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Denmark´s Second National Communication on Climate Change

7.1 Climate change in Denmark

Climate models and climate scenarios

The future climate is projected using climate models. Global climate models are mathematical equations based on physical laws and empirical relationships describing the climate system. State-of-the-art climate models are very complex, and running the models requires considerable computer resources.

Confidence in climate models has increased in recent years. The best models are now able to simulate the most important features of the present climate, including seasonal and large-scale geographical variations. Model resolution is limited by the available computer resources, however, and it is presently not possible with global models to describe regional details with sufficient accuracy. Methods to "interpret" global model results regionally have therefore been developed and used to establish climate scenarios for Denmark based on recent global model experiments.

The "business as usual" scenario

According to the latest findings of the IPCC, the global mean surface temperature is projected to increase by 2 °C by 2100, and sea level will rise 50 cm using the "business as usual" emission scenario (IS92a). For Denmark the annual mean temperature is projected to increase by almost 2 °C by 2050. Precipitation will increase in winter and decrease in summer. A sea level rise of 9 - 18 cm is projected, with the largest rise in the southern part of the country.

By the end of next century the annual mean temperature in Denmark is projected to be 2.8 °C warmer than today. In contrast to earlier estimates, the annual mean precipitation is projected to decrease slightly. Sea level is projected to rise 33 - 46 cm.

Unpleasant surprises such as weakening of the Gulf Stream have not been taken into account.

7.1.1 Impacts and possibilities of adaptation in Denmark

Denmark and the World

Different areas of the World will be differently affected by possible climate changes. Evaluation of the global impact must therefore necessarily be based on individual investigations. Parallel to, and to some extent as a basis for international activities, individual countries therefore carry out national investigations.

Such an investigation has recently been carried out in Denmark based on the above mentioned climate change scenarios.

Denmark only accounts for about 1/1000 of the World population and covers even less of the global land surface. Climatic impacts in Denmark are therefore insignificant in economic evaluations of the seriousness of the total global impact, and in discussions of which degree of intervention is justified. On the other hand, it is imperative to know as much as possible about impacts in Denmark when planning a policy for adaptation to unavoidable impacts. Such information can also be useful in evaluations for other countries.

Human health and well-being

The predicted climate changes will by and large mean that the climate of Denmark will become like the present climate of southern England. In itself this is unlikely to pose any health problems. A certain risk of diseases transferable by insects (malaria) cannot be excluded, and there may be infections as a consequence of increased tourism and immigration. The generally high degree of hygiene in Denmark combined with an expected development of vaccines will probably render this problem insignificant.

The "technical" sectors

In the energy, industry and transport sectors it will be possible to adjust to changing climate as part of normal maintenance and renewal. Moreover, in Denmark these sectors will mainly benefit from a milder climate.

Water resources

Temperature changes per se are not usually decisive, but must be considered in connection with changes in precipitation - not only the total amount - but also the time pattern. Here soil properties and the possibilities of runoff play a significant role. Since the new scenarios for Denmark - in contrast to the previous scenarios - imply a slight reduction in precipitation, problems cannot be excluded.

In a case study, an increase in evaporation of 7% up to year 2100 has been calculated. With a simultaneous reduction in precipitation this results in a reduction in runoff of 40%. The largest effect appears during the summer and will presumably result in a much bigger need for irrigation. During winter and spring, when the temperature and precipitation increase, the soil water deposits will be saturated; this will result in increased evaporation and surface run-off; thereby reducing total formation of groundwater. Runoff from more sandy soils will be dominated by surface runoff and variation in the water flow will increase.

Agriculture

The expected climate changes may lead to a rise in potential yield in Danish agriculture.This will be largest in vegetative crops and unchanged or slightly reduced for most cerial and seed crops. In combination with the fertilisation effect of the increasing carbon dioxide levels, this may lead to an increase in potential yield of 5 - 20% by the middle of the next century. Simultaneously, however, there may be increasing problems with attempts to reduce nitrogen leaching and the use of pesticides.

