Editing Baltic Sea (section)

== Geography ==
=== Geophysical data ===
[[File:Baltic drainage basins (catchment area).svg|thumb|Baltic drainage basins (catchment area), with depth, elevation, major rivers and lakes]]
[[File:Curonian Lagoon and Memel - panoramio.jpg|thumb|[[Curonian Lagoon]], [[Curonian Spit|Spit]] and [[Klaipėda]]]]

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 states|Baltic countries]] to the southeast.

It is about {{convert|1600|km|abbr=on}} long, an average of {{convert|193|km|abbr=on}} wide, and an average of {{convert|55|m|ft}} deep. The maximum depth is {{convert|459|m|ft|abbr=on}} which is on the Swedish side of the center. The surface area is about {{convert|349644|km2|abbr=on}} <ref>{{cite web|title=EuroOcean |url=http://www.eurocean.org/np4/323.html |access-date=14 April 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140415124200/http://www.eurocean.org/np4/323.html |archive-date=15 April 2014}}</ref> and the volume is about {{convert|20000|km3|abbr=on}}. The periphery amounts to about {{convert|8000|km|abbr=on}} of coastline.<ref>{{cite web|url=http://www.envir.ee/baltics/geograph.htm |title=Geography of the Baltic Sea Area |access-date=27 August 2005 |url-status=dead |archive-url=https://web.archive.org/web/20060421230004/http://www.envir.ee/baltics/geograph.htm |archive-date=21 April 2006 }} at envir.ee. (archived) (21 April 2006). Retrieved on 23 June 2011.</ref>

The Baltic Sea is one of the largest [[brackish water|brackish]] inland seas by area, and occupies a basin (a ''[[Zungenbecken]]'') formed by glacial erosion during the last few [[ice age]]s.

{| class="wikitable sortable"
|+Physical characteristics of the Baltic Sea, its main sub-regions, and the transition zone to the Skagerrak/North Sea area<ref>{{Cite web |url=http://www.helcom.fi/Lists/Publications/BSEP104.pdf |title=p. 7 |access-date=27 November 2015 |archive-date=8 December 2015 |archive-url=https://web.archive.org/web/20151208055840/http://www.helcom.fi/Lists/Publications/BSEP104.pdf |url-status=live }}</ref>
! style="background: DarkCyan; color: white; text-align: center" rowspan=2| Sub-area
! style="background: DarkCyan; color: white; text-align: center" colspan=2| Area
! style="background: DarkCyan; color: white; text-align: center" colspan=2| Volume
! style="background: DarkCyan; color: white; text-align: center" colspan=2| Maximum depth
! style="background: DarkCyan; color: white; text-align: center" colspan=2| Average depth
|-
! style="background: DarkCyan; color: white; text-align: center"| km<sup>2</sup>
! style="background: DarkCyan; color: white; text-align: center"| sq mi
! style="background: DarkCyan; color: white; text-align: center"| km<sup>3</sup>
! style="background: DarkCyan; color: white; text-align: center"| cu mi
! style="background: DarkCyan; color: white; text-align: center"| m
! style="background: DarkCyan; color: white; text-align: center"| ft
! style="background: DarkCyan; color: white; text-align: center"| m
! style="background: DarkCyan; color: white; text-align: center"| ft
|-
| style="text-align: left"|Baltic proper
|{{convert|211,069|km2|sqmi|disp=table|sortable=on}}
|{{convert|13,045|km3|cumi|disp=table|sortable=on}}
|{{convert|459|m|ft|disp=table|sortable=on}}
|{{convert|62.1|m|ft|disp=table|sortable=on}}
|-
| style="text-align: left"|Gulf of Bothnia
|{{convert|115,516|km2|sqmi|disp=table|sortable=on}}
|{{convert|6,389|km3|cumi|disp=table|sortable=on}}
|{{convert|230|m|ft|disp=table|sortable=on}}
|{{convert|60.2|m|ft|disp=table|sortable=on}}
|-
| style="text-align: left"|Gulf of Finland
|{{convert|29,600|km2|sqmi|disp=table|sortable=on}}
|{{convert|1,100|km3|cumi|disp=table|sortable=on}}
|{{convert|123|m|ft|disp=table|sortable=on}}
|{{convert|38.0|m|ft|disp=table|sortable=on}}
|-
| style="text-align: left"|Gulf of Riga
|{{convert|16,300|km2|sqmi|disp=table|sortable=on}}
|{{convert|424|km3|cumi|disp=table|sortable=on}}
|{{convert|60|m|ft|disp=table|sortable=on}}
|{{convert|26.0|m|ft|disp=table|sortable=on}}
|-
| style="text-align: left"| Belt Sea/Kattegat
|{{convert|42,408|km2|sqmi|disp=table|sortable=on}}
|{{convert|802|km3|cumi|disp=table|sortable=on}}
|{{convert|109|m|ft|disp=table|sortable=on}}
|{{convert|18.9|m|ft|disp=table|sortable=on}}
|-
! style="text-align: left"| Total Baltic Sea
!{{convert|415,266|km2|sqmi|disp=table|sortable=on}}
!{{convert|21,721|km3|cumi|disp=table|sortable=on}}
!{{convert|459|m|ft|disp=table|sortable=on}}
!{{convert|52.3|m|ft|disp=table|sortable=on}}
|}

