Studies of how natural decadal and multi-decadal climatic fluctuations have affected marine ecosystems (e.g. Ottersen and Stenseth, 2001; Titov, 2001; Boitsov and Orlova, 2004; Titov and Ozhigin, 2005; Drinkwater, 2006), have provided insight into what can be expected given the suggested continued warming of the Barents Sea (Ellingsen et al., 2008). Historically, an increase in temperature of only 2 ºC has been documented to have significant impacts on oceanographic features (frontal zones, salt and heat budgets, thermohaline circulation) that drive ecosystem structure and function (Renaud et al., 2008).
Changes in these drivers are already apparent; but it is difficult at present to separate natural fluctuations from human induced climatic changes.
It is expected that thinning of the annual sea ice will continue. A marked increase in the melting of sea ice during summer will result in an increased width of the area with seasonal ice cover (Ellingsen et al., 2008). Thus, the area covered by annually formed ice will reach farther into the Arctic Ocean (Carmack and Wassmann, 2006). Reduced sea ice cover and thickness combined with a prolonged ice free period may increase primary production (Ellingsen et al., 2008), and support an increased biomass of benthos in the eastern and northern parts of the Barents Sea (Cochrane et al., 2009). A complicating factor when predicting how primary productivity will respond to a warmer climate is that warming of the water column and the associated increase in melting of sea ice may lead to increased stratification of the water column, thus reducing supply rates of nutrient because mixing of nutrient rich deepwater layers with layers higher in the water column where primary production occurs is reduced. Expansion of the area covered with seasonal ice will nevertheless increase the biological production associated with the marginal ice zone (Carmack and Wassmann, 2006). Specifically, nutrient-limited diatom blooms that follow ice melting and stratification of the water masses in the summer (Wassmann et al., 1999; Falk-Petersen et al., 2000) will be positively impacted. These blooms support high densities of suspension-feeding zooplankton and large aggregations of forage fish (i.e. capelin) as well as top predators such as cod, polar bears, whales, seals and seabirds. However, reduction in the extent of sea ice will have negative impacts on ice-associated flora and fauna. Of special concern is the expected negative impacts on several ice-dependent mammal species which have already been severely reduced by human over-harvesting (Kovacs and Lydersen, 2008; Wiig et al., 2008).
The biomass of zooplankton in the Barents Sea is thought to be linked to climate-forced transport of warm Atlantic Water from the Norwegian Sea (Skjoldal et al., 1992; Boitsov and Orlova, 2004). Climate warming might increase the advection of zooplankton. However, recent studies of the zooplankton-advection relation (Ellingsen et al., 2008; Stenevik and Sundby, 2007; Tande et al., 2000; Dalpadado et al., 2003; Loeng and Drinkwater, 2007) show that this is a complex process that needs more study.
Recruitment of herring and cod has been shown to be positively related to sea temperature. Higher than normal sea temperatures in the Norwegian and Barents Seas increase the survival of larvae and juveniles, and thus the chance of producing strong year-classes (Ottersen and Sundby, 1995; Toresen and Østvedt, 2000; Klyashtorin et al., 2009). The mechanism behind this relationship is not completely known, but it is probably related to increased abundance of food for the fish larvae during warm years (Ottersen and Stenseth, 2001). It is thus reasonable to expect that increased sea temperature (within limits) will result in higher abundance of juvenile NSS herring and NEA cod in the Barents Sea (Loeng and Drinkwater, 2007). In the 2000s, the recruitment of these species has been less variable than in previous years; this might be related both to high spawning stock levels and high sea temperatures.
Possible consequences of global climate change for the fishing industry exploiting cod and capelin stocks in the Barents Sea have been discussed in some recent scientific publications. According to Titov and Ozhigin (2005) and Titov et al. (2006), the Barents Sea ecosystem will be dominated by the boreal oceanic system, the range of climate variations will be reduced and the cyclic ecological succession will be limited by the two "late" phases characterized by weak cold advection and low ice coverage. In this case no strong capelin year classes can be expected, and the abundance of cod year classes will probably fluctuate between middle and low levels. However, if there will be drastic changes in the Barents Sea ecosystem, involving considerable changes in fish species composition and distribution as well as changes in migration patterns of commercial stocks, this pessimistic scenario may not come true.
