4. Physical and socio-economic environment
4.3 Changes in the ocean and sea ice
4.3.1
Importance of the Barents Sea
The Barents Sea constitutes about 10% of the Arctic Ocean by area and has a mean depth of only 230 m. Despite this limited volume it influences a much larger region. It is favorably located for exchanges of heat between the ocean and atmosphere because it occupies a key position in one of the main gateways to the Arctic Ocean. The Barents Sea dominates the Arctic heat budget and has the strongest ocean-air heat exchanges in the Arctic (Häkakinen and Cavalieri, 1989). It is an important production area for dense water (Ivanov et al., 2004), which leaves the Barents Sea through relatively deep channels and sinks into the Arctic Ocean, thus contributing to the global thermohaline circulation. The sea ice cover is seasonal over the major part of the Barents Sea; in summer the area is ice free except for the very northern margin, whereas in winter, the marginal ice zone (the transition zone between open water and consolidated ice cover) is located north of the polar front (Smedsrud et al., 2013).
The state of the upper ocean is crucially important for weather and climate in the surrounding area. The ocean and atmosphere are continuously interacting, through the exchange of momentum, moisture and heat (Bintanja and Selten, 2014).
Surface waves are the most visible effect of this interaction in the ocean. The properties of the upper mixed layer are a less visible but no less important an outcome since they determine the marine biota. Changes in sea water temperature are reflected in air temperature (for example, the warming effect of the ocean elevates mean winter temperature on Spitsbergen by about 10°C relative to the zonal average at the same latitude), cloudiness and precipitation.
In the Arctic, the ocean-atmosphere interaction is strongly mediated by sea ice, where sea ice properties (concentration and thickness) determine the strength of energy fluxes. In winter, the upward heat flux from the open ocean is about two orders of magnitude higher than through the pack ice (Smith et al., 1990). The area of open water determines fetch length and wave height. Sea ice also influences the underlying water column.
In spring and summer, sea ice controls the amount of solar radiation absorbed, thus limiting warming in ice-covered areas, and ice melt contributes to the freshwater balance. Occasional opening of polynyas (compact ice-free zones in consolidated ice cover) in winter may trigger instant convective mixing of the water column to substantial depths (or cascading of dense water from the shelf) leading to ventilation of deep layers and upwelling of nutrients to surface waters (Marshall and Schott, 1999; Ivanov et al., 2004). Sea ice also provides a habitat for various Arctic species, including plankton, seals,
and polar bears among others, and in the Barents Sea forms an important element of the marine ecosystem. Water temperature and salinity (basic seawater properties) affect the functioning of ecosystems, directly or indirectly through secondary effects such as density stratification and light transmission. Ocean waters are in constant motion, due to winds and tides. This motion drives constant renewal of the water at any given location. Any change in currents or tidal features may in turn affect water properties and nutrient transport, with potential for impacts on the marine ecosystem.
The Barents Sea supports various industrial sectors, including those important for local communities as well as those important for the wider Barents area (such as international shipping routes connecting Europe and Asia, see Danilov et al., 2014). Changes in oceanic and ice conditions in the Barents Sea are likely to have socio-economic consequences, locally and in distant regions. Projections for the future include continued warming and declining sea ice. Less sea ice leads to greater heat release to the atmosphere and reduced vertical stability, as well as a shift in the large-scale atmospheric circulation pattern over Europe in winter (Christensen et al., 2015). Models suggest further reductions in sea ice in the Barents and Kara seas may bring colder winter temperatures in Europe. Recent model simulations suggest that the North Atlantic Oscillation (NAO;
the dominant mode of near-surface pressure variability over the North Atlantic and neighboring land masses) is highly sensitive to the location of sea ice loss, and that its northern center of action shifts westward or eastward depending on whether the sea ice loss occurs in the Atlantic or Pacific sectors of the Arctic (Pedersen et al., 2016).
