The global carbon cycle is subject to intense research, where sources and sinks for greenhouse gases, carbon dioxide in particular, are estimated for various systems and biomes. Lakes have previously been neglected in carbon balance estimations, but have recently been recognized to be significant net sources of CO2.

This thesis estimates emission of carbon dioxide (CO2) from boreal lakes and factors regulating the CO2 saturation from field measurements of CO2 concentration along with a number of chemical, biological and physical parameters. Concentration of dissolved organic carbon (DOC) was found to be the most important factor for CO2 saturation in lake water, whereas climatic parameters such as precipitation, temperature and global radiation were less influential. All lakes were supersaturated with and, thus, sources of CO2. Sediment incubation experiments indicated that in-lake mineralization processes during summer stratification mainly occurred in the pelagial. Approximately 10% of the CO2 emitted from the lake surface was produced in epilimnetic sediments.

The mineralization of DOC and emission of CO2 from freshwaters was calculated on a catchment basis for almost 80,000 lakes and 21 major catchments in Sweden, together with rates of sedimentation in lakes and export of organic carbon to the sea. The total export of terrestrial organic carbon to freshwaters could thereby be estimated and consequently also the importance of lakes for the withdrawal of organic carbon export from terrestrial sources to the sea. Lakes removed 30-80% of imported terrestrial organic carbon, and mineralization and CO2 emission were much more important than sedimentation of carbon. The carbon loss was closely related to water retention time, where catchments with short residence times (<1 year) had low carbon retentions, whereas in catchments with long residence times (>3 years) a majority of the imported TOC was removed in the lake systems.

The Gulf of Bothnia was also studied in this thesis and found to be a net heterotrophic system, emitting large amounts of CO2 to the atmosphere on an annual basis. The rate of CO2 emission was depending on the balance between primary production and bacterial respiration, and the system was oscillating between being a source and a sink of CO2.

The trophic state of lakes is commonly defined by the concentration of nutrients in the water column. High nutrient concentrations generate high phytoplankton production, and lakes with low nutrient concentrations are considered low-productive. This simplified view of lake productivity ignores the fact that benthic primary producers and heterotrophic bacteria can be important basal producers in lake ecosystems.

In this thesis I have studied clear-water and brown-water lakes with respect to primary production, respiration and bacterial production based on allochthonous organic carbon. These processes were quantified in pelagic and benthic habitats on temporal and spatial scales. I also calculated the net ecosystem production of the lakes, defined as the difference between gross primary production (GPP) and respiration (R). The net ecosystem production indicates whether a lake is net heterotrophic (GPP < R), net autotrophic (GPP > R) or in metabolic balance (GPP = R). Net heterotrophic lakes are sources of carbon dioxide (CO₂) to the atmosphere since respiration in these lakes, by definition, is subsidized by an external organic carbon source. External organic carbon is transported to lakes from the terrestrial environment via inlets, and can serve as a carbon source for bacteria but it also limits light availability for primary producers by absorbing light.

On a seasonal scale, four of the clear-water lakes studied in this thesis were dominated by primary production in the soft-bottom benthic habitat and by respiration in the pelagic habitat. Concentrations of dissolved organic carbon (DOC) were low in the lakes, but still high enough to cause the lakes to be net heterotrophic. However, the lakes were not low-productive due to the high production in the benthic habitat. One of the clear-water lakes was studied also during the winter and much of the respiration under ice was supported by the benthic primary production from the previous summer. This is in contrast to brown-water lakes where winter respiration is suggested to be supported by allochthonous organic carbon.

By studying lakes in a DOC gradient (i.e. from clear-water to brown-water lakes) I could draw two major conclusions. The lakes became less productive since benthic primary production decreased with increasing light extinction, and the lakes became larger sources of CO₂ to the atmosphere since pelagic respiration was subsidized by allochthonous organic carbon. Thus, lake carbon metabolism can have an important role in the global carbon cycle due to their processing of terrestrial organic carbon and to their possible feedback effects on the climate system.

A land cover map of two arctic catchments near the Abisko Scientific Research Station was obtained based on a classification from a Sentinel-2 satellite image and a ground survey performed in July 2022. The two contiguous catchments, Miellajokka and Stordalen, are covered by various ecotypes, from boreal forest to alpine tundra and peatland. Two classification algorithms, support vector machine and random forest, were tested and gave very similar results. The percentage of correctly classified pixels was over 88% in both cases. The developed workflow relies solely on open-source software and acquired ground observations. Space organization was directed by the altitude as demonstrated by the intersection of the land cover with the topography. Comparison between this new land cover map and previous ones based on data acquired between 2008 and 2011 shows some trends in vegetation cover evolution in response to climate change in the considered area. This land cover map is key input data for permafrost modeling and, hence, for the quantification of climate change impacts in the studied area.

Soils represent the largest store of carbon in the biosphere with soils at high latitudes containing twice as much carbon (C) than the atmosphere. High latitude tundra vegetation communities show increases in the relative abundance and cover of deciduous shrubs which may influence net ecosystem exchange of CO2 from this C-rich ecosystem. Monitoring soil respiration (Rs) as a crucial component of the ecosystem carbon balance at regional scales is difficult given the remoteness of these ecosystems and the intensiveness of measurements that is required. Here we use direct measurements of Rs from contrasting tundra plant communities combined with direct measurements of aboveground plant productivity via Normalised Difference Vegetation Index (NDVI) to predict soil respiration across four key vegetation communities in a tundra ecosystem. Soil respiration exhibited a nonlinear relationship with NDVI (y = 0.202e3.508 x, p &lt; 0.001). Our results further suggest that NDVI and soil temperature can help predict Rs if vegetation type is taken into consideration. We observed, however, that NDVI is not a relevant explanatory variable in the estimation of SOC in a single-study analysis.

Rivers transport large amounts of allochthonous organic matter (OM) to the ocean every year, but there are still fundamental gaps in how allochthonous OM is processed in the marine environment. Here, we estimated the relative contribution of allochthonous OM (allochthony) to the biomass of benthic and pelagic consumers in a shallow coastal ecosystem in the northern Baltic Sea. We used deuterium as a tracer of allochthony and assessed both temporal variation (monthly from May to August) and spatial variation (within and outside river plume). We found variability in allochthony in space and time and across species, with overall higher values for zoobenthos (26.2 +/- 20.9%) than for zooplankton (0.8 +/- 0.3%). Zooplankton allochthony was highest in May and very low during the other months, likely as a result of high inputs of allochthonous OM during the spring flood that fueled the pelagic food chain for a short period. In contrast, zoobenthos allochthony was only lower in June and remained high during the other months. Allochthony of zoobenthos was generally higher close to the river mouth than outside of the river plume, whereas it did not vary spatially for zooplankton. Last, zoobenthos allochthony was higher in deeper than in shallower areas, indicating that allochthonous OM might be more important when autochthonous resources are limited. Our results suggest that climate change predictions of increasing inputs of allochthonous OM to coastal ecosystems may affect basal energy sources supporting coastal food webs.

There is compelling evidence from experiments and observations that climate warming prolongs the growing season in arctic regions. Until now, the start, peak, and end of the growing season, which are used to model influences of vegetation on biogeochemical cycles, were commonly quantified using above-ground phenological data. Yet, over 80% of the plant biomass in arctic regions can be below ground, and the timing of root growth affects biogeochemical processes by influencing plant water and nutrient uptake, soil carbon input and microbial activity. We measured timing of above- and below-ground production in three plant communities along an arctic elevation gradient over two growing seasons. Below-ground production peaked later in the season and was more temporally uniform than above-ground production. Most importantly, the growing season continued c. 50% longer below than above ground. Our results strongly suggest that traditional above-ground estimates of phenology in arctic regions, including remotely sensed information, are not as complete a representation of whole-plant production intensity or duration, as studies that include root phenology. We therefore argue for explicit consideration of root phenology in studies of carbon and nutrient cycling, in terrestrial biosphere models, and scenarios of how arctic ecosystems will respond to climate warming.

The high-resolution Sentinel-2 data potentially enable the estimation of gross primary productivity (GPP) at finer spatial resolution by better capturing the spatial variation in a heterogeneous landscapes. This study investigates the potential of 10 m resolution reflectance from the Sentinel-2 Multispectral Instrument to improve the accuracy of GPP estimation across Nordic vegetation types, compared with the 250 m and 500 m resolution reflectance from the Moderate Resolution Imaging Spectroradiometer (MODIS). We applied linear regression models with inputs of two-band enhanced vegetation index (EVI2) derived from Sentinel-2 and MODIS reflectance, respectively, together with various environmental drivers to estimate daily GPP at eight Nordic eddy covariance (EC) flux tower sites. Compared with the GPP from EC measurements, the accuracies of modelled GPP were generally high (R-2 = 0.84 for Sentinel-2; R-2 = 0.83 for MODIS), and the differences between Sentinel-2 and MODIS were minimal. This demonstrates the general consistency in GPP estimates based on the two satellite sensor systems at the Nordic regional scale. On the other hand, the model accuracy did not improve by using the higher spatial-resolution Sentinel-2 data. More analyses of different model formulations, more tests of remotely sensed indices and biophysical parameters, and analyses across a wider range of geographical locations and times will be required to achieve improved GPP estimations from Sentinel-2 satellite data.