=== Extent ===
The [[International Hydrographic Organization]] defines the limits of the Baltic Sea as follows:<ref>{{cite web|url=https://iho.int/uploads/user/pubs/standards/s-23/S-23_Ed3_1953_EN.pdf |title=Limits of Oceans and Seas, 3rd edition |year=1953 |publisher=International Hydrographic Organization |access-date=28 December 2020 |archive-url=https://web.archive.org/web/20111008191433/http://www.iho-ohi.net/iho_pubs/standard/S-23/S23_1953.pdf |archive-date=8 October 2011 }}</ref>

:Bordered by the coasts of Germany, Denmark, Poland, Sweden, Finland, Russia, Estonia, Latvia, and Lithuania, it extends north-eastward of the following limits:
:*''In the [[Little Belt]].'' A line joining [[Nieby|Falshöft]] ({{coord|54|47|N|9|57.5|E|display=inline}}) and Vejsnæs Nakke ([[Ærø]]: {{coord|54|49|N|10|26|E|display=inline}}).
:*''In the [[Great Belt]].'' A line joining Gulstav (South extreme of [[Langeland]] Island) and Kappel Kirke ({{coord|54|46|N|11|01|E|display=inline}}) on Island of [[Lolland]].
:*''In the [[Guldborgsund|Guldborg Sound]].'' A line joining Flinthorne-Rev and Skjelby ({{coord|54|38|N|11|53|E|display=inline}}).
:*''In [[Øresund|the Sound]].'' A line joining [[Stevns Peninsula|Stevns]] Lighthouse ({{coord|55|17|N|12|27|E|display=inline}}) and [[Falsterbo|Falsterbo Point]] ({{coord|55|23|N|12|49|E|display=inline}}).

=== Subdivisions ===
[[File:Baltic marine subdivisions and drainage basins.gif|thumb|Regions and basins of the Baltic Sea:<ref>{{cite web|url=http://www.baltic.vtt.fi/demo/baltmap.htm|title=Baltic Sea area clickable map|website=www.baltic.vtt.fi|access-date=11 April 2008|archive-url=https://web.archive.org/web/20071023052150/http://www.baltic.vtt.fi/demo/baltmap.htm|archive-date=23 October 2007|url-status=dead}}</ref><br />1 = [[Bothnian Bay]]<br />2 = [[Bothnian Sea]]<br />1 + 2 = [[Gulf of Bothnia]], partly also 3 & 4<br />3 = [[Archipelago Sea]]<br />4 = [[Åland Sea]]<br />5 = [[Gulf of Finland]]<br />6 = Northern Baltic Proper<br />7 = Western [[Gotland Basin]]<br />8 = Eastern [[Gotland Basin]]<br />9 = [[Gulf of Riga]]<br />10 = [[Bay of Gdańsk]]/Gdansk Basin<br />11 = [[Bornholm]] Basin and [[Hanöbukten|Hanö Bight]]<br />12 = [[Cape Arkona|Arkona]] Basin<br />6–12 = [[Baltic Proper]]<br />13 = [[Kattegat]], not an integral part of the Baltic Sea<br />14 = Belt Sea ([[Little Belt]] and [[Great Belt]])<br />15 = [[Öresund]] (The Sound)<br />14 + 15 = [[Danish straits]], not an integral part of the Baltic Sea]]

The northern part of the Baltic Sea is known as the [[Gulf of Bothnia]], of which the northernmost part is the Bay of Bothnia <!-- redirects to: --> 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 [[Latvia]]n capital city of [[Riga]] and the [[Estonia]]n island of [[Saaremaa]].

The Northern Baltic Sea lies between the [[Stockholm]] area, southwestern Finland, and Estonia. The [[Gotland Basin|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 [[Cape Arkona|Arkona]] Basin extends from Bornholm to the Danish isles of [[Falster]] and [[Zealand (Denmark)|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|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 [[Öresund|The Sound]] (''Öresund''/''Øresund''), connect the Baltic Sea with the [[Kattegat]] and [[Skagerrak]] strait in the [[North Sea]].

