This is Section 18.4 of the Arctic Climate Impact Assessment. Lead Author: Gunter Weller; Contributing Authors: Elizabeth Bush,Terry V. Callaghan, Robert Corell, Shari Fox, Christopher Furgal, Alf Håkon Hoel, Henry Huntington, Erland Källén, Vladimir M. Kattsov, David R. Klein, Harald Loeng, Marybeth Long Martello, Michael MacCracken, Mark Nuttall,Terry D. Prowse, Lars-Otto Reiersen, James D. Reist, Aapo Tanskanen, John E.Walsh, Betsy Weatherhead, Frederick J.Wrona
This assessment has dealt with individual topics that reflect traditional academic and practical organization. However, a strong thread running through the assessment is the interaction between the various topics and processes in the Arctic. This includes, for example, the interactions between physical atmospheric processes and biological processes in the major ecosystems, and the strong albedo and other feedback responses; the physical and biological connections between land, freshwater, and marine environments; and the integrity of the arctic system as a whole. Three important cross-cutting issues that illustrate the interactions within the Arctic and connections with the global system are carbon storage and carbon cycling, biodiversity, and abrupt climate change and extreme events.
Carbon storage and carbon cycling (18.4.1)
Global importance of carbon in the Arctic (18.104.22.168)
The Arctic contains large stores of carbon that have historically been sequestered from the atmospheric carbon pool. Estimates of arctic and boreal soil carbon (C) in the upper meter of soil vary considerably, ranging from 90 to 290 Pg C in upland boreal forest soils, 120 to 460 Pg C in peatland soils, and 60 to 190 Pg C in arctic tundra soils. There is also a general sparsity of high-latitude carbon data for aquatic ecosystems relative to arctic terrestrial systems, but some estimates from boreal lakes indicate that reserves can be significant (120 Pg C). An additional 450 Pg of organic C is stored as dissolved carbon in the Arctic Ocean (see Chapter 9). Estimates of carbon stored in the upper 100 meters of permafrost are as high as 10 000 Pg C. In any case, the carbon stored in northern boreal forests, lakes, tundra, the Arctic Ocean, and permafrost is considerably greater than the global atmospheric pool of carbon, which is estimated at 730 Pg C. In addition, up to 10 000 Pg C in the form of CH4 and CO2 is stored as hydrates in marine permafrost below 100 m (Chapter 9), however, this figure is a maximum of estimates that span several orders of magnitude.
Spatial patterns of carbon storage (22.214.171.124)
Within the Arctic, carbon storage generally decreases from south to north. On land, this represents parallel decreases from boreal forest to tundra to polar desert and from southern isolated, sporadic, and discontinuous permafrost to continuous permafrost in the north; in freshwater ecosystems there is a decrease from peatlands and lakes with high concentrations of dissolved organic carbon to tundra and high-arctic ponds with low dissolved organic carbon; and in the marine environment there is a decrease from areas of high organic matter production and sedimentation in the south and at the ice margin to relatively clear waters in the Arctic Ocean. Marine permafrost and gas hydrates show a different pattern in that they are concentrated in the area of continental shelves, which are particularly extensive along the northern coastlines of the arctic landmasses.
Processes involved in carbon storage and release (126.96.36.199)
Over thousands of years, an imbalance between photosynthesis and decomposition has led to storage of carbon in lake and ocean sediments, and in forest and tundra soils (Chapters 7, 8, 9, and 14). On land, this imbalance was created because low temperatures, particularly when combined with high soil moisture, retarded microbial decomposition more than photosynthesis. In the marine environment, atmospheric carbon is dissolved as inorganic carbon in surface waters and stored at depth as a result of the physical pump; death and decomposition of organisms also lead to carbon storage in the form of dissolved and particulate organic carbon (Chapter 9). Low ocean temperatures have resulted in high solubility of carbon, while extensive sea-ice cover has reduced the duration and area for carbon exchange between air and surface waters (and thus photosynthesis).