It should be noted that 0-group fish may play a significant role in the ecosystem, both as predators and as prey. In years with high abundance, the biomass of the most abundant species may add up to more than 1 million tonnes. Given the high consumption per body weight, the prey consumption by 0-group fish can be significant compared to the consumption by pelagic fish, particularly in the southern and central areas where little capelin is found. This suggests that keeping high spawning stocks may have a positive effect on the ecosystem even though the gain in fish recruitment may be limited compared to at intermediate spawning stock sizes.
Warm periods in the North Atlantic have been associated with rapid northward displacements in the distribution of fish (Drinkwater, 2006) and invertebrates (Renaud et al., 2008). During the period of arctic warming (1930-1950), Atlantic species uncommon to the Barents Sea were found in the region (Zenkevish, 1963). In recent years, distributional changes associated with a warmer climate, have already taken place (see e.g. Sundby and Nakken, 2008).
Based on experience from the warm period in the Barents Sea during the 1920s and 1930s (see Drinkwater, 2006), it can be expected that the major fish species will continue to expand north and east in the Barents Sea. This includes NEA cod, NEA haddock (Melanogrammus aeglefinus), NSS herring and capelin. Major spawning areas for NEA cod will move northwards along the Norwegian coast from the Lofoten area to Troms and Finmark (Sundby and Nakken, 2008). Blue whiting (Micromesistius poutassou), a boreal species, occurred in the Barents Sea in large quantities in 2001-2007, but the abundance of this species has now returned to a low level. This “outbreak” was probably related both to a large stock and high sea temperatures. One could also expect that other boreal species such as mackerel (Scomber scombrus) and grey gurnard (Chelidonichthys gurnardu) will appear more regularly in the western and southern part of the Barents Sea (Yaragina and Dolgov, 2009). So far, however, the mackerel has extended its distribution northwards in the Norwegian Sea rather than moving into the Barents Sea (ICES 2008a and references therein). Benthic taxa characteristic of arctic shelve seas may be displaced northward by advancing boreal taxa, and left with few refugia north of the Arctic Ocean shelf break (Renaud et al., 2008).
In the North Sea and adjacent shelf areas, warming has been associated with a change in plankton communities from cold to warm water species (Beaugrand et al., 2002). Similar changes can be expected in the Barents Sea where arctic species might be replaced by more boreal species. The plastic life-histories of the Calanus species are expected to change as a response to warming (cf. chapter General background description of the ecosystem - Human activities /impact - Oil and gas activities). Such changes might have large impacts on the “match” with the phytoplankton bloom, and with spawning of major fish stocks, particularly those whose smallest life stages depend on Calanus for food (see Edwards and Richardson, 2004).
Climate change also increases the pollution loads to the Barents Sea due to increased precipitation, increased run-off from land and changes in the atmospheric transport of contaminants. The observed trend with a steadily decreasing input of organic pollutants during the last decade may thus be broken and the increased concentrations of POPs like PAH and HCB may be the first sign of a climate induced change in long range transport of air-borne pollutants (see also chapter Current and expected state of the ecosystem - Human activities /impact - Pollution). Changes in water temperatures, ice cover and ocean chemistry (acidification) will also most likely affect degradation processes and uptake of contaminants in biota. However, all direct and indirect effects of climate change can at present not be assessed and the net influence of climate change on contaminant levels cannot therefore be easily predicted.
The current assessment of climate change suggests that warmer temperatures, changes in precipitation and shifts in the presence of snow, ice and water may affect transport of radioactive substances and their routes in the marine environment. For example, movement both into an out of the Barents Sea may become more rapid than today (AMAP, 2009). We may also expect remobilization of radionuclides, including re-suspension and transfer of contaminated sediments from localised sites to the surrounding areas (for example from Chernaya Bay to the Barents Sea). Changes in temperature may also lead to changes in turnover rates of contaminants in cold-blooded animals such as fish.