4.3.2
Past trends and future projections
The Barents Sea is one of the Arctic regions with the greatest sea ice variations (Deser et al., 2000; Francis and Hunter, 2007).
About 50% of the Barents Sea is ice-covered in winter, but most of it is open sea during summer. Daily to annual sea ice variations are mainly caused by variations in wind strength and direction (Kimura and Wakatsuchi, 2001; Kwok et al., 2005; Koenigk et al., 2009). Anomalously northerly winds transport more and thicker ice from the central Arctic into the Barents Sea and further south, mainly through the section between Svalbard and Franz Josef Land. In contrast, southerly winds prevent ice transport southward while simultaneously moving warmer air and water masses from the Atlantic into the Barents Sea, preventing sea ice formation (Sandø et al., 2014b; Sato et al., 2014). The Arctic sea ice cover is influenced by the northward ocean heat transport in the Norwegian Sea (e.g. Sandø et al., 2010; Smedsrud et al., 2010), and the ocean heat transport through Fram Strait and the Barents Sea Opening plays an important role in sea ice variability in the Barents Sea over annual time scales (Schlichtholz, 2011;
Årthun et al., 2012; Nakanowatari et al., 2014; Ivanov et al., 2016) and longer (Koenigk et al., 2009; Alekseev et al., 2015). The observed sea ice decline in the Barents Sea has occurred at the same time as an observed increase in Atlantic heat transport due to both strengthening and warming of the inflow (Årthun et al., 2012). During winter, the ice margin has shifted towards the north and east (Årthun and Schrum, 2010). Autumn sea ice variations and reductions in the Barents Sea have been linked to the North Atlantic Circulation in the following winter, and to
temperature and snowfall extremes over parts of Europe and Asia (Petoukhov and Semenov, 2010; Hopsch et al., 2012; Yang and Christensen, 2012; Liptak and Strong, 2014; Mori et al., 2014).
Observations provide clear evidence of change in Arctic sea ice. First-year sea ice extent decreased by 3.5–4.1% per decade over the period 1979–2012, with the most pronounced reduction occurring during summer at 9.4–13.6% per decade (equivalent to a loss of 0.73–1.07 million km2 per decade), and was 11–16% per decade for multi-year sea ice (Vaughan et al., 2013). In the Barents Sea, observations reveal that ice extent in the ‘cold’ 1965–1975 period reached on average 180,000 km2 in August, while in the ‘warm’ 2001–2012 period ice extent was considerably less at 46,000 km2 (Roshydromet, 2014).
The monthly ice cover anomaly in the Barents Sea reveals a linear decrease of ~7% per decade over the period 1979–2007, but significant interannual variability (Comiso and Nishio, 2008). Submarine data and satellite measurements show mean Arctic sea ice thickness decreased from 3.64 to 1.89 m over the period 1980–2008 (Rothrock et al., 2008; Kwok and Rothrock, 2009). Observations over recent decades show a strong reduction in sea ice volume in the Arctic (Döscher and Vihma, 2014), attributed to increased greenhouse gas concentrations and increased northward ocean heat transport into the Barents Sea (Skagseth et al., 2008; Levitus et al., 2009).
Sea-ice loss has many consequences in the underlying ocean and the overlaying atmosphere. For example, ice decline in winter increases the exposure of relatively warm open water to cold air outbreaks, which in turn leads to stronger turbulent convection in the atmospheric boundary layer. Sea ice may provide a link between changes in the ocean and in atmospheric circulation (Nakanowatari et al., 2014; Sato et al., 2014), and stronger convection will increase boundary layer thickness and cloudiness, which may generate extreme snowfall and unusually strong winds (Tetzlaff et al., 2014).
The physical environment of the Barents Sea is influenced by climate change in terms of changes in sea level, salinity, temperature, and thereby also changes in sea-ice extent and thickness. Changes in temperature and salinity are likely to cause changes in vertical stratification, which has implications for vertical exchange, water chemistry and the
biota. Oceanographic conditions are strongly determined by advection (horizontal movement of mass, heat and salt) and by exchange with the atmosphere (precipitation, evaporation, air-ocean energy fluxes).