Permafrost thaw resulting from climate warming is threatening to release carbon from high latitude peatlands. The aim of this research was to determine subsidence rates linked to permafrost thaw in sub-Arctic peatlands in Sweden using historical orthophotographic (orthophotos), Unoccupied Aerial Vehicle (UAV), and Interferometric Synthetic Aperture Radar (InSAR) data. The orthophotos showed that the permafrost palsa on the study sites have been contracting in their areal extent, with the greatest rates of loss between 2002 and 2008. The surface motion estimated from differential digital elevation models from the UAV data showed high levels of subsidence (maxi-mum of −25 cm between 2017 and 2020) around the edges of the raised palsa plateaus. The InSAR data analysis showed that raised palsa areas had the greatest subsidence rates, with maximum subsidence rates of 1.5 cm between 2017 and 2020; however, all wetland vegetation types showed sub-sidence. We suggest that the difference in spatial units associated with each sensor explains parts of the variation in the subsidence levels recorded. We conclude that InSAR was able to identify the areas most at risk of subsidence and that it can be used to investigate subsidence over large spatial extents, whereas UAV data can be used to better understand the dynamics of permafrost degradation at a local level. These findings underpin a monitoring approach for these peatlands.

Paleoglaciological reconstructions based on glacial geological and geomorphological traces are used to test and constrain numerical models of ice sheet extent and dynamics. MAGIC-DML (“Mapping, Measuring and Modelling Antarctic Geomorphology and Ice Change in Dronning Maud Land”) project is trying to reconstruct the timing and pattern of ice surface elevation changes since the mid-Pliocene across western Dronning Maud Land, East Antarctica. The study area has sparse pre-existing field data and considerable ice sheet model uncertainties. A remote sensing-based mapping of glacial geomorphology on nunataks and structures on the ice sheet surface is presented for a coastal-inland transect including Ahlmannryggen, Borgmassivet, and Kirwanveggen using high-resolution WorldView imagery. The primary aim of the study is to map traces of a thicker ice sheet on nunatak slopes that were formerly partly or entirely covered during ice surface highstands. Panchromatic and multispectral images were analysed in a multi-step procedure using ArcGIS, including image processing and mosaicking, visual feature recognition, and mapping. The identification of key landforms (such as till veneers and erratic boulders) required the adoption of some assumptions to differentiate, for example, till from regolith. Where patterned ground was mapped, we infer a presence of till rather than regolith because subglacial erosion is more likely to produce finer material than subaerial weathering. Very large boulders on plateau surfaces are mapped as erratics because they could not have been delivered by slope processes to local highpoints. However, the reliability of derived paleo-ice sheet reconstructions is limited by both the necessary assumptions and the absence of crosscutting relationships between landforms. At face value, the presence of till cover and erratics above the present ice surface on some nunataks indicate thicker ice in the past. According to the geomorphological mapping of the transect, in Kirwanveggen the former ice elevation was at least 100 m higher, in Borgmassivet the ice lowered more than 600 m and in Ahlmannryggen the ice was at least 300 m thicker. Additional mapping of structures on the ice sheet surface is used to yield target field routes for upcoming field season(s) to potential cosmogenic nuclide (CN) sampling locations. The chronology derived from CN dating will permit the delineation of ice sheet surface elevations as targets for ice sheet modeling.

The geomorphological impact of small fossorial mammals (adapted to digging and living underground), such as rodents can be significant, and both their direct and indirect effects may contribute to landscape formation. This thesis is based on empirical field studies of two burrowing rodent species in sub-polar environments, namely invasive House mice (Mus musculus) on sub-Antarctic Marion Island and Norwegian lemmings (Lemmus lemmus) in sub-Arctic Abisko. The spatial distribution, sediment displacements, impact on vegetation and microclimatic effects of the rodents are documented.

Invasive mice and rats, introduced on sub-Antarctic Islands during the 19th century, lack natural enemies and are shown to have a significant direct and indirect geomorphic impact by direct sediment displacement, vegetation removal by burrowing, grazing and trampling and thereby exposing the sediments for rain, wind and frost processes.  The geomorphic impacts of lemmings are comparatively more limited as they rely on natural hollows and snow cover for protection and do not burrow to the same extent as other fossorial rodents in cold regions. Lemmings are thus suggested to have little impact on landform integrity, but can affect vegetation composition.

A comparison of the findings from this study with published data on seven other rodent species and other physical mass transfer mechanisms in sub-polar and alpine environments suggests that fossorial rodents are a significant and sometimes dominant geomorphic force in sub-polar and alpine environments. The geomorphic work by ground squirrels, ice rats, plateau pikas and zokors is shown to be in the same order of magnitude as solifluction and rock falls. In alpine and periglacial environments these rodents are considered to act as key-stone species and ecosystem engineers through the creation of landforms by  dislocation and of soil and other impacts on soil properties, vegetation and ecosystem function

The northern permafrost regions are experiencing a rapid warming as surface temperatures are rising, causing a disintegration of permafrost and a deepening of the active layer (AL). This releases previously frozen carbon, making it available for decomposition by microbes. The combination of the high microbial activity and overall wetter soils may cause anoxic conditions and in turn methane (CH4) and carbon dioxide (CO2) release into the atmosphere, further contributing to warming of the climate. The main drivers, and therefore, the magnitude of CO2 and CH4 fluxes may vary spatially (CO2/CH4). Thus, the aim of this study is to investigate the influence of spatial variability of site-specific conditions such as vegetation composition, AL depth, water table depth (WTD) on the magnitude of carbon fluxes (CO2/CH4) between and within sites. Multiple replicate measurements of CO2 and CH4 concentrations, WTD, AL depth, air temperature (Tair) and soil temperature (Tsoil) were taken from three different locations in Abisko, Sweden. The three study sites had varying stages of permafrost degradation: Storflaket had a relative stable permafrost, Kursflaket is currently undergoing permafrost degradation and Katterjokk has undergone a complete permafrost loss over last few decades. The results showed significant differences in CO2 and CH4 fluxes between and within the sites. The CH4 emissions and CO2 uptake were significantly higher in the site with completely disintegrated permafrost (Katterjokk), compared to the other two sites with permafrost presence. CH4 fluxes were also significantly higher for wet plots, compared to dry plots. The CH4 emissions were shown to be mainly driven by the WTD and AL depth as well as the abundance of aerenchymateous vegetation. No significant relationship between the investigated variables and CO2 fluxes could be found. However, there was a significant difference in ecosystem respiration (Reco) between the wet and dry plots, indicating that there may be a relationship between WTD and CO2. The results demonstrated that even within the Abisko region, there were considerable variations in carbon fluxes as well as drivers of the fluxes between and within the sites. The differences in carbon fluxes and the site-specific conditions are important to take into consideration when extrapolating and generalising for larger areas. Furthermore, a continued disintegration of permafrost and deepening of the AL, may further alter the sub arctic ecosystem of Abisko and thereby enhance the spatial variability, as site-specific conditions continue to change. Moreover, further permafrost disintegration on a global scale may lead to even more CH4 emissions, amplifying the initial warming.

Mountain centered glaciers have played a major role throughout the last three million years in the Scandinavian mountains. The climatic extremes, like the present warm interglacial or cold glacial maxima, are very short-lived compared to the periods of intermediate climate conditions, characterized by the persistence of mountain based glaciers and ice fields of regional size. These have persisted in the Scandinavian mountains for about 65% of the Quaternary. Mountain based glaciers thus had a profound impact on large-scale geomorphology, which is manifested in large-scale glacial landforms such as fjords, glacial lakes and U-shaped valleys in and close to the mountain range.