===Temperature and ice===
[[File:BalticSea March2000 NASA-S2000084115409.png|thumb|Satellite image of the Baltic Sea in a mild winter]]
[[File:Baltic Sea 1551 (5496745361).jpg|thumb|Traversing Baltic Sea and ice]]
[[File:Skiing on the ice in Haukilahti.jpg|thumb|On particularly cold winters, the coastal parts of the Baltic Sea freeze into ice thick enough to walk or ski on.]]
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 {{convert|0-5|C|F|abbr=on}} during the peak of the winter and rises to {{convert|15-20|C|F|abbr=on}} during the peak of the summer, with an annual average of around {{convert|9-10|C|F|abbr=on}}.<ref name=OurBalticSea>{{cite web | url=http://stateofthebalticsea.helcom.fi/in-brief/our-baltic-sea/ | title=Our Baltic Sea | publisher=HELCOM | access-date=27 July 2018 | archive-date=26 July 2018 | archive-url=https://web.archive.org/web/20180726025617/http://stateofthebalticsea.helcom.fi/in-brief/our-baltic-sea/ | url-status=live }}</ref> 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 {{convert|80|m|ft|abbr=on}}, the temperature typically is {{convert|1-10|C|F|abbr=on}}, and at the bottom of the Gotland Basin, at depths greater than {{convert|225|m|ft|abbr=on}}, the temperature typically is {{convert|4-7|C|F|abbr=on}}.<ref name=OurBalticSea/> Generally, offshore locations, lower latitudes and islands maintain [[oceanic climate|maritime climate]]s, but adjacent to the water [[humid continental climate|continental climate]]s are common, especially on the [[Gulf of Finland]]. In the northern tributaries the climates transition from moderate continental to [[subarctic climate|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 {{convert|70|cm|0|abbr=on}} 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.<ref>''[[Helsingin Sanomat]]'', 16 February 2011, p. A8.</ref>

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.<ref>Derham, William ''Physico-Theology: Or, A Demonstration of the Being and Attributes of God from His Works of Creation'' (London, 1713).</ref> 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 {{convert|400000|km2|abbr=on}}. During the winter of 2010–11, which was quite severe compared to those of the last decades, the maximum ice cover was {{convert|315000|km2|abbr=on}}, 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 {{convert|25|to|100|km|0|abbr=on}} 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.<ref>''[[Helsingin Sanomat]]'', 10 February 2011, p. A4; 25 February 2011, p. A5; 11 June 2011, p. A12.</ref> 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.<ref>[http://www.ssec.wisc.edu/ Sea Ice Survey] {{Webarchive|url=https://web.archive.org/web/20161125040216/http://www.ssec.wisc.edu/ |date=25 November 2016 }} Space Science and Engineering Center, University of Wisconsin.</ref>

[[File:Jäämäed Puhtulaiul 1.JPG|thumb|Piles of drift ice on the shore of Puhtulaid, near [[Virtsu]], Estonia, in late April]]
During winter, [[fast ice]], which is attached to the shoreline, develops first, rendering ports unusable without the services of [[icebreaker]]s. [[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 {{convert|15|m|abbr=on}}. 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 [[Famine of 1866-68|1867]] remnants of ice were observed as late as 17 July near [[Uddskär]].<ref>{{cite web|title=Nödåret 1867|url=http://byar.lulea.se/scripts/historia_visa.asp?By=79&No=114|archive-url=https://web.archive.org/web/20110727130208/http://byar.lulea.se/scripts/historia_visa.asp?By=79&No=114|url-status=dead|archive-date=27 July 2011|publisher=Byar i Luleå}}</ref> 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.<ref>{{cite web|title=Isvintrene i 40'erne|date=19 January 2008|url=http://vejr.tv2.dk/artikel/id-10070756%3Aisvintrene-i-40erne.html/mraid.js|publisher=TV 2|access-date=6 January 2017|archive-date=6 January 2017|archive-url=https://web.archive.org/web/20170106105613/http://vejr.tv2.dk/artikel/id-10070756%3Aisvintrene-i-40erne.html/mraid.js|url-status=live}}</ref><ref>{{cite web|url=http://natmus.dk/historisk-viden/temaer/militaerhistorie/soeofficeren-peter-schioennings-liv/kilder/peter-schioennings-dagbog/1771/|title=1771 – Nationalmuseet|access-date=15 April 2017|archive-url=https://web.archive.org/web/20170416044935/http://natmus.dk/historisk-viden/temaer/militaerhistorie/soeofficeren-peter-schioennings-liv/kilder/peter-schioennings-dagbog/1771/|archive-date=16 April 2017|url-status=dead}}</ref><ref>{{cite web|title=Is i de danske farvande i 1700-tallet|url=https://natmus.dk/historisk-viden/temaer/klima-i-historien/den-lille-istid-og-industrialiseringen/is-i-de-danske-farvande/|publisher=Nationalmuseet|access-date=18 February 2018|archive-date=18 February 2018|archive-url=https://web.archive.org/web/20180218091529/https://natmus.dk/historisk-viden/temaer/klima-i-historien/den-lille-istid-og-industrialiseringen/is-i-de-danske-farvande/|url-status=live}}</ref>

