Effects of changes in climate on landscape and regional processes, and feedbacks to the climate system

Biological and physical processes in the Arctic system operate at various temporal and spatial scales to impact large-scale feedbacks and interactions with the earth system. There are four main potential feedback mechanisms between the impacts of climate change on the Arctic and the global climate system: albedo, greenhouse gas emissions or uptake by ecosystems, greenhouse gas emissions from methane hydrates, and increased freshwater fluxes that could affect the thermohaline circulation. All these feedbacks are controlled to some extent by changes in ecosystem distribution and character and particularly by large-scale movement of vegetation zones. Indications from a few, full annual measurements of CO2 fluxes are that currently the source areas exceed sink areas in geographical distribution. The little available information on CH4 sources indicates that emissions at the landscape level are of great importance for the total greenhouse balance of the circumpolar North. Energy and water balances of Arctic landscapes are also important feedback mechanisms in a changing climate. Increasing density and spatial expansion of vegetation will cause a lowering of the albedo and more energy to be absorbed on the ground. This effect is likely to exceed the negative feedback of increased C sequestration in greater primary productivity resulting from the displacements of areas of polar desert by tundra, and areas of tundra by forest. The degradation of permafrost has complex consequences for trace gas dynamics. In areas of discontinuous permafrost, warming, will lead to a complete loss of the permafrost. Depending on local hydrological conditions this may in turn lead to a wetting or drying of the environment with subsequent implications for greenhouse gas fluxes. Overall, the complex interactions between processes contributing to feedbacks, variability over time and space in these processes, and insufficient data have generated considerable uncertainties in estimating the net effects of climate change on terrestrial feedbacks to the climate system. This uncertainty applies to magnitude, and even direction of some of the feedbacks.     

Synthesis of effects in four Arctic subregions

An assessment of impacts on Arctic terrestrial ecosystems has emphasized geographical variability in responses of species and ecosystems to environmental change. This variability is usually associated with north-south gradients in climate, biodiversity, vegetation zones, and ecosystem structure and function. It is clear, however, that significant east-west variability in environment, ecosystem structure and function, environmental history, and recent climate variability is also important. Some areas have cooled while others have become warmer. Also, east-west differences between geographical barriers of oceans, archipelagos and mountains have contributed significantly in the past to the ability of species and vegetation zones to relocate in response to climate changes, and they have created the isolation necessary for genetic differentiation of populations and biodiversity hot-spots to occur. These barriers will also affect the ability of species to relocate during projected future warming. To include this east-west variability and also to strike a balance between overgeneralization and overspecialization, the ACIA identified four major sub regions based on large-scale differences in weather and climate-shaping factors. Drawing on information, mostly model output that can be related to the four ACIA subregions, it is evident that geographical barriers to species re-location, particularly the distribution of landmasses and separation by seas, will affect the northwards shift in vegetation zones. The geographical constraints--or facilitation--of northward movement of vegetation zones will affect the future storage and release of carbon, and the exchange of energy and water between biosphere and atmosphere. In addition, differences in the ability of vegetation zones to re-locate will affect the biodiversity associated with each zone while the number of species threatened by climate change varies greatly between subregions with a significant hot-spot in Beringia. Overall, the subregional synthesis demonstrates the difficulty of generalizing projections of responses of ecosystem structure and function, species loss, and biospheric feedbacks to the climate system for the whole Arctic region and implies a need for a far greater understanding of the spatial variability in the responses of terrestrial arctic ecosystems to climate change.     

Arctic Climate Tipping Points

There is widespread concern that anthropogenic global warming will trigger Arctic climate tipping points. The Arctic has a long history of natural, abrupt climate changes, which together with current observations and model projections, can help us to identify which parts of the Arctic climate system might pass future tipping points. Here the climate tipping points are defined, noting that not all of them involve bifurcations leading to irreversible change. Past abrupt climate changes in the Arctic are briefly reviewed. Then, the current behaviour of a range of Arctic systems is summarised. Looking ahead, a range of potential tipping phenomena are described. This leads to a revised and expanded list of potential Arctic climate tipping elements, whose likelihood is assessed, in terms of how much warming will be required to tip them. Finally, the available responses are considered, especially the prospects for avoiding Arctic climate tipping points.     

