic states and transatlantic allies.

Council of the Baltic Sea States

The Council of the Baltic Sea States is an intergovernmental political organisation that focuses on regional cooperation. It was established in 1992 and comprises ten European states and the European Union. The organisation serves as a forum for political dialogue in the region and follows three main objectives: Regional Identity, Safe & Secure Region, and Sustainable & Prosperous Region. It holds annual regional and international meetings.

Historical

  • Bogucka, Maria. "The Role of Baltic Trade in European Development from the XVIth to the XVIIIth Centuries". Journal of European Economic History 9 : 5–20.
  • Davey, James. The Transformation of British Naval Strategy: Seapower and Supply in Northern Europe, 1808–1812.
  • Fedorowicz, Jan K. England's Baltic Trade in the Early Seventeenth Century: A Study in Anglo-Polish Commercial Diplomacy.
  • Frost, Robert I. The Northern Wars: War, State, and Society in Northeastern Europe, 1558–1721.
  • Grainger, John D. The British Navy in the Baltic.
  • Kent, Heinz S. K. War and Trade in Northern Seas: Anglo-Scandinavian Economic Relations in the Mid Eighteenth Century.
  • Koningsbrugge, Hans van. "In War and Peace: The Dutch and the Baltic in Early Modern Times". Tijdschrift voor Skandinavistiek 16 : 189–200.
  • Lindblad, Jan Thomas. "Structural Change in the Dutch Trade in the Baltic in the Eighteenth Century". Scandinavian Economic History Review 33 : 193–207.
  • Lisk, Jill. The Struggle for Supremacy in the Baltic, 1600–1725.
  • Roberts, Michael. The Early Vasas: A History of Sweden, 1523–1611.
  • Rystad, Göran, Klaus-R. Böhme, and Wilhelm M. Carlgren, eds. In Quest of Trade and Security: The Baltic in Power Politics, 1500–1990. Vol. 1, 1500–1890. Stockholm: Probus, 1994.
  • Salmon, Patrick, and Tony Barrow, eds. Britain and the Baltic: Studies in Commercial, Political and Cultural Relations.
  • Stiles, Andrina. Sweden and the Baltic 1523–1721.
  • Thomson, Erik. "Beyond the Military State: Sweden's Great Power Period in Recent Historiography". History Compass 9 : 269–283.
  • Tielhof, Milja van. The "Mother of All Trades": The Baltic Grain Trade in Amsterdam from the Late 16th to Early 19th Century. Leiden, The Netherlands: Brill, 2002.
  • Warner, Richard. "British Merchants and Russian Men-of-War: The Rise of the Russian Baltic Fleet". In Peter the Great and the West: New Perspectives. Edited by Lindsey Hughes, 105–117. Basingstoke, UK: Palgrave Macmillan, 2001.