Through a mapping of glacial landforms in the northern Scandinavian mountain range, in particular a striking set of lateral moraines, this thesis offers new insights into Weichselian stages predating the last glacial maximum. The aerial photograph mapping and field evidence yield evidence that these lateral moraines were overridden by glacier ice subsequent to their formation. The lateral moraines were dated using terrestrial cosmogenic nuclide techniques. Although the terrestrial cosmogenic nuclide signature of the moraines is inconclusive, an early Weichselian age is tentatively suggested through correlations with other landforms and stratigraphical archives in the region. The abundance and coherent spatial pattern of the lateral moraines also allow a spatial reconstruction of this ice field. The ice field was controlled by topography and had nunataks protruding also where it was thickest close to the elevation axis of the Scandinavian mountain range. Outlet glaciers discharged into the Norwegian fjords and major valleys in Sweden.

The process by which mountain based glaciers grow into an ice sheet is a matter of debate. In this thesis, a feedback mechanism between debris on the ice surface and ice sheet growth is presented. In essence, the growth of glaciers and ice sheets may be accelerated by an abundance of debris in their ablation areas. This may occur when the debris cover on the glacier surface inhibits ablation, effectively increasing the glaciers mass balance. It is thus possible that a dirty ablation area may cause the glacier to advance further than a clean glacier under similar conditions. An ice free period of significant length allows soil production through weathering, frost shattering, and slope processes. As glaciers advance through this assemblage of sediments, significant amounts of debris end up on the surface due to both mass wastage and subglacial entrainment. Evidence that this chain of events may occur, is given by large expanses of hummocky moraine (local name Veiki moraine) in the northern Swedish lowlands. Because the Veiki moraine has been correlated with the first Weichselian advance following the Eemian, it implies a heavily debris charged ice sheet emanating from the mountain range and terminating in a stagnant fashion in the lowlands.

Relict non-glacial surfaces occur in many formerly glaciated landscapes, where they represent areas that have escaped significant glacial modification. Frequently distinguished by blockfield mantles, relict non-glacial surfaces are important archives of long-term weathering and landscape evolution processes. The aim of this thesis is to examine the distribution, weathering, ages, and formation of relict non-glacial surfaces in the northern Swedish mountains.

Mapping of surfaces from aerial photographs and analysis in a GIS revealed five types of relict non-glacial surfaces that reflect differences in surface process types or rates according to elevation, gradient, and bedrock lithology. Clast characteristics and fine matrix granulometry, chemistry, and mineralogy reveal minimal chemical weathering of the blockfields.

Terrestrial cosmogenic nuclides were measured in quartz samples from two blockfield-mantled summits and a numerical ice sheet model was applied to account for periods of surface burial beneath ice sheets and nuclide production rate changes attributable to glacial isostasy. Total surface histories for each summit are almost certainly, but not unequivocally, confined to the Quaternary. Maximum modelled erosion rates are as low as 4.0 mm/kyr, which is likely to be near the low extreme for relict non-glacial surfaces in this landscape.

The blockfields of the northern Swedish mountains are Quaternary features formed through subsurface physical weathering processes. While there is no need to appeal to Neogene chemical weathering to explain blockfield origins, these surfaces have remained continuously regolith-mantled and non-glacial since their inception. Polygenetic surface histories are therefore indicated, where the large-scale surface morphologies are potentially older than their regolith mantles.

In the boreal forests of the Northern Hemisphere, time series of tree-ring width (TRW) and maximum density in the latewood (MXD) are highly correlated to local instrumental summer-temperature data and are thus widely used as proxies in high-resolution climate reconstructions. Hence, much of our present knowledge about climatic variability in the last millennium is based on tree-rings. However, many tree-ring records have a lack of data in the most recent decades, which severely hampers our ability to place the recent temperature increase in a longer-timescale perspective of natural variability.

The main objective of this thesis is to update and extend the Torneträsk TRW and MXD records in northern Sweden. Local instrumental climate-data is used to calibrate the new tree-ring records. The results show that TRW is mainly forced by temperature in the early growing season (June/July) while MXD has a wider response window (June – August) and has a higher correlation to temperature. Two reconstructions of summer temperature are made for (i) the last 7,400 years based on TRW, and (ii) the last 1,500 years based on a combination of TRW and MXD. The reconstructions show natural variability on timescales from years to several centuries. The 20th century does not stand out as a notably warm period in the long timescale perspective. A medieval period from AD 900 – 1100 is markedly warmer than the 20th century.

The environmental impact from a large explosive volcanic eruption in 1628/1627 BC is analysed in the tree rings of 14C-dated bog pines in south-central Sweden and in absolutely-dated subfossil pines from Torneträsk. The results show evidence of an impact in the southern site at approximately this time but no detectable impact in the North.

Subfossil trees of Fitzroya cupressoides in southern Chile were 14C-dated to approx. 50,000 years BP and amalgamated into a 1,229-year TRW chronology. This tree-ring record is the oldest in the world. The variability in this Last-glacial chronology is similar to the variability in present-day living trees of the same species. These results suggest that the growth–forcing mechanisms 50,000 years ago were similar to those at present.

Tree-ring widths from 880 living, dry dead, and subfossil northern Swedish pines (Pinus syl vestris L.) have been assembled into a continuous and precisely dated chronology (the Torneträsk chronology) covering the period 5407 BC to ad 1997. Biological trends in the data were removed with autoregressive standardization (ARS) to emphasize year-to-year variability, and with regional curve stan dardization (RCS) to emphasize variability on timescales from decades to centuries. The strong association with summer mean temperature (June–August) has enabled the production of a temperature reconstruction for the last 7400 years, providing information on natural summer-temperature variability on timescales from years to centuries. Numerous cold episodes, comparable in severity and duration to the severe summers of the seventeenth century, are shown throughout the last seven millennia. Particularly severe conditions suggested between 600 and 1 BC correspond to a known period of glacier expansion. The relatively warm conditions of the late twentieth century do not exceed those reconstructed for several earlier time intervals, although replication is relatively poor and confidence in the reconstructions is correspondingly reduced in the pre-Christian period, particularly around 3000, 1600 and 330 bc. Despite the use of the RCS approach in chronology construction, the 7400-year chronology does not express the full range of millennial-timescale temperature change in northern Sweden.

Arctic environmental change induces shifts in high-latitude plant community composition and stature with implications for Arctic carbon cycling and energy exchange. Two major components of change in high-latitude ecosystems are the advancement of trees into tundra and the increased abundance and size of shrubs. How future changes in key climatic and environmental drivers will affect distributions of major ecosystem types is an active area of research. Dynamic vegetation models (DVMs) offer a way to investigate multiple and interacting drivers of vegetation distribution and ecosystem function. We employed the LPJ-GUESS tree-individual-based DVM over the Torneträsk area, a sub-arctic landscape in northern Sweden. Using a highly resolved climate dataset to downscale CMIP5 climate data from three global climate models and two 21st-century future scenarios (RCP2.6 and RCP8.5), we investigated future impacts of climate change on these ecosystems. We also performed model experiments where we factorially varied drivers (climate, nitrogen deposition and [CO2]) to disentangle the effects of each on ecosystem properties and functions. Our model predicted that treelines could advance by between 45 and 195 elevational metres by 2100, depending on the scenario. Temperature was a strong driver of vegetation change, with nitrogen availability identified as an important modulator of treeline advance. While increased CO2 fertilisation drove productivity increases, it did not result in range shifts of trees. Treeline advance was realistically simulated without any temperature dependence on growth, but biomass was overestimated. Our finding that nitrogen cycling could modulate treeline advance underlines the importance of representing plant–soil interactions in models to project future Arctic vegetation change.

Traditional optical remote sensing data have been widely used for snow cover detection and monitoring. However, they are limited to daytime detection and often suffer from large data gaps due to frequent cloud obscuration. This is in particular a serious challenge for high-latitude and polar regions where long nights prevail during the winter. Nighttime light sensors have a strong capability of sensing the low-level reflected moonlight. They potentially provide a new way to detect snow cover. In this study, we quantitatively analyzed the moonlight intensity for snow detection and developed a Minimum Error Thresholding (MET) algorithm to detect snow cover from the data collected by Suomi National Polar-orbiting Partnership Visible Infrared Imaging Radiometer Suite (S-NPP VIIRS) satellite data. For the two case study sites, Abisko in the sub-Arctic zone and the Tibetan Plateau, our analysis results suggest that the moonlight provides sufficient illumination to map snow cover for approximately 10 days in a lunar month. Our nighttime snow cover detection method was quantitatively evaluated by comparing our S-NPP VIIRS DNB snow cover estimates with in situ station observations, Interactive Multisensor Snow and Ice Mapping System (IMS) snow cover products, and Moderate Resolution Imaging Spectroradiometer (MODIS) snow cover products over Abisko region and the Tibetan Plateau during the 2017–2018 snow season. The overall accuracy of S-NPP VIIRS snow cover estimates was approximately 80.3% in Abisko region and 76.7% in the Tibetan Plateau. The data gaps in our S-NPP VIIRS DNB snow cover estimates were smaller than those of the MODIS snow cover products by 22.1% and 5.1% over Abisko region and the Tibetan Plateau, respectively. Further, we found that nearly 92.8% and 74.6% of data gaps in the MODIS snow-cover product can be filled up by incorporating our S-NPP VIIRS DNB snow cover estimates in Abisko region and the Tibetan Plateau. The total accuracy of daily MODIS snow cover products can be improved to 91.0% in the Tibetan Plateau. Our results indicate that S-NPP VIIRS DNB nighttime satellite data can provide reliable snow products over polar regions and mid-latitude mountainous areas, which is complementary to the standard MODIS snow cover products.