Biodiversity, distributions and adaptations of Arctic species in the context of environmental change

The individual of a species is the basic unit which responds to climate and UV-B changes, and it responds over a wide range of time scales. The diversity of animal, plant and microbial species appears to be low in the Arctic, and decreases from the boreal forests to the polar deserts of the extreme North but primitive species are particularly abundant. This latitudinal decline is associated with an increase in super-dominant species that occupy a wide range of habitats. Climate warming is expected to reduce the abundance and restrict the ranges of such species and to affect species at their northern range boundaries more than in the South: some Arctic animal and plant specialists could face extinction. Species most likely to expand into tundra are boreal species that currently exist as outlier populations in the Arctic. Many plant species have characteristics that allow them to survive short snow-free growing seasons, low solar angles, permafrost and low soil temperatures, low nutrient availability and physical disturbance. Many of these characteristics are likely to limit species' responses to climate warming, but mainly because of poor competitive ability compared with potential immigrant species. Terrestrial Arctic animals possess many adaptations that enable them to persist under a wide range of temperatures in the Arctic. Many escape unfavorable weather and resource shortage by winter dormancy or by migration. The biotic environment of Arctic animal species is relatively simple with few enemies, competitors, diseases, parasites and available food resources. Terrestrial Arctic animals are likely to be most vulnerable to warmer and drier summers, climatic changes that interfere with migration routes and staging areas, altered snow conditions and freeze-thaw cycles in winter, climate-induced disruption of the seasonal timing of reproduction and development, and influx of new competitors, predators, parasites and diseases. Arctic microorganisms are also well adapted to the Arctic's climate: some can metabolize at temperatures down to -39 degrees C. Cyanobacteria and algae have a wide range of adaptive strategies that allow them to avoid, or at least minimize UV injury. Microorganisms can tolerate most environmental conditions and they have short generation times which can facilitate rapid adaptation to new environments. In contrast, Arctic plant and animal species are very likely to change their distributions rather than evolve significantly in response to warming.     

Status and trends in Arctic vegetation: Evidence from experimental warming and long-term monitoring

Changes in Arctic vegetation can have important implications for trophic interactions and ecosystem functioning leading to climate feedbacks. Plot-based vegetation surveys provide detailed insight into vegetation changes at sites around the Arctic and improve our ability to predict the impacts of environmental change on tundra ecosystems. Here, we review studies of changes in plant community composition and phenology from both long-term monitoring and warming experiments in Arctic environments. We find that Arctic plant communities and species are generally sensitive to warming, but trends over a period of time are heterogeneous and complex and do not always mirror expectations based on responses to experimental manipulations. Our findings highlight the need for more geographically widespread, integrated, and comprehensive monitoring efforts that can better resolve the interacting effects of warming and other local and regional ecological factors.     

Seafood from a changing Arctic

We review current knowledge about climate change impacts on Arctic seafood production. Large-scale changes in the Arctic marine food web can be expected for the next 40–100 years. Possible future trajectories under climate change for Arctic capture fisheries anticipate the movement of aquatic species into new waters and changed the dynamics of existing species. Negative consequences are expected for some fish stocks but others like the Barents Sea cod (Gadus morhua) may instead increase. Arctic aquaculture that constitutes about 2% of global farming is mainly made up of Norwegian salmon (Salmo salar) farming. The sector will face many challenges in a warmer future and some of these are already a reality impacting negatively on salmon growth. Other more indirect effects from climate change are more uncertain with respect to impacts on the economic conditions of Arctic aquaculture.     