Forestry

Contrary to agriculture, forestry is characterised by very long production times with a rotation age for our trees of between 50 and 180 years. The trees currently being planted in Denmark, thus need to be suited for the climate at the end of next century. Moreover, there is a risk of increased attack from pests, which will thrive in a milder climate. With the uncertainties inherent in the climate scenarios, the best strategy is to increase the general adaptability of the forests - both by choice of tree species and by treatment of the stand. Norway spruce, which is the most common tree in Denmark, already appears to be threatened by mild winters. It is therefore important to convert the large Norway spruce plantations in Jutland to more stable mixtures with a large fraction of deciduous trees including oak and beach.

Terrestrial ecosystems

Denmark is mostly a cultivated landscape, with extensive management of the terrestrial vegetation. This means that in Denmark, changes in vegetation in the agricultural part of the landscape as a consequence of human impact on climate will presumably be minimal. Cultivated forests, however, may suffer from short-term anthropogenic climate changes. Natural ecosystems such as coastal heaths, bogs, mires and natural forests may be strongly affected by climate change. Consequently, Denmark has to a large extent both the knowledge and the technology required to meet negative impacts. It should, however, be taken into account that large constructions such as motor ways and urban areas can hamper natural adjustment.

Larger uncertainty lies in the future climatic development. Often it is not the average temperature or precipitation that determine the distribution of a species, but rather extremes such as unusually humid or mild winters, or very dry summers. Since the extremes can change in both directions for the different climatic factors, they may affect species with completely different demands. There is therefore a risk of reduction in the variability of our flora and fauna, but which species will suffer cannot be predicted.

Freshwater ecosystems

The expected climatic changes can result in strongly impaired living conditions for small animals and fish in watercourses. Because of the lower flow in the watercourses, they will generally be more vulnerable to impacts from releases of sewage effluent and land reclamation in the catchment area. The deterioration of water quality will be most pronounced in small watercourses, where frequent summer desiccation will dominate, especially on the island part of Denmark.

Watercourse velocity and reaeration will be reduced and as a consequence, the macroinvertebrates will become impoverished with fewer and more specialised species. The living conditions for salmon and trout will generally be impaired due to reduced spawning and growth conditions, reduced physical space and reduced foodstuf availability. An expected general reduction in diffuse nutrient loss to freshwater will counteract the negative effects of the temperature rise on the environment in lakes. Important countermeasures in freshwater ecosystems are: Reduction of loading by nutrients, organic compounds and other xenobiotic compounds; more efficient water resource housekeeping; and restoration of watercourses, river valleys and lakes.

Natural emissions of greenhouse gases

While changes in the atmospheric concentrations of greenhouse gases can give climatic changes, climate changes can in turn influence natural sources of greenhouse gases and thus create a positive or negative feedback.. It has not hitherto been possible to determine the magnitude or even the direction of such effects in Denmark. It is known, though, that the formation of nitrous oxide is directly related to the use of fertiliser in agriculture.

Coasts and coastal areas

Since the end of the last ice age about 10,000 years ago, the Danish coastline has changed markedly as a consequence of relative land settlements and elevations as well as the constant erosion by the sea. A global sea level rise will increase the problems along the Danish coasts.

An assumed sea level rise of 50 cm by the year 2100 (i.e. in the upper end of what can be expected) will result in coastal retreat over and about that already taking place. However, it will be possible to counteract this by coast nourishment, where sand is recovered from deep areas of the the sea floor and dredged onto the shore. The process must be repeated at regular intervals, however, and will therefore constitute an economic load. The safety level for the existing dyke and storm flood protection will be reduced, but the sea level rise will be so slow that there will be ample time to adjust the constructions to the changed conditions. Moreover, there has been a tendency to abandon old dykes protecting marginal areas. In coastal cities the effectiveness of drainage systems will be reduced. Necessary extensive renovation of worn-out parts of the sewage system must take into account the sea level rise.