Waterborne carbon (C) forms an active and significant part of the global C cycle, which is important in theArctic where greater temperature increases and variability are anticipated relative to the rest of the globe withpotential implications for the C cycle. Understanding and quantification of the current processes governing themovement of C by connecting terrestrial and marine systems is necessary to better estimate future changes ofwaterborne C. This thesis investigates how the sub-arctic landscape influences the waterborne carbon exportby combining data-driven and modeling methods across spatial and temporal scales. First, a study of the stateof total organic carbon monitoring in northern Scandinavia was carried out using national-scale monitoringdata and detailed data from scientific literature. This study, which highlights the consistency in land cover andhydroclimatic controls on waterborne C across northern Scandinavia, was combined with three more detailedstudies leveraging field measurements and modeling. These focused on the Abisko region to provide insightto processes and mechanisms across scales. The thesis highlights that the governing transport mechanismsof dissolved organic and inorganic carbon (DOC and DIC respectively) are fundamentally different due todifferences in release rates associated with the nature of their terrestrial sources (geogenic and organic matterrespectively). As such, the DIC mass flux exhibits a high flow-dependence whereas DOC is relatively flowindependent.Furthermore, these investigations identified significant relationships between waterborne C andbiogeophysical as well as hydroclimatic variables across large to small spatial scales. This thesis demonstratesthat both surface and sub-surface hydrological processes (such as flow pathway distributions) in combinationwith distributions of C sources and associated release rates are prerequisite for understanding waterborne Cdynamics in northern streams.

The empirical quantification of rates of material movement in cryoturbated soils has lagged behind the physical and chemical characterisation of these materials. We applied a novel suite of elemental (C, Hg), stable isotope (C-13) and radioisotope (Cs-137, Pb-210, C-14, Be-10) tracers in conjunction with analytical and numerical models to constrain the rates and patterns of soil movement due to cryoturbation in a non-sorted circle (NSC) near Abisko, Sweden. We present the first observations of the variability of Be-10 across a patterned-ground feature, which facilitate the interpretation of subsurface peaks in soil organic carbon, Hg and C-13 and provide constraints on the surficial histories of cryoturbated materials. Apparent rates of surficial lateral movement across the NSC estimated from Cs-137 and Pb-210 (0-2.55cm year(-1)) decreased with distance from its centre and were an order of magnitude greater than rates of subduction and subsurface movement estimated from C-14 (0.04-0.27cm year(-1)). Novel estimates of the original surficial residence times of cryoturbated parcels based on excess Be-10 and Hg inventories ranged from 238 to 3940years. Our results demonstrate the utility of the spatially explicit application of elemental and radioisotopic tracer suites to constrain cryoturbation rates in Arctic patterned ground.

This thesis investigates how variations in the oxygen isotopic composition of lake waters in northern Fennoscandia are recorded in lake sediment archives, especially diatoms, and how these variations can be used to infer past changes in climate and atmospheric circulation. Results from analyses of the oxygen isotopic composition of lake water samples (δ¹⁸O_lakew) collected between 2001 and 2006 show that δ¹⁸O of northern Fennoscandian lakes is mainly controlled by the isotopic composition of the precipitation (δ¹⁸O_p). Changes in local δ¹⁸O_p depend on variations in ambient air temperature and changes in atmospheric circulation that lead to changes in moisture source, vapour transport efficiency, or winter to summer precipitation distribution. This study demonstrates that the amount of isotopic variation in lake water δ¹⁸O is determined by a combination of the original δ¹⁸O_lakew, the amount and timing of the snowmelt, the amount of seasonally specific precipitation and groundwater, any evaporation effects, and lake water residence time. The fact that the same isotope shifts have been detected in various δ¹⁸O_lakew proxies, derived from hydrologically different lakes, suggests that these records reflect regional atmospheric circulation changes. The results indicate that diatom biogenic silica isotope (δ¹⁸O_diatom) records can provide important information about changes in atmospheric circulation that can help explain temperature and precipitation changes during the Holocene. The reconstructed long-term Holocene decreasing δ¹⁸O_p trend was likely forced by a shift from strong zonal westerly airflow (relatively high δ¹⁸O_p) in the early Holocene to a more meridional flow pattern (relatively low δ¹⁸O_p). The large δ¹⁸O_lakew depletion recorded in the δ¹⁸O records around ca. 500 cal yr BP (AD 1450) may be due to a shift to more intense meridional airflow over northern Fennoscandia resulting in an increasing proportion of winter precipitation from the north or southeast. This climate shift probably marks the onset of the so-called Little Ice Age in this region.

This thesis investigates how variations in the oxygen isotopic composition of lake waters in northern Fennoscandia are recorded in lake sediment archives, especially diatoms, and how these variations can be used to infer past changes in climate and atmospheric circulation. Results from analyses of the oxygen isotopic composition of lake water samples (δ¹⁸O_lakew) collected between 2001 and 2006 show that δ¹⁸O of northern Fennoscandian lakes is mainly controlled by the isotopic composition of the precipitation (δ¹⁸O_p). Changes in local δ¹⁸O_p depend on variations in ambient air temperature and changes in atmospheric circulation that lead to changes in moisture source, vapour transport efficiency, or winter to summer precipitation distribution. This study demonstrates that the amount of isotopic variation in lake water δ¹⁸O is determined by a combination of the original δ¹⁸O_lakew, the amount and timing of the snowmelt, the amount of seasonally specific precipitation and groundwater, any evaporation effects, and lake water residence time. The fact that the same isotope shifts have been detected in various δ¹⁸O_lakew proxies, derived from hydrologically different lakes, suggests that these records reflect regional atmospheric circulation changes. The results indicate that diatom biogenic silica isotope (δ¹⁸O_diatom) records can provide important information about changes in atmospheric circulation that can help explain temperature and precipitation changes during the Holocene. The reconstructed long-term Holocene decreasing δ¹⁸O_p trend was likely forced by a shift from strong zonal westerly airflow (relatively high δ¹⁸O_p) in the early Holocene to a more meridional flow pattern (relatively low δ¹⁸O_p). The large δ¹⁸O_lakew depletion recorded in the δ¹⁸O records around ca. 500 cal yr BP (AD 1450) may be due to a shift to more intense meridional airflow over northern Fennoscandia resulting in an increasing proportion of winter precipitation from the north or southeast. This climate shift probably marks the onset of the so-called Little Ice Age in this region.

Lakes in sub-Arctic regions have the potential of retaining many different aspects of water isotope composition in their sediments which can be used for palaeoclimate reconstruction. It is therefore important to understand the modern isotope hydrology of these lakes. Here we discuss the significance of variations in water isotope composition of a series of lakes located in north-west Swedish Lapland. Climate in this region is forced by changes in the North Atlantic which renders it an interesting area for climate reconstructions. We compare δ¹⁸O_lake and δ²H_lake collected between 2001 and 2006 and show that the lakes in this sub-Arctic region are currently mainly recharged by shallow groundwater and precipitation which undergoes little subsequent evaporation, and that the d¹⁸O and δ²H composition of input to the majority of the lakes varies on a seasonal basis between winter precipitation (and spring thaw) and summer precipitation. Seasonal variations in the isotopic composition of the lake waters are larger in lakes with short residence times (<6 months), which react faster to seasonal changes in the precipitation, compared to lakes with longer residence times (>6 months), which retain an isotopic signal closer to that of annual mean precipitation. Lake waters also show a range of isotope values between sites due to catchment elevation and timing of snow melt. The lake water data collected in this study was supported by isotope data from lake waters, streams and ground waters from1995 to 2000 reported in other studies.