Relevance of Hydro-Climatic Change Projection and Monitoring for Assessment of Water Cycle Changes in the Arctic

Rapid changes to the Arctic hydrological cycle challenge both our process understanding and our ability to find appropriate adaptation strategies. We have investigated the relevance and accuracy development of climate change projections for assessment of water cycle changes in major Arctic drainage basins. Results show relatively good agreement of climate model projections with observed temperature changes, but high model inaccuracy relative to available observation data for precipitation changes. Direct observations further show systematically larger (smaller) runoff than precipitation increases (decreases). This result is partly attributable to uncertainties and systematic bias in precipitation observations, but still indicates that some of the observed increase in Arctic river runoff is due to water storage changes, for example melting permafrost and/or groundwater storage changes, within the drainage basins. Such causes of runoff change affect sea level, in addition to ocean salinity, and inland water resources, ecosystems, and infrastructure. Process-based hydrological modeling and observations, which can resolve changes in evapotranspiration, and groundwater and permafrost storage at and below river basin scales, are needed in order to accurately interpret and translate climate-driven precipitation changes to changes in freshwater cycling and runoff. In contrast to this need, our results show that the density of Arctic runoff monitoring has become increasingly biased and less relevant by decreasing most and being lowest in river basins with the largest expected climatic changes.     

High Arctic ecosystem states: Conceptual models of vegetation change to guide long-term monitoring and research

Vegetation change has consequences for terrestrial ecosystem structure and functioning and may involve climate feedbacks. Hence, when monitoring ecosystem states and changes thereof, the vegetation is often a primary monitoring target. Here, we summarize current understanding of vegetation change in the High Arctic—the World’s most rapidly warming region—in the context of ecosystem monitoring. To foster development of deployable monitoring strategies, we categorize different kinds of drivers (disturbances or stresses) of vegetation change either as pulse (i.e. drivers that occur as sudden and short events, though their effects may be long lasting) or press (i.e. drivers where change in conditions remains in place for a prolonged period, or slowly increases in pressure). To account for the great heterogeneity in vegetation responses to climate change and other drivers, we stress the need for increased use of ecosystem-specific conceptual models to guide monitoring and ecological studies in the Arctic. We discuss a conceptual model with three hypothesized alternative vegetation states characterized by mosses, herbaceous plants, and bare ground patches, respectively. We use moss-graminoid tundra of Svalbard as a case study to discuss the documented and potential impacts of different drivers on the possible transitions between those states. Our current understanding points to likely additive effects of herbivores and a warming climate, driving this ecosystem from a moss-dominated state with cool soils, shallow active layer and slow nutrient cycling to an ecosystem with warmer soil, deeper permafrost thaw, and faster nutrient cycling. Herbaceous-dominated vegetation and (patchy) bare ground would present two states in response to those drivers. Conceptual models are an operational tool to focus monitoring efforts towards management needs and identify the most pressing scientific questions. We promote greater use of conceptual models in conjunction with a state-and-transition framework in monitoring to ensure fit for purpose approaches. Defined expectations of the focal systems’ responses to different drivers also facilitate linking local and regional monitoring efforts to international initiatives, such as the Circumpolar Biodiversity Monitoring Program.     

Effects on the function of Arctic ecosystems in the short- and long-term perspectives