Because low temperatures have been so important for the capture and storage of atmospheric carbon in the Arctic, projected temperature increases have the potential to lead to the release of old and more recently captured carbon to the atmosphere, although the older the stored carbon, the less responsive it will be to projected climate changes. The release of stored carbon will increase atmospheric GHG concentrations and provide a positive feedback to the climate system. However, increased temperatures are also likely to increase the photosynthetic capture of atmospheric carbon if other environmental conditions do not become limiting. On land, plants will grow faster and more productive vegetation will successively replace less productive vegetation at higher latitudes and altitudes (Chapters 7 and 14). In freshwater ecosystems, reduced duration of ice cover over lakes and ponds and increased temperatures are likely to increase primary production (Chapter 8). In the marine environment, primary production is expected to increase as areas where production has been limited by sea-ice cover become more restricted in extent. Also, it is likely that more carbon will be buried as deposition shifts from the continental shelves where primary production is currently concentrated to the deeper slope and basin region as the ice edge retreats (Chapter 9).
The balance between the opposing processes of increased carbon capture and release will determine future changes in the carbon feedback from the Arctic to global climate. However, there are great uncertainties in calculating this balance across permafrost, terrestrial soil, ocean, and freshwater systems and no quantitative integrative assessment has been performed to date.
Projected changes in carbon storage and release to the atmosphere (188.8.131.52)
There is a consensus from the trace-gas measurement researchers that the terrestrial Arctic is presently a source of carbon and radiative forcing, but is likely to become a weak sink of carbon during future warming (Chapter 7). Modeling approaches suggest that circumpolar mean carbon uptake is likely to increase from the current 12 g C/m2/yr to 22 g C/m2/yr by 2100 and that carbon storage is likely to increase by 12 to 31 Pg C depending on the ACIA climate scenario used. However, the uncertainties are great: the projections are limited to terrestrial ecosystems and do not include carbon stored in permafrost and gas hydrates. Potential increases in human and natural disturbances are further uncertainties. The marine environment has been suggested as a weak sink, but the amount of carbon that the Arctic Ocean can sequester is likely to increase significantly under scenarios of decreased sea-ice cover, both through surface uptake and increased biological production, although there may be an abrupt release of CO2 and CH4 from thawing permafrost in marine sediments.
In the marine environment, there are vast stores of CH4 and CO2 (at least 10 000 Pg C in the form of gas hydrates in marine permafrost below 100 m;). As there are currently about 4 Pg C in CH4 in the atmosphere, even the release of a small percentage of CH4 from gas hydrates could result in an abrupt and significant climate forcing (Chapter 9). The process of CH4 release from gas hydrates under continental shelves could already be occurring due to the warming of earlier coastal landmasses during Holocene flooding. On land, however, natural gas hydrates are found only at depths of several hundreds of meters and are relatively inert.
The diversity of species in terrestrial, freshwater, and marine ecosystems of the Arctic is fundamental to the life support of the residents of the region and to commercial interests such as fishing at lower latitudes.
Diversity is also important to the functioning of arctic ecosystems: productivity, carbon emissions, and albedo are all related to specific characteristics of current arctic species.While the Arctic contains some specialist species that are well adapted to the harsh arctic environment, it also contains species that migrate and contribute to the biodiversity of more southerly latitudes. Each year, whales, dolphins, and hundreds of millions of birds migrate from the Arctic to warmer latitudes. The Arctic is an area of relatively undisturbed and natural biodiversity because of generally lower human impacts than elsewhere on earth. However, at its southern border, human impacts are greater and particular areas, such as old growth forests on land, preserve biodiversity that is endangered in managed areas.
Patterns of diversity in the Arctic (184.108.40.206)
The diversity of living organisms at any one time in the Arctic is a snapshot of complex, dynamic physical and biological processes that create habitats and opportunities or constraints for species, and genetically distinct populations of particular species, to colonize them.