Observed trends are likely to continue or strengthen in the future, and recent climate model simulations (CMIP5; IPCC, 2013) suggest the Barents Sea will be the first Arctic region ice-free all year round. An evaluation of how well the most recent GCMs capture past trends suggests there is a tendency for models to slightly overestimate sea-ice extent in the Arctic (by about 10%) in winter and spring (Flato et al., 2013). Projections indicate that surface air temperature in the Barents Sea and Arctic Ocean will increase by about twice as much as the global mean, with accompanying decreases in sea-ice extent (IPCC, 2013). The air-ocean heat fluxes will thus show considerable change, principally in response to a warmer ocean due to increased uptake of solar heat following the decline in ice cover and increased heat transport into the region. The Barents Sea will be ice-free all year round by mid-century according to many climate models, and recent analyses of future projections suggest that increased oceanic heat transport will be a major contributory factor to sea-ice decline in this area (Koenigk and Brodeau, 2014).
The latest assessment by the Intergovernmental Panel on Climate Change (AR5) confirms the findings of its previous assessment (IPCC, 2007) in terms of change in Arctic sea-ice extent to the end of the century, despite a wide spread in model results. The rate of decrease in mean sea-ice cover is greatest in September, but there are major differences in the multi-model averages depending on RCP used. The projected decline in sea-ice extent ranges from 8% (RCP2.6) to 34% (RCP8.5) in February and 43% (RCP2.6) to 94% (RCP8.5) in September (Collins et al., 2013). Due to a substantial reduction in sea-ice thickness, the corresponding losses in sea sea-ice volume are expected to be much higher.
Regional effects of climate change can be heavily modulated by internal variability and may either mitigate or worsen the impacts of global warming. Interannual variability in sea-ice extent is largely determined by the inflow of relatively warm Atlantic Water through the Barents Sea Opening (Sandø et al., 2010;
Årthun et al., 2012, Smedsrud et al., 2013, Nakanowatari et al., Figure 4.12 Change in sea surface temperature in March for downscaled GISS (left), NCAR (middle) and NorESM (right). The left and middle plots are from Sandø et al. (2014a) and show change between present (1981–2000, data from the 20C3M control run) and future (2046–2065, A1B scenario). The right plots shows change between 2010–2019 and 2060–2069 using the RCP4.5 scenario (Bentsen et al., 2013).
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Change in SST, °C
GISS NCAR NorESM
2014; Sandø et al., 2014a,b), but observations show none of the CMIP5 GCMs are able to simulate suffi cient infl ow of heat through this region (Sandø et al., 2014a). Results from ocean-downscaling of two CMIP3 control climate simulations (20C3M) were analyzed by Melsom et al. (2009) and Sandø et al. (2014a).
Th ey found that sea ice and hydrographic conditions in the Barents Sea are reproduced well in the downscaling experiments, despite large regional biases in the GCMs used for boundary conditions. Th is improvement is attributed to a more realistic ocean circulation and infl ow of Atlantic Water in the Barents Sea Opening due to higher resolution in the regional models.
Can similar improvements be expected if the future scenarios from the GCMs are downscaled? Comparing the downscaled CMIP3 GCMs for the A1B scenario shows relatively good agreement for future temperature rise, but large diff erences for future salinity (Sandø et al., 2014a). Th ese diff erences were attributed to deviations in the GCMs that were transferred to
the regional models through initial and boundary conditions.