Peatlands play an important role in the global carbon cycle as they contain a large soil carbon stock. However, current climate change could potentially shift peatlands from being carbon sinks to carbon sources. Remote sensing methods provide an opportunity to monitor carbon dioxide (CO2) exchange in peatland ecosystems at large scales under these changing conditions. In this study, we developed empirical models of the CO2 balance (net ecosystem exchange, NEE), gross primary production (GPP), and ecosystem respiration (ER) that could be used for upscaling CO2 fluxes with remotely sensed data. Two to three years of eddy covariance (EC) data from five peatlands in Sweden and Finland were compared to modelled NEE, GPP and ER based on vegetation indices from 10 m resolution Sentinel-2 MSI and land surface temperature from 1 km resolution MODIS data. To ensure a precise match between the EC data and the Sentinel-2 observations, a footprint model was applied to derive footprint-weighted daily means of the vegetation indices. Average model parameters for all sites were acquired with a leave-one-out-cross-validation procedure. Both the GPP and the ER models gave high agreement with the EC-derived fluxes (R2 = 0.70 and 0.56, NRMSE = 14% and 15%, respectively). The performance of the NEE model was weaker (average R2 = 0.36 and NRMSE = 13%). Our findings demonstrate that using optical and thermal satellite sensor data is a feasible method for upscaling the GPP and ER of northern boreal peatlands, although further studies are needed to investigate the sources of the unexplained spatial and temporal variation of the CO2 fluxes.

Metabolism is one of the most fundamental ecosystem processes, but the drivers of variation in metabolic rates among lakes dominated by benthic primary producers remain poorly constrained. Here, we report the magnitudes and potential drivers of whole-lake metabolism across 43 Swedish arctic–alpine lakes, based on the free-water diel oxygen technique with sondes deployed during the open-water season near the surface and bottom of the lakes. Gross primary production (GPP) and ecosystem respiration (R) were strongly coupled and ranged from 0.06 to 0.45 mg and 0.05 to 0.43 mg L⁻¹ d⁻¹ among lakes. On average, GPP and R decreased eightfold from relatively shallow to deep lakes (mean depth 0.5–10.9 m) and twofold from concave to convex lakes (mean depth: maximum depth 0.2–0.5). We attribute this to light limitation and shape-specific sensitivity of benthic GPP to disturbance by lake ice. Net ecosystem production (GPP-R) ranged from −0.09 to 0.14 mg L⁻¹ d⁻¹ and switched, on average, from positive to negative towards deeper lakes and lakes richer in dissolved organic carbon (DOC; 0.5–7.4 mg DOC L⁻¹). Uncertainties in metabolism estimates were high (around one and three times mean R and GPP), especially in deep lakes with low insulation and diurnally variable wind speed. Our results confirm the role of DOC in stimulating net heterotrophy and highlight novel effects of lake shape on productivity in benthic-dominated lake ecosystems and its response to changes in lake ice cover.

Treeline advance during the past 100 years was assessed by repeat in situ measurement at 14 locations distributed along the entire Swedish Scandes, c. 800 km from south to north. Concerned species were mountain birch (Betula pubescens ssp. czerepanovii), Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and rowan (Sorbus aucuparia). Treeline was narrowly defined as the highest elevation with trees, at least 2 m tall. In order to elucidate the full effect and power of climate change on the treeline area, focus is on sites with the most extensive treeline shifts, according to prior regional surveys. Thereby, local constraints (topoclimate) were minimized. Betula. Picea and Sorbus accomplished treeline rise predominantly by phenotypic height growth increment of old-established krummholz, while Pinus responded by establishment and growth of new specimens. The largest upshifts, irrespective of species, were in the order of 200 m (max. 245 m). In perspective of historical treeline shifts, the new and higher treelines are close to the position prevailing about 7000 years ago. In contrast to previous generalizations, no obvious differences existed between southern and northern Scandes. Based on a common temperature lapse rate of 0.6°C per 100 m altitude and recorded regional and centennial summer warming of 1.7°C, the observed rise appears as a fully expected response. This kind of performance indicates that, at optimal sites, treelines are in equilibrium with climate at a centennial scale, in cases of climate warming.

Extensive glacier recession has prevailed for almost 100 years in the Scandes and other parts of the world. At the lower fringe and forefields of shrinking alpine glaciers and ice patches, a plethora of ancient tree remnants is recovered. This is presumably the first time of exposure since burial by ice thousands of years ago. These remains represent prior stands of virtually all boreal tree species, currently growing in northern Scandinavia. As a consequence, a previously unexpected and patchily treed high-mountain landscape emerges, in some cases 600-700 m higher than present-day treelines. This difference in treeline positions between then and now (corrected for land uplift) indicates that summer temperatures have declined by about 3 °C since the early Holocene treeline maximum. Radiocarbon-dating reveals that the age of the tree remnants ranges between c. 16 800 and 2000 cal. yr BP. Initially,the high-mountain peaks stood out as nunataks in a surrounding for long glaciated landscape at lower elevations. As the ice sheet gradually shrinked, glacier cirques and hollows became filled with tree groves, in a matrix of alpine tundra. I naddition to Betula pubescens ssp. czerepanovii, Picea abies and Pinus sylvestris, these high-elevation enclaves contained tree species, previously unknown to such high positions and so early. These are Picea abies and a species currently considered as exotic to Scandinavia, namely Larix sibirica. In response to gradual climatecooling since the middle Holocene, the tree stands declined and dead trees were eventually entombed by glacier ice, which is currently disintegrating.

Mapping alpine vegetation at a meso-scale (catchment level) using remote sensing presents difficulties due to a patchy distribution and heteregeneous spectral appearance of the plant cover. We discuss issues of generalization and accuracy assessment in this case study when using a digital CIR air photo for an automatic classification of the dominant plant communities. About 10 plant communities could be classified with acceptable accuracy where the chosen classification scheme determined the final outcome. If a high resolution pixel mosaic is generalized to units that match the positional accuracy of simple GPS this generalization may also influence the information content of the image

Accurate description of snow and firn processes is necessary for estimating the fraction of glacier surface melt that contributes to runoff. Most processes in snow and firn are to a great extent controlled by the temperature therein and in the absence of liquid water, the temperature evolution is dominated by the conductive heat exchange. The latter is controlled by the effective thermal conductivity k. Here we reconstruct the effective thermal conductivity of firn at Lomonosovfonna, Svalbard, using an optimization routine minimizing the misfit between simulated and measured subsurface temperatures and densities. The optimized k* values in the range from 0.2 to 1.6 W (m K)⁻¹ increase downwards and over time. The results are supported by uncertainty quantification experiments, according to which k* is most sensitive to systematic errors in empirical temperature values and their estimated depths, particularly in the lower part of the vertical profile. Compared to commonly used density-based parameterizations, our k values are consistently larger, suggesting a faster conductive heat exchange in firn.

Glaciers are one of the main sources of freshwater in cold regions. The glacier melting process can significantly impact the glacier mass balance (GMB) and contribute a large amount of runoff in cold regions. This study applied the recently developed semi-distributed glacio-hydrological conceptual model (FLEXG) to understand the glacier melting process and the effect of topography on GMB in the Torne River basin, northern Sweden. The study simulated glacier and snow accumulation and ablation, as well as runoff from the glacier and non-glacier areas of the basin using the FLEXG model for the time period 1989–2018. The FLEXG model considers the influence of topography on runoff generation, and in this study the basin was classified into 143 zones depending on elevation and aspect. In order to gain a comprehensive view of the performance of the FLEXG model, the classical lumped hydrological model HBV was used and compared with the FLEXG model in simulating total streamflow and peak runoff at the outlet of the basin. Our results revealed that the FLEXG model performed well in reproducing the streamflow (also better than the HBV model) with metric Kling-Gupta Efficiency (KGE) of 0.80 and 0.71 for the calibration and validation periods, respectively. We also found that the FLEXG model performs better in peak runoff simulation than the HBV model. The FLEXG simulated snow cover area proportion agreed well with the MODIS satellite snow cover product (R2 = 0.60 and RMSE = 28%). The GMB in different elevation zones was simulated, and a downward trend was found for GMB changes during the study period because of climate change.

Arctic and sub-Arctic freshwaters are currently experiencing substantial ecosystem changes due to the effects of global warming. Global warming effects on these freshwaters include increasing water temperatures, altered hydrological patterns, shifts in riparian vegetation and changes in the export of nutrients and carbon from soils. How these alterations to the physical and chemical hab-itat will affect stream ecosystem functioning largely depends on the responses by autotrophic pro-ducers and heterotrophic primary consumers. In this thesis, I explore how key stream ecosystem processes such as metabolic rates and nutrient cycling vary as a function of climate and landscape drivers, particularly light, temperature, and nutrient and carbon availability. To do this I leveraged natural gradients in vegetation, altitude, disturbance, and precipitation throughout the year in northern Sweden, as well as long- and short-term manipulations of nutrient availability. I also synthesized nutrient limitation data from lakes and streams to more holistically assess the re-sponses of boreal to Arctic freshwaters to changes in nutrients and climate variables. I found that nutrient availability, and especially nitrogen (N), is a main driver of spatial and temporal patterns of biofilm productivity, whole system metabolic rates, and short term N uptake in Arctic and sub-Arctic streams. I also show the importance of light and temperature constraints during early spring and late autumn, which set the limit for the aquatic growing season and annual productivity pat-terns. I present a first comparison of combined drivers of lake and stream responses to nutrient addition, which points to a shared importance of N and phosphorus (P) rather than light or tem-perature in driving the magnitude of nutrient limitation across these systems. Ultimately, I pro-pose that across large ranges in habitat variables, widespread nutrient limitation of Arctic fresh-waters constrain other climate change effects on ecosystem functions. The results presented in this thesis will promote better predictions of climate change effects on Boreal to Arctic stream ecosystem functioning.