The current diversity of organisms in the Arctic has been shaped by major climatic and associated changes in physical and chemical conditions of the land, wetlands, and oceans over past glacial and interglacial periods. Changes are presently occurring that are also driven by direct human activities such as fishing, hunting, and gathering, changes in land use, and habitat fragmentation, in addition to indirect human activities such as anthropogenic climate change, stratospheric ozone depletion, and transboundary movement of contaminants.
On land, and in freshwater and the marine environment, the fauna and flora are young in a geological context. Recent glaciations resulted in major losses of biodiversity, and recolonization has been slow because of the extreme environmental conditions and overall low productivity of the arctic system. On land, of at least 12 large herbivores and six large carnivores present in steppe–tundra areas at the last glacial maximum, only four and three respectively survive today and of these, only two herbivores (reindeer and musk ox) and two carnivores (brown bear, Ursus arctos and wolf, Canis lupus) presently occur in the arctic tundra biome. Arctic marine mammals to a large extent escaped the mass extinctions that affected their terrestrial counterparts at the end of the Pleistocene because of their great mobility. However, hunting in historical times had severe impacts on several species that were exterminated (great auk, Pinguinus impennis; Steller sea cow, Hydrodamalis gigas) or almost harvested to extinction (walrus; bowhead whale; sea otter, Enhydra lutris). Polar bears, all the Great Whales, white whales, and many species of colonially nesting birds were dramatically reduced.
The youth of arctic flora and fauna, together with the harsh physical environment of arctic habitats and to some extent over-harvesting, have resulted in lower species diversity in the Arctic compared to other regions. This results in arctic ecosystems, in a global sense, being “simple”. Some of the species are specialists that are well adapted to the Arctic’s physical environment; others were pre-adapted to the arctic environment and moved north during deglaciation. Overall however, many arctic species – marine, freshwater, and terrestrial – possess a suite of characteristics that allows them to survive in extreme environments. However, these characteristics, together with low diversity and simple relationships between species in food webs, render arctic species and ecosystems vulnerable to the environmental changes now occurring in the Arctic and those projected to occur in the future.
Although diversity of arctic species is relatively low, in absolute terms it can be high: about 6000 marine, freshwater, and terrestrial species have been catalogued in and around Svalbard and about 7200 terrestrial and freshwater species have been recorded in a subarctic area of northern Finland. About 3% (around 5900 species) of the global flora occurs in the Arctic. The diversity of arctic terrestrial animals beyond the latitudinal treeline (6000 species) is nearly twice as great as that of vascular plants and bryophytes. The arctic fauna accounts for about 2% of the global total. In the arctic region as defined by CAFF (Conservation of Arctic Flora and Fauna), which includes forested areas, some 450 species of birds have been recorded breeding, and around 280 species migrate. The diversity of vertebrate species in the arctic marine environment is less than on land. Species diversity differs from group to group: primitive species of land plants such as mosses and lichens are well represented in the Arctic whereas more advanced flowering plants are not; primitive species of land animals such as springtails are well represented whereas more advanced beetles and mammals are not. In contrast, although most taxonomic groups of freshwater organisms in the Arctic are not diverse, some groups such as fish have high diversity at and below the species level. One consequence of the generally low species diversity is that species will be susceptible to damage by new insect pests, parasites, and diseases. For example, low diversity of boreal trees together with low diversity of parasites and predators that control populations of insect pests exaggerates the impacts of the pests.