7.2 Climate change in Greenland

An exception to the general trend in arctic climate change

The overall assumption of the most recent IPCC climate change report (Houghton et al. 1996) is that the on-going increase in atmospheric CO₂ leads to atmospheric warming, most drastic at high latitudes. Arctic and sub-Arctic regions are predicted to experience greater warming than the global average, although Greenland may be an exception as a result of changes in thermohaline circulation in the North Atlantic sea and its generally more maritime climate than other High Arctic regions. Statements in the present Chapter are further discussed in Heide-Jøregnsen & Johnsen (1997).

Climate scenario

Climate change in Greenland is predicted to cause an increase in the mean yearly temperature of between 1.8 - 3.6°C by the end of the next century. The greatest increase is expected at mid- and high latitudes in West Greenland. The increase will primarily be an increase in winter temperatures. A slight diminution of the north-south temperature gradient is expected. The predicted maximum increase in July temperature is about +2°C for Ilulissat (Jakobshavn). The frequency of extreme low temperatures is expected to decrease. Warm extremes may occur more frequently both winter and summer.

The ice cap will respond to warming through increased melt rates at the margins and accumulation rates in the interior. Melt rates will probably dominate. However, precipitation and melt rate predictions are not as reliable as temperature predictions. Precipitation is predicted to increase by 2 - 24 mm per month, with most of the increase in the summer on the south and west coast but in the winter or all year round on the east coast and at high latitudes. Half the change will occur within the next 40 - 50 years. Other consequences of climate change include: lengthening of the snow-free season by a month or more, a slight increase in the length of the growing season by 1 - 2 weeks, a deepening of the soil active layer, and a shorter northward movement of the permafrost boundary.

Uncertainty for Southern Greenland

There is considerable uncertainty regarding predictions for Southern Greenland, which has experienced a cooling of 1 - 1.6°C in the past 60 years. Ocean models predict a cold centre SW of Greenland. The cooling effect around this centre will counteract and may even neutralise greenhouse warming in SEGreenland. This cooling may be related to the 80% reduction in deep water formation observed in the Greenland Sea during the 1980s. Hence, less warm Atlantic water is streaming north. The maximum temperature increase in South Greenland may therefore represent a return to the mean summer temperatures of 60 years ago, and the possibility of a temperature fall must be considered.

Ozone and UV-B

Depletion of the stratospheric ozone layer will continue in the first half of the next century, causing increased UV-B radiation, particularly towards the north, where the effect is increased by snow and ice albedo.

Sea surface temperature

At sea, a decrease in the north-south gradient of sea surface temperatures is expected. Changes in sea level are still unclear, but in Greenland a world-wide increase in sea level will most likely be counterbalanced by a land raise. Reductions in sea ice thickness, surface area and duration are also expected.

7.2.1 Ecological implications

Long-term predictions of Arctic plant performance require a knowledge of the natural variation and dynamics in Greenland ecosystems, as well as a not yet available understanding of feedback mechanisms on element cycling, microclimate, etc. Near-future changes (10 - 20 years) are expected to be modest, particularly in the south, but later the following changes may occur leading to warming:

7.2.2 Changes in patterns of terrestrial ecosystems

New dominant ecosystems

Initial changes at the population level in existing communities and ecosystems are expected to be followed by major changes in community structure, resulting in new types of dominant ecosystems. Disintegration of plant communities in the Arctic will also result in drastic changes for animal population.

Lichens and mosses may become less frequent in heath and wetland ecosystems, and dwarf shrub and shrub vegetation may be favoured at the expense of grasses and herbs. Consequently, graminoid-dominated wetland may become restricted, causing a decline in plant species diversity and the abundance of grazing animals and their predators. Certain dwarf shrubs possess chemical defence against herbivores.

Evergreen perennial plants may respond slowly or not at all with increased biomass and suffer a competitive disadvantage in communities with aggressive deciduous species, such as dwarf birch.

As the summer-winter temperature difference tends to decrease, coastal heath types, dominated by crowberry may expand in coastal regions.

Longer growing season, colonisation and migration

Increased length of the growing season will cause present distribution boundaries for a number of plant species and vegetation types to move northward and to higher altitudes. A direct result of such a migration could be competitive exclusion of northern species by southern species. Animals will experience extended feeding areas and season in more productive high latitudes.