The ice cover is the main habitat for two large mammals, the [[grey seal]] (''Halichoerus grypus'') and the Baltic [[ringed seal]] (''Pusa hispida botnica''), 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===
[[File:Ostseetiefen.png|thumb|Depths of the Baltic Sea in meters]]
The Baltic Sea flows out through the [[Danish straits]]; however, the flow is complex. A surface layer of brackish water discharges {{convert|940|km3|abbr=on}} 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 {{convert|475|km3|abbr=on}} 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 {{convert|40|to|70|m|abbr=on}} deep. The general circulation is anti-clockwise: northwards along its eastern boundary, and south along with the western one .<ref name=Alhonen>Alhonen, p. 88</ref>

The difference between the outflow and the inflow comes entirely from fresh [[water]]. More than 250 streams drain a basin of about {{convert|1600000|km2|abbr=on}}, contributing a volume of {{convert|660|km3|abbr=on}} per year to the Baltic. They include the major rivers of north Europe, such as the [[Oder]], the [[Vistula]], the [[Neman River|Neman]], the [[Daugava River|Daugava]] and the [[Neva]]. Additional fresh water comes from the difference of [[precipitation (meteorology)|precipitation]] less evaporation, which is positive.

An important source of salty water is infrequent inflows (also known as [[Major Baltic Inflow|major Baltic inflows]] or MBIs) 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 Basin|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.<ref>{{Cite journal |last=Mohrholz |first=Volker |date=2018 |title=Major Baltic Inflow Statistics – Revised |journal=Frontiers in Marine Science |volume=5 |page=384 |doi=10.3389/fmars.2018.00384 |issn=2296-7745 |doi-access=free |bibcode=2018FrMaS...5..384M }}</ref><ref>{{Cite journal |last1=Lehmann |first1=Andreas |last2=Myrberg |first2=Kai |last3=Post |first3=Piia |last4=Chubarenko |first4=Irina |last5=Dailidiene |first5=Inga |last6=Hinrichsen |first6=Hans-Harald |last7=Hüssy |first7=Karin |last8=Liblik |first8=Taavi |last9=Meier |first9=H. E. Markus |last10=Lips |first10=Urmas |last11=Bukanova |first11=Tatiana |date=16 February 2022 |title=Salinity dynamics of the Baltic Sea |url=https://esd.copernicus.org/articles/13/373/2022/ |journal=Earth System Dynamics |language=English |volume=13 |issue=1 |pages=373–392 |doi=10.5194/esd-13-373-2022 |bibcode=2022ESD....13..373L |issn=2190-4979 |access-date=24 July 2023 |archive-date=21 July 2023 |archive-url=https://web.archive.org/web/20230721123134/https://esd.copernicus.org/articles/13/373/2022/ |url-status=live |doi-access=free }}</ref>

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 {{convert|17 to 19|cm|in|1|abbr=on}} in the Gulf of Finland.<ref>{{Cite journal|last1=Medvedev|first1=I. P.|last2=Rabinovich|first2=A. B.|last3=Kulikov|first3=E. A.|date=September 2013|title=Tidal oscillations in the Baltic Sea|url=http://link.springer.com/10.1134/S0001437013050123|journal=Oceanology|language=en|volume=53|issue=5|pages=526–538|doi=10.1134/S0001437013050123|bibcode=2013Ocgy...53..526M|s2cid=129778127|issn=0001-4370|access-date=27 September 2021|archive-date=4 December 2022|archive-url=https://web.archive.org/web/20221204034253/https://link.springer.com/article/10.1134/S0001437013050123|url-status=live}}</ref>

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 {{convert|0.5|m|abbr=on}} .<ref name=Alhonen/> According to the media, during a storm in January 2017, an extreme wave above {{convert|14|m|ft|0|abbr=on}} has been measured and significant wave height of around {{convert|8|m|ft|0|abbr=on}} has been measured by the [[Finnish Meteorological Institute|FMI]]. A numerical study has shown the presence of events with {{convert|8 to 10|m|ft|0|abbr=on}} significant wave heights. Those extreme waves events can play an important role in the coastal zone on erosion and sea dynamics.<ref>{{Cite journal|last1=Rutgersson|first1=Anna|last2=Kjellström|first2=Erik|last3=Haapala|first3=Jari|last4=Stendel|first4=Martin|last5=Danilovich|first5=Irina|last6=Drews|first6=Martin|last7=Jylhä|first7=Kirsti|last8=Kujala|first8=Pentti|last9=Guo Larsén|first9=Xiaoli|last10=Halsnæs|first10=Kirsten|last11=Lehtonen|first11=Ilari|date=6 April 2021|title=Natural Hazards and Extreme Events in the Baltic Sea region|url=https://esd.copernicus.org/preprints/esd-2021-13/|journal=Earth System Dynamics Discussions|volume=13|issue=1|language=English|pages=251–301|doi=10.5194/esd-2021-13|s2cid=233556209|issn=2190-4979|doi-access=free|access-date=29 September 2021|archive-date=29 September 2021|archive-url=https://web.archive.org/web/20210929141707/https://esd.copernicus.org/preprints/esd-2021-13/|url-status=live}}</ref>