Historically, the function of Arctic ecosystems in terms of cycles of nutrients and carbon has led to low levels of primary production and exchanges of energy, water and greenhouse gases have led to low local and regional cooling. Sequestration of carbon from atmospheric CO2, in extensive, cold organic soils and the high albedo from low, snow-covered vegetation have had impacts on regional climate. However, many aspects of the functioning of Arctic ecosystems are sensitive to changes in climate and its impacts on biodiversity. The current Arctic climate results in slow rates of organic matter decomposition. Arctic ecosystems therefore tend to accumulate organic matter and elements despite low inputs. As a result, soil-available elements like nitrogen and phosphorus are key limitations to increases in carbon fixation and further biomass and organic matter accumulation. Climate warming is expected to increase carbon and element turnover, particularly in soils, which may lead to initial losses of elements but eventual, slow recovery. Individual species and species diversity have clear impacts on element inputs and retention in Arctic ecosystems. Effects of increased CO2 and UV-B on whole ecosystems, on the other hand, are likely to be small although effects on plant tissue chemisty, decomposition and nitrogen fixation may become important in the long-term. Cycling of carbon in trace gas form is mainly as CO2 and CH4. Most carbon loss is in the form of CO2, produced by both plants and soil biota. Carbon emissions as methane from wet and moist tundra ecosystems are about 5% of emissions as CO2 and are responsive to warming in the absence of any other changes. Winter processes and vegetation type also affect CH4 emissions as well as exchanges of energy between biosphere and atmosphere. Arctic ecosystems exhibit the largest seasonal changes in energy exchange of any terrestrial ecosystem because of the large changes in albedo from late winter, when snow reflects most incoming radiation, to summer when the ecosystem absorbs most incoming radiation. Vegetation profoundly influences the water and energy exchange of Arctic ecosystems. Albedo during the period of snow cover declines from tundra to forest tundra to deciduous forest to evergreen forest. Shrubs and trees increase snow depth which in turn increases winter soil temperatures. Future changes in vegetation driven by climate change are therefore, very likely to profoundly alter regional climate.     

Carbon budgets for catchments across a managed landscape mosaic in southeast Sweden: contributing to the safety assessment of a nuclear waste repository

Ecosystem budgets of matter contribute to the assessment of transport and accumulation of bioavailable contaminants in a landscape, since flows of matter and energy ultimately determine the rates at which contaminants will be partitioned in the environment. This study compares ecosystem properties, such as net primary production (NPP), sequestration of matter and fluxes to food sources for humans, which are of potential interest to describe fluxes and accumulation of bioavailable radionuclides in 14 catchments within a larger catchment area in southeast Sweden. The carbon budgets, used as a proxy for organic matter, are mainly based on local estimates of pools and fluxes, which have been distributed across a landscape mosaic of different vegetation types and management regimes using a geographical information system (GIS). NPP varied by a factor close to two (432 - 709 g x Cx m(-2)x y(-1)), while net ecosystem production ranged between -124 and 159 gx C x m(-2) x y (-1) for the different catchments. Carbon sequestration mainly occurred in the vegetation while the soil organic carbon pool was mainly a source of carbon. Large herbivores consumed on average 4.5 % of the above-ground green tissue production. When arable land was present in the catchment, the flux of carbon to humans was highest from crops and, in decreasing order, milk and beef, followed by the flux from hunting and berry/fungus picking. The results can be used to estimate the potential assimilation of radionuclides in vegetation and the potential exposure to humans of bioavailable radionuclides.     

Forests under climate change and air pollution: gaps in understanding and future directions for research

Forests in Europe face significant changes in climate, which in interaction with air quality changes, may significantly affect forest productivity, stand composition and carbon sequestration in both vegetation and soils. Identified knowledge gaps and research needs include: (i) interaction between changes in air quality (trace gas concentrations), climate and other site factors on forest ecosystem response, (ii) significance of biotic processes in system response, (iii) tools for mechanistic and diagnostic understanding and upscaling, and (iv) the need for unifying modelling and empirical research for synthesis. This position paper highlights the above focuses, including the global dimension of air pollution as part of climate change and the need for knowledge transfer to enable reliable risk assessment. A new type of research site in forest ecosystems ("supersites") will be conducive to addressing these gaps by enabling integration of experimentation and modelling within the soil-plant-atmosphere interface, as well as further model development.     