As seed plants play a minor role in the High Arctic, lichens and mosses are expected to be the first to benefit from higher temperatures due to the existence of a dormant propagule bank. More dramatic changes are expected where colonizable bare ground exists, unless water supply is a limiting factor.

Arctic deserts, semi-deserts, and fellfields may be colonised by invading plants and animals, improving living conditions for man. Fast invasion of both barren and occupied areas may occur from existing, milder microhabitats. Species from outside such protected habitats are expected to immigrate much more slowly.

Due to strong physical barriers, it is unlikely that Arctic plants can migrate fast enough to keep up with the speed of climate change. Eventually, however, immigration may lead to increased species diversity (decades, centuries).

Small scale forestry and farming

If south Greenland becomes warmer there may be a potential for small scale forestry and farming with highland cattle as the tree-line moves north and the risk of frost damage decreases. The northward migration of forest may be very slow, slower than in most other Arctic areas, because only a few copses exist in the southernmost, sub-Arctic Greenland. Migration rates for alder and birch are about 130 - 1000 m per year.

Loss of diversity

A loss of species and biodiversity is predicted for the first many decades. Extreme changes in soil moisture and species competition may be direct causes, the latter influenced by nutrient availability, temperature, CO₂, and UV-B.

Endangered plants

Only a few High Arctic plants are in danger of being exterminated as a direct consequence of temperature increase. Ranunculus sabinei, which is currently limited to the narrow outer coastal zone of north Greenland, has nowhere to mitigate to avoid a warmer and drier climate. For individual species, plant or animal, temperature responses will be greatest closest to their climatic distributional limit.

Fossil evidence of the survival of arctic eco- systems

Fossil evidence from the relatively warm Pleistocene shows, first, that High Arctic lowland fens can be restored from openings in boreal forest. Second, the occurrence of arctic species in Pleistocene remains indicates that although the treeline and foresttundra will move northward as warming proceeds, High Arctic ecosystems will not disappear from high latitudes with short growing seasons. In undisturbed fens, establishment of new plants is extremely slow.

UV-B sensitive lichens and plants

Lichens and some arctic mosses and higher plants are sensitive to UV-B radiation. The long-term changes, given an increasing UV-B irradiance trend in the Arctic, may be a reduction in cover and frequency. Interactions between increased CO₂ and UV-B radiation may reduce the nutritional value of plants to herbivores. Data are lacking on UV-B as a threat to skin, vision and immune systems.

Herbivores, birds and insects

Arctic animals depend on stable winter climate with unbroken frost. Increasing snow depth and changed species composition of plant communities in the north may result in mass mortality of musk-ox and caribou. Rising winter temperatures could also be harmful to large herbivores, Arctic hares, and small rodents, such as lemmings, living beneath the snow cover. Repeated freezing and thawing results in ice-crust formation, making it difficult to reach vegetation in winter. Herbivores may therefore experience periods of severe starvation. A decrease in herbivore populations will affect predators (fox and wolf) as well.

Increased snow cover, ice-crust formation and a prolonged thawing period will have a great impact on migratory birds, depending on the availability of insects or plants when they arrive in spring. If these food sources are not available in time, the birds will starve.

The size of insect populations are controlled by temperature. The expected rise in winter temperature, therefore, may increase egg survival. Aphid populations, for example, may increase strongly considerably.

Four of Greenland's five butterfly species are confined to the High Arctic or become more rare towards the south. They are expected to move north when warming occurs. Beetles depend on higher temperatures and may benefit from a warming and extend their range. Since insects are very sensitive to temperature changes, they could be useful as indicators of such changes.

7.2.3 Change in processes of terrestrial ecosystems

Permafrost melt, drainage and soil drying

A marked impact on vegetation is expected from permafrost melt, resulting in waterlogging or drought, depending on precipitation. As permafrost melts, there will be land subsidence (thermokarst erosion). This in turn leads to the formation of ponds and lakes. The changes in landscape, sea ice distribution, lake and river ice may not only have significant biological impacts, such as changes in caribou and polar bear migration routes, but would affect indigenous peoples.