The number of species generally decreases with increasing latitude. The steep temperature gradient that has such a strong influence on species diversity occurs over much shorter distances in the Arctic than in other biomes. North of the treeline in Siberia, mean July temperature decreases from 12 to 2 ºC over 900 km, whereas a 10 ºC decline in July temperature is spread over 2000 km in the boreal zone, and July temperature decreases by less than 10 ºC from the equator to the southern boreal zone. Patterns of species diversity in the Arctic also differ according to geography.With its complicated relief, geology, and biogeographic history, there are more species on land in Beringia at a given temperature than on the Taymir Peninsula. Taymir biodiversity values are intermediate between the higher values for Chukotka and Alaska, which have a more complicated relief, geology, and floristic history, and the lower values in the eastern Canadian Arctic with its impoverished flora resulting from relatively recent glaciation.Within any region, biological hot spots occur, for example below predictable leads in the sea ice, polynyas, oceanographic fronts, areas of intense mixing, and the marginal ice zone in the marine environment; in delta areas that lie at the interface between rivers and lakes or oceans; and at the ecotone between tundra and taiga on land where elements of both forest and tundra floras and faunas mix. Such hotspots are centers from which species with restricted distributions can expand during climatic warming.
An important consequence of the general decline in numbers of species with increasing latitude is an increase in abundance and dominance. For example, on land, one species of collembolan, Folsomia regularis, may constitute 60% of the total collembolan density in the polar desert.
In freshwater ecosystems, midge and mosquito larvae are particularly abundant but species-level diversity is low. These “super-dominant” species, such as lemmings in peak years of their population cycles, are generally highly plastic, occupy a wide range of habitats, and generally have large effects on ecosystem processes. Similarly, arctic fish communities of the marine environment are dominated by a few species, several of which are commercially important, while the abundance of fish, marine mammals, and birds attracted hunters and fishing enterprises in historical times. Loss/reduction of one or more of these species, particularly fish species, will have disproportionate impacts on economy and ecosystem function.
Characteristics of arctic species related to the arctic environment (220.127.116.11)
Several physical factors make arctic marine systems unique from other oceanic regions including: a very high proportion of continental shelves and shallow water; a dramatic seasonality and overall low level of sunlight; extremely low water temperatures (but not compared with arctic terrestrial habitats); presence of extensive permanent and seasonal sea-ice cover; and a strong freshwater influence from rivers and ice melt. Arctic freshwater environments are also characterized by extreme seasonality and low levels of incident radiation, much of which is reflected due to the high albedo of snow and ice. In addition, the thermal energy of a substantive part of this incoming radiation is used to melt ice, rendering it unavailable to biota. However, large arctic rivers with headwaters south of the Arctic act as conduits of heat and biota.
On land, low solar angles, long snow-covered winters, cold soils with permafrost, and low nutrient availability in often primitive soils limit survival and productivity of organisms. Many species of marine and terrestrial environments migrate between relatively warm wintering grounds in the south and the rich, but short-lived, feeding and breeding grounds in the Arctic. In freshwater ecosystems, some fish are highly migratory, moving in response to environmental cues. Those species that do not migrate have a suite of characteristics (behavior, physiology, reproduction, growth, development) that allow them to avoid the harshest weather or to persist. Two characteristics common to marine, freshwater, and terrestrial arctic organisms are a protracted life span with slow development over several years to compensate for the brevity and harshness of each growing/feeding/ breeding season, and low reproductive rates. These characteristics render arctic organisms in general vulnerable to disturbance and environmental change.
Responses of biodiversity to climate and UV-B radiation change (18.104.22.168)
The past and present patterns of biodiversity in the Arctic, the characteristics of arctic species, and the experimental and modeling assessments described in Chapters 7, 8, and 9, together make it possible to infer the following likely changes to arctic biodiversity:
The total number of species in the Arctic will increase as new species move northward during warming. Large, northward-flowing rivers are conduits for species to move northward. New communities will form.
Present arctic species will relocate northward, as in the past, rather than adapt to new climate envelopes, particularly as the projected rate of climate change exceeds the ability of most species to adapt. However, some species, particularly freshwater species, may already be pre-adapted to acclimate successfully to rapid climate change.
Locally adapted species may be extirpated from certain areas while arctic specialists and particular groups of species that are poorly represented in the Arctic – some through loss of species during earlier periods of climate warming – will be at risk of extinction. Examples of arctic specialists at risk include polar bears, and seals of the ice margins in the marine environment, and large ungulates and predators on land.