===Salinity===
[[File:Sunny day on the coast on Baltic sea.jpg|thumb|Baltic Sea near [[Klaipėda]] ([[Karklė]]).]]

The Baltic Sea is the world's largest [[brackish]] sea.<ref name=Leijonmalm2017>{{cite book | author1=Snoeijs-Leijonmalm P. | author2=E.Andrén | year=2017 | chapter=Why is the Baltic Sea so special to live in? | editor1=P. Snoeijs-Leijonmalm | editor2=H. Schubert | editor3=T. Radziejewska | title=Biological Oceanography of the Baltic Sea | publisher=Springer, Dordrecht | pages=23–84 | isbn=978-94-007-0667-5 }}</ref> Only two [[List of brackish bodies of water|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]] (only a small fraction is inland). The [[Caspian Sea]] is larger in water volume, but—despite its name—it is a lake rather than a sea.<ref name=Leijonmalm2017/>

The Baltic Sea's [[salinity]] is much lower than that of ocean water (which averages 3.5%), as a result of abundant freshwater runoff from the surrounding land (rivers, streams and alike), 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 {{convert|21000|km3|abbr=on}} and yearly runoff is about {{convert|500|km3|abbr=on}}.{{citation needed|date=July 2018}}

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 (the former located roughly between southeast [[Zealand]] and Bornholm, and the latter directly east of Bornholm). It gradually falls further east and north, reaching the lowest in the [[Bothnian Bay]] at around 0.3%.<ref name=Hydrogeography&Oxygen>{{cite web | author=Viktorsson, L. | date=16 April 2018 | url=http://www.helcom.fi/baltic-sea-trends/environment-fact-sheets/hydrography/hydrography-and-oxygen-in-the-deep-basins/ | title=Hydrogeography and oxygen in the deep basins | publisher=HELCOM | access-date=27 July 2018 | archive-date=27 July 2018 | archive-url=https://web.archive.org/web/20180727225916/http://www.helcom.fi/baltic-sea-trends/environment-fact-sheets/hydrography/hydrography-and-oxygen-in-the-deep-basins | url-status=live }}</ref> Drinking the surface water of the Baltic as a means of survival would actually hydrate the body instead of [[dehydration|dehydrating]], as is the case with ocean water.<ref group="note">A healthy serum concentration of sodium is around 0.8–0.85%, and healthy kidneys can concentrate salt in urine to at least 1.4%.</ref>{{citation needed|date=July 2018}}

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.<ref>{{cite web|url=http://www.ices.dk/projects/balticsea/baltic%20sea-rse%20thulin%20and%20andrusaitis.pdf |title=The Baltic Sea: Its Past, Present and Future |url-status=dead |archive-url=https://web.archive.org/web/20070606211738/http://www.ices.dk/projects/balticsea/baltic%20sea-rse%20thulin%20and%20andrusaitis.pdf |archive-date=6 June 2007 }} &nbsp;{{small|(352&nbsp;KB)}}, Jan Thulin and Andris Andrushaitis, Religion, Science and the Environment Symposium V on the Baltic Sea (2003).</ref> 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 (equaling depths greater than {{convert|40|m|ft|abbr=on|disp=or}}) and Bornholm Basin (depths greater than {{convert|80|m|ft|abbr=on|disp=or}}) 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 (depths greater than {{convert|120|m|ft|abbr=on|disp=or}}) where it is slightly below 0.4%, or only marginally higher than the surface in the same region.<ref name=Hydrogeography&Oxygen/>

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.<ref name=Hydrogeography&Oxygen/> 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 (and eventually the Atlantic).<ref name=Muus1999>{{cite book |author1=Muus, B. |author2=J.G. Nielsen |author3=P. Dahlstrom |author4=B. Nystrom |year=1999 |title=Sea Fish |publisher=Scandinavian Fishing Year Book |isbn=978-8790787004 }}</ref> 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 Inflow|major Baltic inflows (MBIs)]].