Improved drainage may result due to thickening of the unfrozen zone between the seasonal frost and permafrost layers. General drying-out of the soils will greatly affect wetland areas.

In areas subject to soil drying, herbivore productivity will be reduced. The absence of herbivores favours mosses, which act as an insulating layer over the soil, preserving water and slowing decomposition and nutrient cycling.

Where soils become wetter, lichens and mosses will play a more important role in ecosystem carbon fixation and control of water loss from the soil to the atmosphere. The largest effect may be seen where drying accompanies warming, since replacement of mosses by deciduous species will result in increased rates of carbon and nutrient cycling.

Increased N₂-fixation

N₂- fixation rates (primarily by cyanobacteria) are predicted to increase by a factor of 1.5 - 2 as a result of temperature, moisture, and CO₂ changes. This will lead to a 25 - 50% increase in nitrogen input to arctic ecosystems, thus affecting the abundant N-deficient ecosystems in Greenland.

Increased nutrient availability, combined with higher temperatures, may result in a shift towards an ecosystem composition and structure having higher annual nutrient requirements, litter quality and tissue turnover rates.

CO₂ and CH₄ release

As the area of Greenland wetland soils is small relative to the area of global tundra soils, it is believed that enhanced release of CO₂ and CH₄ from increasing peat decomposition will have a relatively modest impact on global climate. It is unclear whether enhanced net primary production will offset increased decomposition rates, and thus whether the Arctic will continue to serve as a carbon sink.

Mineralization and nutrient availability

Decomposition and soil mineralization rates are expected to increase due to higher fluxes of oxygen to the soil organic matter, higher soil temperatures and higher nitrogen fixation rates. This will improve conditions for plant growth and soil nutrient mineralization.

High Arctic ecosystems are presently more limited by temperature than by nutrient availability, while the opposite characterises Low Arctic ecosystems. Low Arctic plants show a greater response to nutritional increases than High Arctic plants, whereas plant response to temperature elevation is greatest in the High Arctic, stimulating development, reproduction and seed germination.

Arctic vegetation is generally nutrient-limited, and almost all the nitrogen and phosphorous in the soil-vegetation system is bound in plants and soil microorganisms. Nutrients released by increased decomposition and mineralization would not necessarily be available to plants, since they are rather efficiently immobilised by soil microorganisms. It is uncertain, whether shrubs having mycorrhiza may circumvent microbial nutrient immobilisation.

Plant productivity in polar deserts

In polar deserts, herb barrens, and heaths in northern Greenland, plant productivity and long-term differentiation of vegetation types are strongly correlated with increases in precipitation. In such areas, reduced moisture may lead to greater mortality, decreased seed germination and seedling survival.

Food quality of plants

If CO₂-fixation increases without a matching increase in nutrient uptake, the quality of plant tissue as a food resource is likely to decrease, due to a greater C/N ratio. This may reduce the growth and abundance of invertebrates and retard decomposition rates in soil microorganisms, both of which are involved in litter breakdown. It may also affect herbivory, as herbivores would have to increase consumption in order to compensate for poor food quality and avoid malnutrition.

7.2.4 Marine ecosystems

Changes due to fresh water input and to UV-B

The influx of freshwater from melting ice and river runoff may cause a shift in the structure of biological communities in the upper ocean layers (e.g. coccolithophorids to diatoms).

Increased UV-B radiation may induce a change in species composition of both zooplankton and phytoplankton towards dominance of toxic species and species of low food value. This could cause major changes in food chains. Increased nitrogen demand may reduce productivity. Inhibition of photosynthesis might occur in some species, while bacteria may be stimulated because of an increase in substance availability.

Sea surface temperature and productivity

A rise in sea surface temperatures at high latitudes will result in a longer growing season and higher productivity. It may also result in the extinction of some species while others might proliferate. In southern waters, temperature may not rise and the return of the cod may fail.

Polynyas

Reduced supply of relatively warm Atlantic water to upwelling sites will cause decreased ice-edge primary production, a general nutrient loss, and a decrease in bioproductivity. A potential risk is that the polynyas of north Greenland may freeze, resulting in drastic changes for marine life including sea mammals and birds overwintering there.