Presently super-abundant species will be restricted in range and abundance with severe impacts on commercial fisheries, indigenous hunting, and ecosystem function. Examples on land include lemmings, mosses and lichens, and some migratory birds.
On land, shifts in major vegetation zones such as forests and tundra will be accompanied by changes in the species associated with them. For example, tree seed-eating birds, and wood-eating insects will move northward with trees.
Low biodiversity will render ecosystems more susceptible to disturbance through insect pest infestations, parasites, pathogens, and disease.
At the small scale, changes will be seen in the representation of different genetically separate populations within species. In cases such as Arctic char, the species may remain but become geographically or ecologically marginalized with the potential loss of particular morphs.
Changes in UV-B radiation levels are likely to have small effects on biodiversity compared with climate warming. However, UV-B radiation has harmful effects on some fish larvae, on those amphibians that might colonize the Arctic, and on some microorganisms and fungi. In freshwater ecosystems, increased UV-B radiation levels could potentially reduce biodiversity by disadvantaging sensitive species and changing algal communities.
All the projected changes in biodiversity resulting from changes in climate and UV-B radiation levels are likely to be modified by direct human activities. Protection and management of some areas have led to the recovery of some previously declining species while deforestation, extractive industries, and pollution have prevented forests and associated species from moving northward during recent warming in some areas. Protection of ecosystems from the impacts of changes in climate and UV-B radiation in the long term is difficult and perhaps impossible.
Abrupt climate change and extreme events (18.4.3)
Human activities are causing atmospheric concentrations of CO2 and other GHGs and aerosols to change slowly from year to year, thereby causing the radiative forcing that drives climate change to shift slowly. However, the resulting changes in climate and associated impacts do not necessarily have to change slowly and smoothly. First, the natural interactions of the atmosphere, oceans, snow and ice, and the land surface, both within and outside the Arctic, can cause climatic conditions to fluctuate. These variations can cause months, seasons, years, and even decades to be warmer or cooler, wetter or drier, and even more settled or more changeable than the multi-decadal average conditions. Intermittent volcanic eruptions and variations or cycles in the intensity of solar radiation can also cause such fluctuations. These types of fluctuations can be larger than the annual or even decadal increment of long-term anthropogenic global climate change. Present model simulations project a slow and relatively steady change in baseline climate while natural factors create fluctuations on monthly to decadal scales. As the baseline climate changes, the ongoing fluctuations are very likely to cause new extremes to be reached and the occurrence of conditions that currently create stress (e.g., summer temperatures greater than 30 ºC) are likely to increase significantly.
The climatic history of the earth shows that instead of climate changing steadily and gradually, change can be intermittent and abrupt in particular regions – even very large regions. Reconstructions of climatic variations over the last glacial cycle and the early part of the current interglacial some 8000 to 20000 years ago suggest that temperature changes of several degrees in the largescale, long-term climate occurred over a relatively short period. For example, ice cores indicate that temperatures over Greenland dropped by as much as 5 ºC within a few years during the period of warming following the last glacial. These changes were apparently driven by a sharp change in the thermohaline circulation of the ocean (also referred to as the Atlantic meridional overturning circulation), which probably also prompted changes in the atmospheric circulation that caused large climatic changes over land areas surrounding the North Atlantic and beyond. Over multi-decadal time periods, persistent shifts in atmospheric circulation patterns, such as the North Atlantic and Arctic Oscillations, have also caused changes in the prevailing weather regimes of arctic countries, contributing, for example, to warm decades, such as the 1930s and 1940s, and cool decades, such as the 1950s and 1960s.