Climate change in the Arctic is altering the delivery of nutrients from terrestrial to aquatic ecosystems. The impact of these changes on downstream lakes and rivers is influenced by the capacity of small streams to retain such inputs. Given the potential for nutrient limitation in oligotrophic Arctic streams, biotic demand should be high, unless harsh environmental conditions maintain low biomass standing stocks that limit nutrient uptake capacity.

We assessed the drivers of nutrient uptake in two contrasting headwater environments in Arctic Sweden: one stream draining upland tundra and the other draining an alluvial valley with birch forest. At both sites, we measured nitrate (NO₃⁻) uptake biweekly using short-term slug releases and estimated rates of gross primary production (GPP) and ecosystem respiration from continuous dissolved oxygen measurements.

Catchment characteristics were associated with distinct stream chemical and biological properties. For example, the tundra stream maintained relatively low NO₃⁻ concentrations (average: 46 µg N/L) and rates of GPP (0.2 g O₂ m⁻² day⁻¹). By comparison, the birch forest stream was more NO₃⁻ rich (88 µg N/L) and productive (GPP: 1.7 g O₂ m⁻² day⁻¹). These differences corresponded to greater areal NO₃⁻ uptake rate and increased NO₃⁻ use efficiency (as uptake velocity) in the birch forest stream (max 192 µg N m⁻² min⁻¹ and 96 mm/hr) compared to its tundra counterpart (max 52 µg N m⁻² min⁻¹ and 49 mm/hr) during 2017. Further, different sets of environmental drivers predicted temporal patterns of nutrient uptake at these sites: abiotic factors (e.g. NO₃⁻ concentration and discharge) were associated with changes in uptake in the tundra stream, while metabolic activity was more important in the birch forest stream.

Between sites, variation in uptake metrics suggests that the ability to retain pulses of nutrients is linked to nutrient supply regimes controlled at larger spatial and temporal scales and habitat properties that promote biomass accrual and thus biotic demand.

Overall, constraints on biotic potential imposed by the habitat template determined the capacity of these high latitude streams to respond to future changes in nutrient inputs arising from climate warming or human land use.

Climate change has the potential to alter hydrological regimes and to expand saturated areas in permafrost environments, which are important sources of organic carbon. The sources, transfer zones, and delivery mechanisms of carbon into the stream network are controlled by the morphometric properties of the catchment; however, the utility and limitations of these properties as predictors of dissolved organic carbon concentrations have rarely been systematically evaluated. This study tested the relationships between 18 morphometric indicators and observed dissolved organic carbon (DOC) concentrations in the Stordalen catchment, Sweden. Geospatial and explorative statistics were combined to assess the topographical, areal, and linear indicators influencing the distribution of DOC in the catchment. The results suggest that catchment morphometric indicators can be used as proxies to predict DOC concentrations along a longitudinal continuum in subarctic climate regions (R2 up to 0.52). Morphometry indicators that best served as predictors of DOC concentration in the model were as follows: relief, slope length and steepness factor (LS-factor), sediment transport capacity, and catchment area. Due to the influence that catchment form exerts in DOC spatial patterns and processing, a morphometric approach can serve as a first approximation of DOC spatial patterns within a catchment. The initial step in identifying carbon sources based on the catchment topography has the potential to allow for quick and multilevel comparison within and between catchments.

The prospect of sea level rise due to melting ice sheets affirms the urgency of gaining knowledge on ice sheet dynamics during deglaciation. The Fennoscandian Ice Sheet serves as an analogue, whose retreat can be reconstructed from the geomorphological record. The recent development of a high-resolution LiDAR-derived elevation model can reveal new relationships between landforms, even for well-studied areas such as the Torneträsk region in northwestern Sweden. Therefore, this study aims to refine the reconstruction of the deglaciation in this region based on an updated glacial geomorphological map. A range of glacial landforms were mapped, which by means of an inversion model were utilized to form swarms representing spatially and temporally coherent ice sheet flow systems. Additionally, glacial lake traces allowed for the identification of ice margins that dammed lakes in Torneträsk, Rautasjaure, and other (former) lake basins. Eight glacial lake stages were identified for the Torneträsk basin, where final drainage occurred through Tornedalen. Over 20 glacial lake stages were identified for the Rautasjaure basin, where drainage occurred along the margins of a thinning ice lobe. The disparity between the glacial lake systems results from different damming mechanisms in relation to the contrasting topography of the basins. A strong topographic control on the retreat pattern is evident, as the ice sheet retreated southward in an orderly fashion in the premontane region, but disintegrated into ice lobes in the montane region. The temporal resolution of current dating techniques is insufficient to constrain the timing of ice retreat at the spatial scale of this study. Precise dating of the Pärvie fault would pinpoint the age of the ice margin which at the time of rupture was located between two glacial lake stages of Torneträsk. Collectively, this study provides data for better understanding the final retreat of the ice sheet and associated processes, such as interactions between glacial lakes and ice dynamics.

The Arctic is warming rapidly, especially in winter, which is causing large-scale reductions in snow cover. Snow is one of the main controls on soil thermodynamics, and changes in its thickness and extent affect both permafrost thaw and soil biogeochemistry. Since soil respiration during the cold season potentially offsets carbon uptake during the growing season, it is essential to achieve a realistic simulation of the effect of snow cover on soil conditions to more accurately project the direction of arctic carbon–climate feedbacks under continued winter warming.

The Lund–Potsdam–Jena General Ecosystem Simulator (LPJ-GUESS) dynamic vegetation model has used – up until now – a single layer snow scheme, which underestimated the insulation effect of snow, leading to a cold bias in soil temperature. To address this shortcoming, we developed and integrated a dynamic, multi-layer snow scheme in LPJ-GUESS. The new snow scheme performs well in simulating the insulation of snow at hundreds of locations across Russia compared to observations. We show that improving this single physical factor enhanced simulations of permafrost extent compared to an advanced permafrost product, where the overestimation of permafrost cover decreased from 10 % to 5 % using the new snow scheme. Besides soil thermodynamics, the new snow scheme resulted in a doubled winter respiration and an overall higher vegetation carbon content.

This study highlights the importance of a correct representation of snow in ecosystem models to project biogeochemical processes that govern climate feedbacks. The new dynamic snow scheme is an essential improvement in the simulation of cold season processes, which reduces the uncertainty of model projections. These developments contribute to a more realistic simulation of arctic carbon–climate feedbacks.

The choice of coordinate system to calculate eddy covariance fluxes becomes particularly relevant at complex measurement sites. The traditional way is to perform double rotation (DR) of the coordinate system i.e., to calculate turbulent fluxes in a coordinate system that is aligned with the flow streamlines within the flux averaging period (e.g., Kaimal and Finnigan, 1994). The second approach, the so-called planar-fitted (PF) coordinate system, averages the flow over a longer period of time, in practice a month or more. The PF method allows to derive an intercept coefficient of the vertical wind speed which can be attributed to the offset of the sonic anemometer or the average vertical flow related to meteorological conditions. We evaluated the variants of the PF methods using data from a variety of sites ranging from complex urban and forest sites to nearly ideal forest and peatland sites. At complex sites, we found that the intercept of the vertical wind speed derived from the PF method is a function of wind direction, time of day and/or stability. The sector-wise PF (SPF) method frequently led to insignificant statistical relationships. We tested a continuous PF (CPF) method where the relationship establishing the coordinate frame was represented as the continuous function in the form of Fourier series. The method enabled to obtain the PF with lower uncertainty as compared to the SPF method, by selecting necessary number of harmonics for each site based on confidence intervals of estimated parameters. Therefore, we recommend to use the CPF method in cases when the number of observations in some wind direction interval is low or the obtained SPF is insignificant due to large variance in measurements. We also showed that significant systematic difference can exist in cumulative turbulent fluxes between the DR and PF methods over a longer period of time. Derived vertical advection of carbon dioxide exhibited large variability with wind direction due to topography at complex sites and therefore, without considering horizontal advection, cannot be used to improve the net ecosystem exchange estimation during nocturnal, low turbulence conditions.