Polar bears and ringed seals

Reduced sea ice will improve access by ships to harbours all over Greenland, but will cause problems for polar bears and seals. The polar bear migrates all over Greenland, but resident populations occur primarily in NW and NE Greenland. The breeding areas have stable winter climates, permanent snow cover, ice-covered inlets and drift ice, with abundant ringed seals which depend on sea ice for breeding, resting, and as diving platforms.

It is expected that the southern limit for resident polar bear populations will move northward since a decrease or periodic disappearance of sea ice will reduce the abundance of the ringed seal. During a prolonged ice-free period, polar bears would have less time to build up fat reserves. This may result in declining body condition, thereby lowering survival rate through the ice-free period, reducing reproductive rates and reducing cub survival.

More rain in late winter and early spring is another threat to both polar bears and ringed seals. The rain may cause seal birth lairs and maternity dens of polar bears to collapse. The dens are situated so deep in the snow that the weight of the snow above may crush females and cubs. Collapse of seal birth lairs in the upper snow layers can cause increased exposure of pups to predation by polar bears and Arctic foxes. Thaw and melt events in a milder winter may also damage the dens and birth lairs.

Other marine mammals

Other mammals which are more dependent on open water and less well adapted to the extreme arctic climate than the ringed seal may benefit from a prolonged ice-free period as long as their food chains are intact. Such animals are the walrus, harbour seal, harp seal and bearded seal. However, some of the seals may be forced to seek areas for breeding and shedding hairs further north. Fewer incidents of ice entrapment of whales are expected.

The danger of seal plagues and other diseases may increase. High temperatures combined with large densities of seals may be responsible for the seal plagues caused by viruses earlier this century.

7.3 Climate change in the Faroe Islands

Climate scenario

The North Atlantic region, including south Greenland and the Faroe Islands, is expected to warm less or at a slower rate than elsewhere in the northern hemisphere. All atmospheric circulation models seem to agree that the North Atlantic region, including the Faroe Islands, will experience the slowest rate of temperature increase. Adding the cooling effect of the reduced North Atlantic Current, it is unlikely that the annual mean air temperature increase will exceed 1 - 2°C within the next century. The rise in winter temperature may be twice the rise in summer temperature. The risk of frost in the high mountains may be reduced. An increase in yearly precipitation is expected to be less than 4%. Gale frequencies are expected to increase.

The sea

Sea surface temperature may drop due to reduced thermohaline circulation. The sea level may rise at a similar rate of 5 cm per decade as predicted for the British Isles and is not expected to be compensated for by a land rise. Estimates of sea level rise vary from two to five times the rate of 10 - 25 cm observed over the past century.

7.3.1 Ecological implications

Reduced species diversity

Only minor changes in terrestrial ecosystems are expected. The isolation of the Faroe Islands in the Atlantic Ocean may have the consequence that climate-induced changes in plant and animal life will be unbalanced. Thus, the rate of possible species loss from terrestrial ecosystems may not be counterbalanced by a similar immigration rate, resulting in reduced species diversity.

Unpredictable changes in marine ecosystems

The greatest changes are expected at sea, although the uncertainty is also the greatest here as long as the fate of the North Atlantic Current has not been clarified. Warmer deep water could result in a redistribution of pelagic and benthic communities.

Impacts on plankton will be similar to those mentioned for Greenland. Fish species that settle in shallow waters in the early spring such as flatfish, lumpfish, and species with pelagic drifting eggs and larvae will have a high risk of UV-B induced damages.

Effects on marine mammals and seabirds are expected mainly to concern spatial shifts in areas of food production and primary productivity (changes in upwelling sights), nesting and rearing sites, and increases in diseases and oceanic biotoxin production (both from temperature increase and current changes).

The reappearance of the cod seems highly dependent on what happens to sea currents. That there have been 3 - 4 times as many storms as normal in recent years has contributed to the disappearance of the cod by blowing the fry towards waters too cold for their survival. A reduction in water arriving from the south will worsen the present lack of the fry's favourite food.

 
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