Differentiating the effects of climate and land use change on European biodiversity: A scenario analysis

Current observed as well as projected changes in biodiversity are the result of multiple interacting factors, with land use and climate change often marked as most important drivers. We aimed to disentangle the separate impacts of these two for sets of vascular plant, bird, butterfly and dragonfly species listed as characteristic for European dry grasslands and wetlands, two habitats of high and threatened biodiversity. We combined articulations of the four frequently used SRES climate scenarios and associated land use change projections for 2030, and assessed their impact on population trends in species (i.e. whether they would probably be declining, stable or increasing). We used the BIOSCORE database tool, which allows assessment of the effects of a range of environmental pressures including climate change as well as land use change. We updated the species lists included in this tool for our two habitat types. We projected species change for two spatial scales: the EU27 covering most of Europe, and the more restricted biogeographic region of ‘Continental Europe’. Other environmental pressures modelled for the four scenarios than land use and climate change generally did not explain a significant part of the variance in species richness change. Changes in characteristic bird and dragonfly species were least pronounced. Land use change was the most important driver for vascular plants in both habitats and spatial scales, leading to a decline in 50–100% of the species included, whereas climate change was more important for wetland dragonflies and birds (40–50 %). Patterns of species decline were similar in continental Europe and the EU27 for wetlands but differed for dry grasslands, where a substantially lower proportion of butterflies and birds declined in continental Europe, and 50 % of bird species increased, probably linked to a projected increase in semi-natural vegetation. In line with the literature using climate envelope models, we found little divergence among the four scenarios. Our findings suggest targeted policies depending on habitat and species group. These are, for dry grasslands, to reduce land use change or its effects and to enhance connectivity, and for wetlands to mitigate climate change effects.     

Hydrological change--climate change impact simulations for Sweden

Climate change resulting from the enhanced greenhouse effect is expected to give rise to changes in hydrological systems. This hydrological change, as with the change in climate variables, will vary regionally around the globe. Impact studies at local and regional scales are needed to assess how different regions will be affected. This study focuses on assessment of hydrological impacts of climate change over a wide range of Swedish basins. Different methods of transferring the signal of climate change from climate models to hydrological models were used. Several hydrological model simulations using regional climate model scenarios from Swedish Regional Climate Modelling Programme (SWECLIM) are presented. A principal conclusion is that subregional impacts to river flow vary considerably according to whether a basin is in northern or southern Sweden. Furthermore, projected hydrological change is just as dependent on the choice of the global climate model used for regional climate model boundary conditions as the choice of anthropogenic emissions scenario.     

Modelling tundra vegetation response to recent arctic warming

The Arctic land area has warmed by > 1 °C in the last 30 years and there is evidence that this has led to increased productivity and stature of tundra vegetation and reduced albedo, effecting a positive (amplifying) feedback to climate warming. We applied an individual-based dynamic vegetation model over the Arctic forced by observed climate and atmospheric CO(2) for 1980-2006. Averaged over the study area, the model simulated increases in primary production and leaf area index, and an increasing representation of shrubs and trees in vegetation. The main underlying mechanism was a warming-driven increase in growing season length, enhancing the production of shrubs and trees to the detriment of shaded ground-level vegetation. The simulated vegetation changes were estimated to correspond to a 1.75 % decline in snow-season albedo. Implications for modelling future climate impacts on Arctic ecosystems and for the incorporation of biogeophysical feedback mechanisms in Arctic system models are discussed.     

Ecosystem response to climatic change: the importance of the cold season

Winter climate and snow cover are the important drivers of plant community development in polar regions. However, the impacts of changing winter climate and associated changes in snow regime have received much less attention than changes during summer. Here, we synthesize the results from studies on the impacts of extreme winter weather events on polar heathland and lichen communities. Dwarf shrubs, mosses and soil arthropods were negatively impacted by extreme warming events while lichens showed variable responses to changes in extreme winter weather events. Snow mould formation underneath the snow may contribute to spatial heterogeneity in plant growth, arthropod communities and carbon cycling. Winter snow cover and depth will drive the reported impacts of winter climate change and add to spatial patterns in vegetation heterogeneity. The challenges ahead lie in obtaining better predictions on the snow patterns across the landscape and how these will be altered due to winter climate change.     