This thesis presents an assessment of the variation in solifluction occurrence, morphometry and movement rates in the Abisko region, northern Sweden. Variations in movement rates are analyzed both on a regional and local scale. The main methodological contributions of this thesis have been to provide new techniques of analyzing spatial and temporal variations of solifluction in order to detect long term temporal trends and to regionalize the variations in movement rates. The spatial analysis is achieved by using a combination of field measurements, GIS and remote sensing techniques and statistical analysis. The results are presented in six papers, focusing on the morphometry of solifluction landforms (paper I), the occurrence of permafrost (paper II), the spatial and temporal variations of lobe front movement rates using aerial photographs (paper III), the temporal, regional and local spatial variations in movement rates (paper IV – VI) and statistical modelling of the occurrence of solifluction landforms and calculation of geomorphic work (paper V and VI).

The results show that, on a regional scale, vegetation patterns are a major control on the occurrence of turf-banked solifluction landforms, with high NDVI-values (vegetation) associated with the presence of forms. Elevation is also a major control on a regional scale with a decrease in lobe dimensions and movement rates with increased elevation. High soil moisture values are associated with larger landforms and increased movement rates. Movement rates are generally higher in the western part of the region and appear to increase with higher MAAT. Equally, geomorphic work is greatest in the western part of the region. The important controls on a local scale vary from site to site, but include vegetation, slope angle and soil moisture. The photo analysis indicates that annual movement rates of lobe fronts in Kärkevagge and Låktatjåkka valley over the period 1959-2000 ranges from not-detectable to 63mm/yr. The permafrost model shows probabilities >0.8 for permafrost at elevations above 1300 m a.s.l. in the western part of the region, decreasing to altitudes over 850 m a.s.l. in the eastern part of the region. Calculated geomorphic work suggests that solifluction is a significant denudational agent in the sub-Arctic mountains of northern Sweden, but less so than previously estimated.

We use a pro-glacial oxygen isotope record of diatom silica (δ¹⁸O_diatom) and a sedimentary proxy for glacier flutuations to determine centennial-millennial scale climate change during the last 5000 yeras in northern Sweden. We show that the lake water isotopic composition åredominantly reflects the isotopic composition of the precipitation. Superimposed on a general depletion trend of 3.5‰ over the past 5000 years we found that the isotopic composition of precipitation became depleted (> 1‰ excursions) during four occasions centered at 4400, 3000, 2000 and, after 1200 cal yr BP. Climate simultaneously sustained a positive glacier mass balance, taht caused the catchment glacier to advance. A peristan cgange in the atmopheric circulation pattern could potentially have caused the registered chnages in the δ¹⁸O_diatom because different air masses hold characteristics δ¹⁸O signatures of their precipitation. The glacier mass balance primarily responds to the influence of summer temperature on ablation. We suggest that the most likely cause for the recorded chnages in both these proxies is a steadily increasing but fluctuating dominance of colder and δ¹⁸O depleted air masses from the north/northeast during the past 5000 years. Theδ¹⁸O_diatom depletion and glacier events all occur at times of relative ice-rafted-debris maxima in the North Atlanic, consistent with cold conditions and changes in surface wind directions. Our results confirm that changes towards a predominace of north/northeasterly winds occured at these time intervals.

Lake sediments are frequently used as archives of climate and environmental change. Minerogenic sediment variability in alpine lakes is often used to reconstruct past glacier and slope process activity. Alpine lake sediments can however have many different origins, which may induce errors in paleoenvironmental reconstructions. The aim of this project was to enhance the understanding of minerogenic lake sedimentation in alpine lakes and improve their use as environmental archives.

Catchment geomorphology and Holocene sediment sequences were analysed for five alpine lakes. Several minerogenic sediment sources were detected in catchments and sediment sequences. Slope-, fluvial-, periglacial-, nival- and aeolian sediment transportation processes contribute to create complex lake sediment patterns. Large variations in sedimentation rates were discovered within and between lakes, which has implications for sampling strategies and age-model constructions. Similar fine-grained minerogenic laminations were found in four of the investigated lakes, despite large differences in setting. The demonstrated similarity between glacial and non-glacial lakes may complicate interpretations of glaciolacustrine sediment signals.

The main conclusion is that lake sedimentation in alpine environments is highly dependent on several geomorphological factors. All lakes should therefore be viewed as unique and the geomorphology should be thoroughly investigated before environmental reconstructions are based on lake sediment proxies. This study has confirmed the multi-source origin of alpine lake sediment, which also opens possibilities of more multi-faceted paleoenvironmental studies. Different process-proxies could potentially be used to separate different climate signals, e.g. precipitation, temperature and wind, in lake sediments. Analysis of grain-size distribution, detailed mineralogy and magnetic mineralogy in combination with X-ray radiography are suggested methods for such reconstructions.

Longwave radiation is often the dominant source of energy for snowmelt in forests. Measurements at forest sites of varying density in Sweden and Finland show that downwelling longwave radiation is enhanced under forest canopies, even for sparse canopies and particularly for clear skies. Canopy density must be estimated accurately to predict this enhancement. Linear regression with above-canopy longwave radiation and air temperature as predictors of sub-canopy radiation gives good predictions of sub-canopy longwave radiation with weightings for transmission through canopy gaps that are close to measured sky view fractions. Air temperature serves here as a proxy for effective canopy radiative temperature. Adding above-canopy shortwave radiation as a predictor gives little improvement in the predictions, suggesting that daytime heating of trunks above the air temperature (“hot trees”) has limited influence on longwave radiation under these continuous canopies. The influence of canopy temperatures falling below the above-canopy air temperature (“cold trees”) on calm, clear nights, however, is apparent. Decoupling of canopy and above-canopy air temperatures in an energy balance model of the type used in large-scale land surface modeling allows this cooling.

Paleoglaciological reconstructions based on glacial geological and geomorphological evidence areused to constrain and test numerical models of ice sheet extent and dynamics. The MAGIC-DMLresearch project (“Mapping, Measuring and Modelling Antarctic Geomorphology and Ice Change, inDronning Maud Land”) is trying to reconstruct the timing and pattern of ice surface elevation changessince the mid-Pliocene across western Dronning Maud Land, East Antarctica. This reconstructionwill work as the basis for testing and constraining ice sheet numerical models to improve climateunderstanding in Antarctica.

This master thesis project contributes to MAGIC-DML by adopting a high-resolution remote sensingbasedmapping of glacial geomorphology and ice sheet surface structures, for a coast-inland transectincluding the Ahlmannryggen, Borgmassivet, and Kirwanveggen nunatak ranges. The primary aimof this study is to investigate the glaciology and paleoglaciology of the study area, in order to mapevidence for a former thicker ice sheet on nunatak slopes and plateaus, and patterns of ice flow of thecurrent ice sheet surface. Meso-scale glacial landforms and ice flow features were identified andmapped using different remote sensing data sets: the LANDSAT Image Mosaic of Antarctica(LIMA), DigitalGlobe Worldview-2 (WV02) and Worldview-3 (WV03) panchromatic andmultispectral images, the Radarsat Antarctica Mapping Project (RAMP) Ice Surface Digital ElevationModel (DEM) version 2, and the Bedmap2 datasets. The satellite imagery was analysed in a multistepprocedure using ArcGIS, including image processing and mosaicking, visual feature recognition,and mapping. The identification of some key landforms required the adoption of assumptions, forexample in order to distinguish till cover from regolith or boulders derived from rock fall from glacialerratics. Present-day ice flow directions were traced according to the distribution of ice surfacefeatures such as blue ice areas, crevasse fields, longitudinal surface structures, and supraglacialmoraines. The occurrence of till cover and erratics above the present-day ice surface on somenunataks slopes and plateaus was considered indicative of a thicker ice sheet in the past. Paleo-iceflow directions were inferred from the proximity of locations to the closest ice streams, since thatlatter have been active since the Oligocene.