Diff erences in the representation of the hydrological cycle in the GCM simulations lead to large diff erences in the ocean salt budget that regional downscaling cannot change much. Th e ocean is too inert and the impact of the GCM results from the initial and boundary conditions is too large. So, despite improvements due to increased resolution in regional models, unrealistic biases in the global model projections will infl uence the fi nal regional results. Figure 4.12 shows the projected change in sea-surface temperature from the downscaled NorESM4.5 model. Th e downscaled RCP4.5 results from GISS-AOM, NCAR-CCSM, and NorESM show the greatest temperature increase in the Barents Sea will occur in March, which diff ers from the rest of the Arctic Ocean. Like the two downscaling studies reported by Sandø et al. (2014a), this model also shows a warming of 1–2°C for March in most of the Barents Sea. Th is warming is refl ected in the sea-ice extent data shown in Figure 4.13, where reductions relative to the present ice concentration can be seen Figure 4.13 Change in sea ice concentration and thickness in March. Downscaled GISS sea ice concentration (upper left ), NCAR sea ice concentration (upper right), NorESM sea ice concentration (lower left ), and NorESM sea ice thickness (lower right). Th e upper plots are from Sandø et al. (2014a) and show change between present (1981–2000, data from the 20C3M control run) and future (2046–2065, A1B scenario). In the lower plots, change is between 2010–2019 and 2060–2069 using the RCP4.5 scenario (Bentsen et al., 2013).
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Change in sea-ice concentration
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Change in sea-ice concentration
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Change in sea-ice thickness, m
in the central and northern parts of the Barents Sea. Th ese results support the idea of oceanic heat transport having a critical role in sea-ice decay in this region. Th e decreasing trend in average sea-ice thickness for the 50-year period 2010–2070 (Figure 4.13) is less than observed from 1980 to 2008. Th is may be due to natural variability, which is typically stronger on a decadal scale than a multi-decadal scale.
4.3.3
Water temperature and salinity
A continued northward shift in the ice edge in the Barents Sea will aff ect thermohaline properties in the upper mixed layer.
Prolonged exposure of the open sea surface to the atmosphere will lead to a substantial increase in the uptake of short-wave solar radiation and consequent warming of surface waters (Sandø et al., 2010). Together with reduced surface salinity, as indicated by the downscaled NorESM results the warming strengthens density stratifi cation over most of the Barents Sea (Figure 4.14). Th e excess freshwater input at the surface is due to increased high latitude precipitation, as projected in almost all CMIP5 models (Collins et al., 2013).
Th e signifi cance of the large-scale infl ow of warm and saline Atlantic-origin water in shaping thermohaline conditions in the Barents Sea is well-established (Smedsrud et al., 2013; Sandø et al., 2014b). Under conditions of gradually shrinking ice cover, the eff ect of Atlantic-origin water infl ow is expected to strengthen and extend further east, since cooling and freshening of the Atlantic-origin water en route will slow, as there will be less ice to melt. Signs to support this idea have recently been reported (Årthun et al., 2012; Dmitrenko et al., 2015). Warmer and saltier Atlantic-origin water further to the north-east in the ice-free Barents Sea will provide more heat for release to the atmosphere in winter, and the associated heat loss will increase water density and favor the development of deeper convection. Th is phenomenon results in the formation of a well-ventilated water column and enhances nutrient transport to the surface waters.
In shallow waters, convection may extend to the seabed, providing the prerequisite conditions for cascading (down-slope gravity-driven current), which transports dense water
from the shelf to the deep ocean (Shapiro et al., 2003). Dense water formation on the shallow banks of the Barents Sea and western shelf of Novaya Zemlya Archipelago is well-documented (Midttun, 1985; Ivanov et al., 2004). As long as ice forms on the shallow shelves in winter, dense water formation will continue and may even increase (Bitz et al., 2006; Ivanov and Watanabe, 2013; Moat et al., 2014). One of the reasons for this is eff ective salinization of cold shallow water near the marginal ice zone, as described by Ivanov and Shapiro (2005).