A recent example of a rapid change in arctic climate was the so-called regime shift in the Bering Sea in 1976, which had serious consequences and impacts on the environment. In 1976, mean annual temperatures in Alaska experienced a step-like increase of 1.5 ºC to a lasting new high level, shown as the average of several measuring stations. Sea-ice extent in the Bering Sea showed a similar step-like decrease of about 5%. An analysis by Ebbesmeyer et al. gave statistical measures of deviation from the normal of 40 environmental parameters in the North Pacific region, as a consequence of this rapid change. The parameters included air and water temperatures, chlorophyll, geese, salmon, crabs, glaciers, atmospheric dust, coral, CO2, winds, ice cover, and Bering Strait transport. The authors concluded that “apparently one of the Earth’s large ecosystems occasionally undergo large abrupt shifts”.
As anthropogenic climate change continues, the potential exists for oceanic and atmospheric circulations to shift to new or unusual states.Whether such changes, perhaps brought on when a temperature or precipitation threshold is crossed, will occur abruptly (i.e., within a few years) or more gradually (i.e., within several decades or more) remains to be determined. Such shifts could cause the relatively rapid onset of various types of impacts. A warm and wet anomaly might accelerate the onset of pests or infectious diseases.Warming exceeding about 3 ºC might initiate the long-term deterioration of the Greenland Ice Sheet as temperatures above freezing spread across the plateau in summer. The tentative indication of an initial slowing of the thermohaline circulation could change into a significant slowdown, greatly reducing the northward transport of tropical warmth that now moderates European winters. The likelihood of any such shifts or changes occurring is not yet well established, but if the future is like the past, the possibility for abrupt change and new extremes is real.
Chapter 18: Summary and Synthesis of the ACIA
18.2. A summary of ACIA conclusions
18.3. A synthesis of projected impacts in the four regions
18.4. Cross-cutting issues in the Arctic
18.5. Improving future assessments
NOTE:This chapter is a summary based on the seventeen preceding chapters of the Arctic Climate Impact Assessment and a full list of references is provided in those chapters. Only references to major publications and data sources, including integrative regional assessments, and some papers reporting the most recent developments, are listed.
- ^Semiletov, I.P., 1999. On aquatic sources and sinks of CO2 and CH4 in the polar regions. Journal of Atmospheric Sciences, 56(2):286–306.
- ^IPCC, 2001. Climate Change 2001: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, 1032p.
- ^Semiletov, 1999, Op. cit.
- ^Prestrud, P., S. Hallvard and H.V. Goldman, 2004. A catalogue of the terrestrial and marine animals of Svalbard. Skrifter 201. Norwegian Polar Institute,Tromsø, Norway, 137p.
- ^Callaghan,T.V., M. Johansson, O.W. Heal, N.R. Sælthun, L.J. Barkved, N. Bayfield, O. Brandt, R. Brooker, H.H. Christiansen, M.C. Forchhammer,T.T. Høye, O. Humlum, A. Järvinen, C. Jonasson, J. Kohler, B. Magnusson, H. Meltofte, L. Mortensen, S. Neuvonen, I. Pearce, M. Rasch, L.Turner, B. Hasholt, E. Huhta, E. Leskinen, N. Nielsen and P. Siikamäki, 2004. Environmental Changes in the North Atlantic Region: SCANNET as a collaborative approach for documenting, understanding and predicting changes. Ambio Special Report, 13:35–58.
- ^Weller, G., P. Anderson and B.Wang (eds.), 1999. Preparing for a Changing Climate:The Potential Consequences of Climate Change and Variability. A Report of the Alaska Regional Assessment Group for the U. S. Global Change Research Program, University of Alaska Fairbanks, 42p.
- ^Ebbesmeyer, C.C., D.R. Cayan, D.R. McLain, F.H. Nichols, D.H. Peterson and K.T. Redmond, 1991. 1976 step in the Pacific climate: forty environmental changes between 1968–1975 and 1977–1984. In: J.L. Betancourt and V.L.Tharp (eds). Proceedings of the Seventh Annual Pacific Climate (PACLIM) Workshop, April 1990. California Dept. of Water Resources. Interagency Ecological Studies Program, Technical Report 26.