Ecosystem Response to Climatic Change: The Importance of the Cold Season

Winter climate and snow cover are the important drivers of plant community development in polar regions. However, the impacts of changing winter climate and associated changes in snow regime have received much less attention than changes during summer. Here, we synthesize the results from studies on the impacts of extreme winter weather events on polar heathland and lichen communities. Dwarf shrubs, mosses and soil arthropods were negatively impacted by extreme warming events while lichens showed variable responses to changes in extreme winter weather events. Snow mould formation underneath the snow may contribute to spatial heterogeneity in plant growth, arthropod communities and carbon cycling. Winter snow cover and depth will drive the reported impacts of winter climate change and add to spatial patterns in vegetation heterogeneity. The challenges ahead lie in obtaining better predictions on the snow patterns across the landscape and how these will be altered due to winter climate change.     

Modelling Tundra Vegetation Response to Recent Arctic Warming

The Arctic land area has warmed by >1 °C in the last 30 years and there is evidence that this has led to increased productivity and stature of tundra vegetation and reduced albedo, effecting a positive (amplifying) feedback to climate warming. We applied an individual-based dynamic vegetation model over the Arctic forced by observed climate and atmospheric CO₂ for 1980–2006. Averaged over the study area, the model simulated increases in primary production and leaf area index, and an increasing representation of shrubs and trees in vegetation. The main underlying mechanism was a warming-driven increase in growing season length, enhancing the production of shrubs and trees to the detriment of shaded ground-level vegetation. The simulated vegetation changes were estimated to correspond to a 1.75 % decline in snow-season albedo. Implications for modelling future climate impacts on Arctic ecosystems and for the incorporation of biogeophysical feedback mechanisms in Arctic system models are discussed.     

The challenges of marine spatial planning in the Arctic: Results from the ACCESS programme

Marine spatial planning is increasingly used to manage the demands on marine areas, both spatially and temporally, where several different users may compete for resources or space, to ensure that development is as sustainable as possible. Diminishing sea-ice coverage in the Arctic will allow for potential increases in economic exploitation, and failure to plan for cross-sectoral management could have negative economic and environmental results. During the ACCESS programme, a marine spatial planning tool was developed for the Arctic, enabling the integrated study of human activities related to hydrocarbon exploitation, shipping and fisheries, and the possible environmental impacts, within the context of the next 30 years of climate change. In addition to areas under national jurisdiction, the Arctic Ocean contains a large area of high seas. Resources and ecosystems extend across political boundaries. We use three examples to highlight the need for transboundary planning and governance to be developed at a regional level.     

Implications of climate and land-use change for landscape processes,biodiversity, ecosystem services,and governance

This introduction to the Special Issue summarizes the results of 14 scientific articles from the interdisciplinary research program Ekoklim at Stockholm University, Sweden. In this program, we investigate effects of changing climate and land use on landscape processes, biodiversity, and ecosystem services, and analyze issues related to adaptive governance in the face of climate and land-use change. We not only have a research focus on the 22 650 km² Norrström catchment surrounding lake Mälaren in south-central Sweden, but we also conduct research in other Swedish regions. The articles presented here show complex interactions between multiple drivers of change, as well as feedback processes at different spatiotemporal scales. Thus, the Ekoklim program highlights and deals with issues relevant for the future challenges society will face when land-use change interacts with climate change.     

Spitsbergen landscape under 20th century climate change: Sørkapp Land

A reaction of the European Arctic landscape to a climate change on the scale of a typical middle-sized region is outlined. A wide scope of the methods was used, first of all field mapping and observations. Glaciers are important in the S?rkapp Land landscape because they cover the majority of its territory and undergo quick recessions as a result of the 20th century warming. Glacial recession influence intensively: relief with Quaternary deposits, waters, animals, vegetation and soils. The most important landscape changes in the 20th century are: uplift of the equilibrium line altitude on glaciers by 100-200 m; large glacial recession in both surface and volume; significant decrease of the land area due to recession of tidewater glaciers; lengthening of the coastline, and especially of glacial cliffs; development of the land water network; start of the plant succession in areas abandoned by glaciers. No isostatic uplift has taken place in S?rkapp Land since the Little Ice Age.