=== Major tributaries ===
{{See also|List of rivers of the Baltic Sea}}
The rating of [[Discharge (hydrology)|mean discharges]] differs from the ranking of hydrological lengths (from the most distant source to the sea) 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:

{| class="wikitable sortable" style="padding:6px; spacing:6px;"
!rowspan=2| Name 
!colspan=2| Mean discharge
!colspan=2| Length 
!colspan=2| Basin area 
!rowspan=2| States sharing the basin 
!rowspan=2| Longest watercourse
|-
!{{nowrap|m<sup>3</sup>/s}}
!{{nowrap|cu ft/s}}
!km
!mi
!{{nowrap|km<sup>2</sup>}}
!{{nowrap|sq mi}}
|-
|[[Neva]] (nominal) ||{{convert|2500|m3/s|cuft/s|disp=table|sortable=on}} || {{convert|74|km|mi|disp=table|sortable=on}} ||{{convert|281,000|km2|sqmi|disp=table|sortable=on}} ||rowspan=2|[[Russia]], [[Finland]] (Ladoga-affluent [[Vuoksi]]) ||rowspan=2| [[Suna River|Suna]] ({{convert|280|km|abbr=on|disp=semicolon}}) → [[Lake Onega]] ({{convert|160|km|abbr=on|disp=semicolon}}) →<br />[[Svir River|Svir]] ({{convert|224|km|abbr=on|disp=semicolon}}) → [[Lake Ladoga]] ({{convert|122|km|abbr=on|disp=semicolon}}) → Neva
|-
|Neva (hydrological) || {{convert|2500|m3/s|cuft/s|disp=table|sortable=on}} ||{{convert|860|km|mi|disp=table|sortable=on}} || {{convert|281,000|km2|sqmi|disp=table|sortable=on}}
|-
| [[Vistula]] || {{convert|1080|m3/s|cuft/s|disp=table|sortable=on}} || {{convert|1047|km|mi|disp=table|sortable=on}} || {{convert|194,424|km2|sqmi|disp=table|sortable=on}} || [[Poland]], tributaries: [[Belarus]], [[Ukraine]], [[Slovakia]] || [[Bug (river)|Bug]] ({{convert|774|km|abbr=on|disp=semicolon}}) → [[Narew]] ({{convert|22|km|abbr=on|disp=semicolon}}) → Vistula ({{convert|156|km|abbr=on|disp=semicolon}}) total 1{{convert|204|km|abbr=on|disp=semicolon}}
|-
| [[Daugava River|Daugava]] || {{convert|678|m3/s|cuft/s|disp=table|sortable=on}} || {{convert|1020|km|mi|disp=table|sortable=on}} || {{convert|87,900|km2|sqmi|disp=table|sortable=on}} || [[Russia]] (source), [[Belarus]], [[Latvia]] ||
|-
| [[Neman River|Neman]] || {{convert|678|m3/s|cuft/s|disp=table|sortable=on}} || {{convert|937|km|mi|disp=table|sortable=on}} || {{convert|98,200|km2|sqmi|disp=table|sortable=on}} || [[Belarus]] (source), [[Lithuania]], [[Russia]] ||
|-
| [[Kemijoki]] (main river) ||{{convert|556|m3/s|cuft/s|disp=table|sortable=on}} || {{convert|550|km|mi|disp=table|sortable=on}} ||{{convert|51,127|km2|sqmi|disp=table|sortable=on}} ||rowspan=2| [[Finland]], [[Norway]] (source of [[Ounasjoki]]) ||rowspan=2| longer tributary [[Kitinen]]
|-
| Kemijoki (river system) || {{convert|556|m3/s|cuft/s|disp=table|sortable=on}} || {{convert|600|km|mi|disp=table|sortable=on}} || {{convert|51,127|km2|sqmi|disp=table|sortable=on}}
|-
| [[Oder]] || {{convert|540|m3/s|cuft/s|disp=table|sortable=on}} || {{convert|866|km|mi|disp=table|sortable=on}} || {{convert|118,861|km2|sqmi|disp=table|sortable=on}} || [[Czech Republic]] (source), [[Poland]], [[Germany]] ||[[Warta]] ({{convert|808|km|abbr=on|disp=semicolon}}) → Oder ({{convert|180|km|abbr=on|disp=semicolon}}) total: {{convert|928|km|abbr=on|disp=semicolon}}
|-
| [[Lule älv]] || {{convert|506|m3/s|cuft/s|disp=table|sortable=on}} || {{convert|461|km|mi|disp=table|sortable=on}} || {{convert|25,240|km2|sqmi|disp=table|sortable=on}} || [[Sweden]] ||
|-
| [[Narva River|Narva]] (nominal) ||{{convert|415|m3/s|cuft/s|disp=table|sortable=on}} || {{convert|77|km|mi|disp=table|sortable=on}} ||{{convert|56,200|km2|sqmi|disp=table|sortable=on}} ||rowspan=2| [[Russia]] (source of Velikaya), [[Estonia]] ||rowspan=2| [[Velikaya River|Velikaya]] ({{convert|430|km|abbr=on|disp=semicolon}}) → [[Lake Peipus]] ({{convert|145|km|abbr=on|disp=semicolon}}) → Narva
|-
| Narva (hydrological) ||{{convert|415|m3/s|cuft/s|disp=table|sortable=on}} || {{convert|652|km|mi|disp=table|sortable=on}} || {{convert|56,200|km2|sqmi|disp=table|sortable=on}}
|-
| [[Torne älv]] (nominal) ||{{convert|388|m3/s|cuft/s|disp=table|sortable=on}} || {{convert|520|km|mi|disp=table|sortable=on}} ||{{convert|40,131|km2|sqmi|disp=table|sortable=on}} ||rowspan=2| [[Norway]] (source), [[Sweden]], [[Finland]] ||rowspan=2| Válfojohka → Kamajåkka → Abiskojaure → [[Abiskojokk]]<br />(total {{convert|40|km|abbr=on|disp=semicolon}}) → [[Torneträsk]] ({{convert|70|km|abbr=on|disp=semicolon}}) → Torne älv
|-
| Torne älv (hydrological) || {{convert|388|m3/s|cuft/s|disp=table|sortable=on}} ||{{convert|630|km|mi|disp=table|sortable=on}} || {{convert|40,131|km2|sqmi|disp=table|sortable=on}}
|}