Later, however, together with declining sea ice formation in winter, bottom water formation in the Barents Sea is expected to slow, both in terms of open ocean convection and cascading (e.g. Årthun and Schrum, 2010). Recent (2008) measurements confi rm that the density of Atlantic-origin water in the Barents Sea and bottom water in St Anna Trough (through which dense water enters Nansen Basin) have remained higher than those measured in the 1990s (Lien and Trofi mov, 2013), potentially indicating greater dense water formation in the ice-depleted conditions of the 2000s.
Th e existence of large-scale open water area in winter caused by increased infl ux of Atlantic-origin water, might also impact on the atmosphere both locally and remotely. For the hypothetical case of a totally ice-free Arctic Ocean in winter, simple calculations by Newson (1973) suggest that weakening of the meridional temperature gradient would lead to a weakening of westerly winds, atmospheric blocking and general cooling in the mid-latitudes. Th e veracity of this foresight was recently confi rmed by more sophisticated model studies (Petoukhov and Semenov, 2010; Hopsch et al., 2012; Yang and Christensen, 2012; Liptak and Strong, 2014; Mori et al., 2014).
4.3.4
Sea level and surface waves
Sea level is the combined result of many factors and local sea level will be aff ected diff erently depending on location. Th ese factors include melting ice over land (but not melting sea ice), thermal expansion as water warms, prevailing winds, distribution of land and ice masses, and the shape of the ocean basin. Land rebound following the disappearance of the Fennoscandian ice sheet is
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Change in salinity Change in seawater density, kg/m3
Figure 4.14 Change in salinity (left ) and stratifi cation (right) in the upper 50 m in March based on downscaled NorESM data. Present (2010-2019) and future (2060-2069) using the RCP4.5 scenario.
also a major factor in the Baltic Sea region. Simpson et al. (2015) observed that relative sea level projections can differ by as much as 0.50 m from place to place depending on vertical uplift rates.
Analysis of changes in local sea level must take into account the glacial isostatic effect. The adjustment-corrected rate from Arctic tide gauges for the period 1993–2014 varies along the Norwegian mainland: Vardø (2.7±1.6 mm/y), Honningsvåg (2.9±1.6 mm/y), Hammerfest (3.8±1.7 mm/y), Tromsø (3.7±1.8 mm/y), Andenes (3.7±1.7 mm/y), Harstad (3.4±1.7 mm/y), Kabelvåg (4.0±1.8 mm/y), and Bodø (3.3±2.0 mm/y).
Future wave conditions in the Barents Sea will depend on surface wind and ice conditions, and the open sea is subject to strong wind fetch (Lynch et al., 2008). Based on model simulations for the 21st century, Khon et al. (2014) reported a significant increase in wave height across the Arctic due to reduced sea-ice cover and stronger regional winds. An opposite tendency, a slight reduction in wave height, may appear over the Atlantic sector and Barents Sea. Rutgersson et al.
(2015) found no trend in wind statistics, but pronounced decadal variations.
An important implication of stronger wave-induced vertical mixing under ice-free conditions is a deepening of the upper mixed layer and a rise in salinity due to the influx of deeper more-saline water (Kraus and Turner, 1967). This additional salt flux from below may partly compensate for the additional freshwater input through increased precipitation. This could result in a spatially intermittent weakening of vertical density stratification accompanied by localized winter convection rather than massive overturning events.
4.3.5
Ocean acidification
Many marine species incorporate calcium carbonates into their body armor (shells, exoskeletons, claws). Ocean acidification leads to less favorable conditions for the formation of these mineral-based features. Currently, surface waters are generally supersaturated with respect to calcium carbonates, but saturation state decreases when more CO2 is dissolved in the water. Understanding how saturation state could change with respect to these minerals is therefore
Many marine species incorporate calcium carbonates into their body armor (shells, exoskeletons, claws). Ocean acidification leads to less favorable conditions for the formation of these mineral-based features. Currently, surface waters are generally supersaturated with respect to calcium carbonates, but saturation state decreases when more CO2 is dissolved in the water. Understanding how saturation state could change with respect to these minerals is therefore