Underwater noise

Within the Baltic Sea, the underwater noise environment comprises ambient noise (i.e. sound from rain falling on the surface, waves, marine animals, etc.) that ranges in frequency between approximately 50 Hz and 200 Hz, and noise from distinct and identifiable anthropogenic sources (i.e. sound from shipping, mechanical installations, construction activities, etc.). The noise generated from these sources comes from all directions and varies in magnitude, frequency, location and time. However, it is estimated to dominate at a frequency between 10 Hz and 100 Hz /94/.

The sound pressure level (SPL) of underwater sources varies. Generally, lightning strikes, seismic eruptions and underwater explosions are considered to be some of the loudest sound sources and generate SPLs of 260-280 dB re 1 μPa at 1 m (decibels, sound intensity level relative to 1 microPascal at 1 m). Loud ships can also generate SPLs of up to 190 dB re 1 μPa at 1 m.

Sound sources can also be biological; dolphins have been known to produce SPLs of approximately

230 dB re 1 μPa at 1 m, whilst cod, when they grunt, produce an SPL of approximately 150 dB re 1 μPa at 1 m /94/. Quieter sound sources include wind and rain, which generate SPLs of 40-90 dB re 1 μPa.

As part of an ongoing project to study the influence of anthropogenic noise on the Baltic Sea (Baltic Sea Information on the Acoustic Soundscape (BIAS) project), a series of measurements were undertaken over one year (2014) at 38 locations covering the whole Baltic Sea (except the German landfall area). The results of these measurements have been extracted using the BIAS soundscape planning tool and are shown in Figure 9-9 /94/.

In general, the noise levels within the main shipping lanes were approximately 100-130 dB re 1μPa, whilst levels outside the shipping lanes ranged between approximately 60-90 dB re 1μPa. Underwater noise monitoring in Germany during construction of NSP in 2010 revealed mean SPLs of 112 dB re 1 μPa at 1 m for shipping lanes and 102 dB re 1 μPa at 1 m for remote parts of Greifswalder Bodden and Pomeranian Bay, respectively /95/. Most of the Baltic marine area is impacted at least by a level of noise that has been estimated to mask the communication of animals. Noise levels causing an avoidance reaction in mobile organisms are likely to occur only in areas with construction works, such as between Helsinki and Tallinn (resulting from cable construction) and at wind farm construction sites, e.g. in Kemi in the Bothnian Bay and Malmö in the Sound /96/.

Figure 9-9 Underwater soundscape map of noise in the Baltic Sea measured during June 2014 under the BIAS project. Centred frequency 125 Hz third octave band, depth interval 0 m to bottom. Exceeded sound level L10 (10% of time). These results have been extracted using the BIAS soundscape planning tool, which was prepared within the EU LIFE project /97/.

Climate and air quality 9.2.3

9.2.3.1 Climate

Current climate

Meteorological forces over the sea, together with hydrographical processes, have a strong influence on the environmental conditions of the Baltic Sea. These processes influence the water temperature and ice conditions, regional river run-off and the atmospheric deposition of contaminants on the sea surface. Moreover, they also govern water exchange with the North Sea and between the sub-basins, as well as the transport and mixing of water within the various sub-regions of the Baltic marine area /90/.

The Baltic Sea is located in the temperate climate zone, which is characterised by large seasonal contrasts. The climate is influenced by major air pressure systems, particularly the North Atlantic Oscillation during winter, which affects the atmospheric circulation and precipitation in the Baltic Sea basin.

The near-surface wind climate exerts a strong impact on the ecosystem of the Baltic Sea. Storms are essential for the ventilation and mixing of the strongly stratified Baltic Sea, and inflow events importing salt and oxygen from the North Sea are very dependent on the wind climate and pressure differences between these two seas.

Surface air temperatures have shown an overall increase in the Baltic Sea region over the past 140 years. Since 1871, the annual mean temperature trends show an increase of 0.11°C per decade north of 60°N and 0.08°C south of 60°N, while the trend of the global mean temperature was about 0.05°C per decade for the period 1861-2000. The daily temperature cycle is also changing, and there has been an increase in temperature extremes. These changes are resulting in seasonal changes, e.g. the length of the growing season has increased and the length of the cold season has decreased /98/.

The amount of precipitation in the Baltic Sea area during the past century has varied between regions and seasons, with both increasing and decreasing precipitation. A tendency of increasing precipitation in winter and spring has been detected during the second half of the 20th century /98/.

In the Baltic Sea, ice can appear as fast ice or as drift ice. Fast ice is smooth and stationary and can be attached to islands, islets and shallow reefs. Fast ice usually appears at a water depth of up to 15 m /99/, /100/. In deeper waters in the open sea, ice is more dynamically formed, consisting of drift ice that moves along with the currents and winds. On stormy days, drift ice can move 20-30 km. Drift ice and deformed ice can easily get packed against each other or other obstacles, which can result in pack ice or in vast ice ridges /99/, /100/. In shallow areas, packing of drift ice can result in ice packs that grow vertically downwards to the sea bottom. This kind of seabed-attached pack ice has been observed down to water depths of 20 m /99/.

In Atlas Map CL-01-Espoo, the maximum ice cover is shown for a severe winter (2010-2011), an average winter (2012-2013) and a mild winter (2014-2015). As would be expected, the most severe ice conditions prevail in the most north-eastern part of the Baltic Sea, i.e. in the Gulf of Finland.

Future climate

NSP2 has been designed for an operational life of at least 50 years. The purpose of this section is to describe how projected global climate change can be expected to affect the Baltic Sea region during this time.

Surface waters in the Baltic Sea have warmed since 1985, where the annual mean sea-surface temperature has increased by up to 1°C per decade from 1990 to 2008. At the same time, the annual maximum ice extent of the Baltic Sea has decreased about 20% over the past 100 years, and the length of the ice season has decreased by approximately 18 days per century in the Bothnian Bay and 41 days per century in the eastern Gulf of Finland /98/.

An oceanographic study carried out by the Swedish Meteorological and Hydrological Institute (SMHI) showed that average sea surface temperatures for the entire Baltic Sea could increase by some 2-4ºC by the end of the 21st century /101/ (see Atlas Map CL-02-Espoo). This is estimated to decrease the ice extent in the Baltic Sea by 50%-80%. The average duration of ice cover for the period 1961-1990 is shown together with the expected duration of ice cover at the end of the 21st century in Atlas Map CL-03-Espoo.

Increased freshwater inflow and increased mean wind speeds may cause the Baltic Sea to reach a new steady state with significantly lower salinity. In the southern Baltic, oxygen concentrations may decrease and phosphate concentrations increase, resulting in increased biomass and cyanobacteria concentrations with a higher cyanobacteria-to-phytoplankton ratio.

A recent report issued by HELCOM largely confirms these findings /98/. It concluded that the summer sea surface temperature is likely to increase 2-4°C by the end of this century, and that there will be a marked decrease in the sea ice cover in the Baltic Sea. Model projections indicate that precipitation will increase in the entire Baltic Sea run-off region during winter, and extremes of precipitation are projected to increase. Atlas Map CL-04-Espoo shows the expected changes in

winter and summer precipitation during the 21st century. A sea level rise of 0.6-1.1 m is expected (see Atlas Map CL-05-Espoo), as well as a decrease in sea surface salinity. Increasing areas of hypoxia and anoxia are anticipated.

The mean and extreme wave heights at the end of the 21st century will probably have increased compared with today. The changes can be expected to be largest in the Bothnian Bay and the Bothnian Sea because of reduced ice coverage, causing unstable marine atmospheric boundary layers with increased surface speed /102/.