=== Islands and archipelagoes ===
{{main|List of islands in the Baltic Sea}}
[[File:Skerries which are part of the Åland Islands.jpg|thumb|[[skerry|Skerries]] form an integral and typical part of many of the [[archipelago]]s of the Baltic Sea, such as these in the archipelago of [[Åland]], [[Finland]].]]
[[File:Korsö Kroksö Sandön February 2013.jpg|thumb|[[Stockholm archipelago]]]]
[[File:Bornholm luftaufnahme.jpg|thumb|Aerial view of [[Bornholm]], Denmark]]
[[File:Cloud cumulonimbus at baltic sea(1).jpg|thumb|At [[Öland]], Sweden]]
* [[Åland]] ([[Finland]], [[autonomous entity|autonomous]])
* [[Archipelago Sea]] ([[Finland]])
** [[Pargas]]
** [[Nagu]]
** [[Korpo]]
** [[Houtskär]]
** [[Kustavi]]
** [[Kimito]]
* [[Blekinge archipelago]] ([[Sweden]])
* [[Bornholm]], including [[Christiansø]] ([[Denmark]])
* [[Falster]] ([[Denmark]])
* [[Gotland]] ([[Sweden]])
* [[Hailuoto]] ([[Finland]])
* [[Kotlin Island|Kotlin]] ([[Russia]])
* [[Lolland]] ([[Denmark]])
* [[Kvarken]] archipelago, including [[Valsörarna]] ([[Finland]])
* [[Møn]] ([[Denmark]])
* [[Öland]] ([[Sweden]])
* [[Rügen]] ([[Germany]])
* [[Stockholm archipelago]] ([[Sweden]])
** [[Värmdön]] ([[Sweden]])
* [[Usedom]] or Uznam (split between [[Germany]] and [[Poland]])
* [[West Estonian archipelago]] ([[Estonia]]):
** [[Hiiumaa]]
** [[Muhu]]
** [[Saaremaa]]
** [[Vormsi]]
* [[Wolin]] ([[Poland]])
* [[Zealand]] ([[Denmark]])

===Coastal countries===
[[File:Population density in the Baltic Sea catchment area.svg|thumb|Population density in the Baltic Sea catchment area]]

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&nbsp;million people live in the Baltic drainage basin, 15&nbsp;million within {{convert|10|km|0|abbr=on}} of the coast and 29&nbsp;million within {{convert|50|km|0|abbr=on}} of the coast. Around 22&nbsp;million live in population centers of over 250,000. 90% of these are concentrated in the {{convert|10|km|0|abbr=on}} band around the coast. Of the nations containing all or part of the basin, Poland includes 45% of the 85&nbsp;million, Russia 12%, Sweden 10% and the others less than 6% each.<ref>{{Cite journal|last=Sweitzer|first=J|date=May 2019|title=Land Use and Population Density in the Baltic Sea Drainage Basin: A GIS Database|url=https://www.researchgate.net/publication/237511351|journal=Ambio|volume=25|pages=20|via=ResearchGate|access-date=11 July 2019|archive-date=30 May 2020|archive-url=https://web.archive.org/web/20200530150109/https://www.researchgate.net/publication/237511351_LAND_USE_AND_POPULATION_DENSITY_IN_THE_BALTIC_SEA_DRAINAGE_BASIN_A_GIS_DATABASE|url-status=live}}</ref>

==== Cities ====
{{Main|List of cities and towns around the Baltic Sea}}
[[File:Spb 06-2017 img01 Spit of Vasilievsky Island.jpg|thumb|[[Vasilyevsky Island]] in [[Saint Petersburg]], Russia]]
[[File:Stockholm 8721-B (9870833193).jpg|thumb|[[Stockholm]] in Sweden]]
[[File:Riga Petrikirche Blick vom Turm 8.JPG|thumb|[[Riga]] in Latvia]]
[[File:Paasitorni by the sea.jpg|thumb|[[Helsinki]] in Finland]]
[[File:Calle Dlugie Pobrzeze, Gdansk, Polonia, 2013-05-20, DD 07.jpg|thumb|[[Gdańsk]] in Poland]]
[[File:Таллин. - panoramio.jpg|thumb|[[Tallinn]] in Estonia]]
'''The biggest coastal cities (by population):'''
{{colbegin|colwidth=30em}}
* [[Saint Petersburg]] (Russia) 5,392,992 (metropolitan area 6,000,000)
* [[Stockholm]] (Sweden) 962,154 (metropolitan area 2,315,612)
* [[Helsinki]] (Finland) 665,558 (metropolitan area 1,559,558)
* [[Riga]] (Latvia) 614,618 (metropolitan area 1,070,00)
* [[Gdańsk]] (Poland) 462,700 ([[Tricity, Poland|metropolitan area]] 1,041,000)
* [[Tallinn]] (Estonia) 458,398 (metropolitan area 542,983)
* [[Kaliningrad]] (Russia) 431,500
* [[Szczecin]] (Poland) 413,600 (metropolitan area 778,000)
* [[Espoo]] (Finland) 306,792 (part of Helsinki metropolitan area)
* [[Gdynia]] (Poland) 255,600 ([[Tricity, Poland|metropolitan area]] 1,041,000)
* [[Kiel]] (Germany) 247,000<ref>[http://www.kiel.de/rathaus/statistik/statistische_berichte/statistische_kurzinformationen/2012/Kurzinfo_Nr._151_-_Amtliche_Einwohnerzahl_im_Dezember_2011.pdf Statistische Kurzinformation] {{webarchive|url=https://web.archive.org/web/20121111143033/http://www.kiel.de/rathaus/statistik/statistische_berichte/statistische_kurzinformationen/2012/Kurzinfo_Nr._151_-_Amtliche_Einwohnerzahl_im_Dezember_2011.pdf |date=11 November 2012 }} (in German). Landeshauptstadt Kiel. Amt für Kommunikation, Standortmarketing und Wirtschaftsfragen Abteilung Statistik. Retrieved on 11 October 2012.</ref>
* [[Lübeck]] (Germany) 216,100
* [[Rostock]] (Germany) 212,700
* [[Klaipėda]] (Lithuania) 194,400
* [[Oulu]] (Finland) 191,050
* [[Turku]] (Finland) 180,350
{{colend}}
'''Other important ports:'''
{{colbegin|colwidth=15em}}
* ''Estonia:''
** [[Pärnu]] 44,568
** [[Maardu]] 16,570
** [[Sillamäe]] 16,567
* ''Finland:''
** [[Pori]] 83,272
** [[Kotka]] 54,887
** [[Kokkola]] 46,809
** [[Port of Naantali]] 18,789
** [[Mariehamn]] 11,372
** [[Hanko, Finland|Hanko]] 9,270
* ''Germany:''
** [[Flensburg]] 94,000
** [[Stralsund]] 58,000
** [[Greifswald]] 55,000
** [[Wismar]] 44,000
** [[Eckernförde]] 22,000
** [[Neustadt in Holstein]] 16,000
** [[Wolgast]] 12,000
** [[Sassnitz]] 10,000
* ''Latvia:''
** [[Liepāja]] 85,000
** [[Ventspils]] 44,000
* ''Lithuania:''
** [[Palanga]] 17,000
* ''Poland:''
** [[Kołobrzeg]] 44,800
** [[Świnoujście]] 41,500
** [[Police, West Pomeranian Voivodeship|Police]] 34,284
** [[Władysławowo]] 15,000
** [[Darłowo]] 14,000
* ''Russia:''
** [[Vyborg]] 79,962
** [[Baltiysk]] 34,000
* ''Sweden:''
** [[Norrköping]] 144,932
** [[Gävle]] 103,619
** [[Trelleborg]] 30,818
** [[Karlshamn]] 19,000
** [[Oxelösund]] 11,000
{{colend}}