Geomorphological and ice flow direction maps were obtained and used to infer the paleoglaciologyof the three nunatak ranges. Ice sheet thinning reconstructions reveal a minimum ice surface loweringof ~400–500 m in the Ahlmannryggen and Borgmassivet nunatak ranges, of ~300 m north of theKirwanveggen escarpment and of ~100 m on the edge of Amundsenisen polar plateau. The paleo-icesheet flow pattern probably differed from today, because ice flow has locally been influenced by anincreased topographical complexity, due to the thinning of the ice sheet and the emerging of nunatakoutcrops. According to dating studies conducted elsewhere in DML, the inferred ice surface decreasewas probably initiated in the Late Pliocene/Early Pleistocene, and continued after the Last GlacialMaximum interruption across the coastal sector of the ice sheet. The reliability of derived paleo-icesheet reconstructions, based on the mapping and interpretation of landforms, needs to be verified infuture field studies. This master thesis project has identified 34 well-suited locations for the samplingof erratic boulders and bedrock surfaces for cosmogenic nuclide (CN) surface exposure dating duringthe MAGIC-DML 2017/18 field season. The chronology derived from CN dating and fieldverification of the presented mapping will permit the delineation of ice sheet surface elevations astargets for ice sheet modelling.

In the face of climate change, it is important to estimate changes in key ecosystem properties such as plant biomass and gross primary productivity (GPP). Ground truth estimates and especially experiments are performed at small spatial scales (0.01-1 m(2)) and scaled up using coarse scale satellite remote sensing products. This will lead to a scaling bias for non-linearly related properties in heterogeneous environments when the relationships are not developed at the same spatial scale as the remote sensing products. We show that unmanned aerial vehicles (UAVs) can reliably measure normalized difference vegetation index (NDVI) at centimeter resolution even in highly heterogeneous Arctic tundra terrain. This reveals that this scaling bias increases most at very fine resolution, but UAVs can overcome this by generating remote sensing products at the same scales as ecological changes occur. Using ground truth data generated at 0.0625 m(2)and 1 m(2)with Landsat 30 m scale satellite imagery the resulting underestimation is large (8.9%-17.0% for biomass and 5.0%-9.7% for GPP(600)) and of a magnitude comparable to the expected effects of decades of climate change. Methods to correct this upscaling bias exist but rely on sub-pixel information. Our data shows that this scale-dependency will vary strongly between areas and across seasons, making it hard to derive generalized functions compensating for it. This is particularly relevant to Arctic greening with a predominantly heterogeneous land cover, strong seasonality and much experimental research at sub-meter scale, but also applies to other heterogeneous landscapes. These results demonstrate the value of UAVs for satellite validation. UAVs can bridge between plot scale used in ecological field investigations and coarse scale in satellite monitoring relevant for Earth System Models. Since future climate changes are expected to alter landscape heterogeneity, seasonally updated UAV imagery will be an essential tool to correctly predict landscape-scale changes in ecosystem properties.

Accumulation and ablation rates over an Antarctic blue-ice area spanning the 14year period 1988–2002 are presented. Data were obtained by direct stake measurements. Large spatial and temporal variations in the net balance were observed without any clear trend over the entire period. There are marginally significant increases in snow accumulation, and in ablation in the blue-ice area farthest from the equilibrium zone (both at the 95% confidence level). The snow/blue-ice transition zone shows no change over the entire period of observation,and the blue-ice area near the zone shows no change in ablation rate over the 14 year period. The mass-balance gradient in Scharffenbergbotnen may have increased during the period 1988–2002. However, the changes are small, especially when compared with the changes observed elsewhere in Antarctica even relatively close to the blue-ice area. This may indicate that the blue-ice areas are relatively stable to changes in accumulation rate, and possibly temperature.

Permafrost thaw in Arctic regions is increasing methane (CH4) emissions into the atmosphere, but quantification of such emissions is difficult given the large and remote areas impacted. Hence, Earth observation (EO) data are critical for assessing permafrost thaw, associated ecosystem change and increased CH4 emissions. Often extrapolation from field measurements using EO is the approach employed. However, there are key challenges to consider. Landscape CH4 emissions result from a complex local-scale mixture of micro-topographies and vegetation types that support widely differing CH4 emissions, and it is difficult to detect the initial stages of permafrost degradation before vegetation transitions have occurred. This study considers the use of a combination of ultra-high-resolution unoccupied aerial vehicle (UAV) data and Sentinel-1 and Sentinel-2 data to extrapolate field measurements of CH4 emissions from a set of vegetation types which capture the local variation in vegetation on degrading palsa wetlands. We show that the ultra-high-resolution UAV data can map spatial variation in vegetation relevant to variation in CH4 emissions and extrapolate these across the wider landscape. We further show how this can be integrated with Sentinel-1 and Sentinel-2 data. By way of a soft classification and simple correction of misclassification bias of a hard classification, the output vegetation mapping and subsequent extrapolation of CH4 emissions closely matched the results generated using the UAV data. Interferometric synthetic-aperture radar (InSAR) assessment of subsidence together with the vegetation classification suggested that high subsidence rates of palsa wetland can be used to quantify areas at risk of increased CH4 emissions. The transition of a 50 ha area currently experiencing subsidence to fen vegetation is estimated to increase emissions from 116 kg CH4 per season to emissions as high as 6500 to 13 000 kg CH4 per season. The key outcome from this study is that a combination of high- and low-resolution EO data of different types provides the ability to estimate CH4 emissions from large geographies covered by a fine mixture of vegetation types which are vulnerable to transitioning to CH4 emitters in the near future. This points to an opportunity to measure and monitor CH4 emissions from the Arctic over space and time with confidence.

Microtopography can be a key driver of heterogeneity in the ground thermal and hydrological regime of permafrost landscapes. In turn, this heterogeneity can influence plant communities, methane fluxes, and the initiation of abrupt thaw processes. Here we have implemented a two-tile representation of microtopography in JULES (the Joint UK Land Environment Simulator), where tiles are representative of repeating patterns of elevation difference. Tiles are coupled by lateral flows of water, heat, and redistribution of snow, and a surface water store is added to represent ponding. Simulations are performed of two Siberian polygon sites, (Samoylov and Kytalyk) and two Scandinavian palsa sites (Stordalen and Iškoras).

The model represents the observed differences between greater snow depth in hollows vs. raised areas well. The model also improves soil moisture for hollows vs. the non-tiled configuration (“standard JULES”) though the raised tile remains drier than observed. The modelled differences in snow depths and soil moisture between tiles result in the lower tile soil temperatures being warmer for palsa sites, as in reality. However, when comparing the soil temperatures for July at 20 cm depth, the difference in temperature between tiles, or “temperature splitting”, is smaller than observed (3.2 vs. 5.5 ∘C). Polygons display small (0.2 ∘C) to zero temperature splitting, in agreement with observations. Consequently, methane fluxes are near identical (+0 % to 9 %) to those for standard JULES for polygons, although they can be greater than standard JULES for palsa sites (+10 % to 49 %).

Through a sensitivity analysis we quantify the relative importance of model processes with respect to soil moisture and temperatures, identifying which parameters result in the greatest uncertainty in modelled temperature. Varying the palsa elevation between 0.5 and 3 m has little effect on modelled soil temperatures, showing that using only two tiles can still be a valid representation of sites with a range of palsa elevations. Mire saturation is heavily dependent on landscape-scale drainage. Lateral conductive fluxes, while small, reduce the temperature splitting by ∼ 1 ∘C and correspond to the order of observed lateral degradation rates in peat plateau regions, indicating possible application in an area-based thaw model.

Meteorological parameters determine the physical properties of snow precipitating from the atmosphere, but snow layers also continue to develop within the snow pack after the precipitation event. New characteristics form depending on temperature fluctuations, interaction with the soil, overburden compression, rain-on-snow events and more. As climate change is evidenced across the globe and particularly in the Arctic, understanding the relationship between snow and climate is important. In this project, a set of observed data of snow layer characteristics, collected every two weeks each winter over a 50+ year period at Abisko Scientific Research Station, northern Sweden, is co-studied with a multi-layer snow pack model which is able to reproduce additional snow properties. Data is presented in long time series as well as in high resolution to capture both trends and details. Comparison between modelled and observed data is made where possible. Physical processes are discussed and potential trends in the data are evaluated. Results show good agreement for snow pack depth between model and observations, while modelled snow density is largely confirmed by comparison with other records of density measured at Abisko. Modelled outputs illustrate snow pack temperature fluctuations, percolation of melt water and densification of snow layers within the profiles; observed data show variations in snow layer hardness, grain compactness, grain size and dryness. Long-term trends indicate an increase in snow layer hardness and a decrease in snow grain size since the beginning of the record.

Rising temperatures can influence ecosystem processes both directly and indirectly, through effects on plant species and communities. An improved understanding of direct versus indirect effects of warming on ecosystem processes is needed for robust predictions of the impacts of climate change on terrestrial ecosystem carbon (C) dynamics.To explore potential direct and indirect effects of warming on C dynamics in arctic tundra heath, we established a warming (open top chambers) and dominant plant species (Empetrum hermaphroditum Hagerup) removal experiment at a high and low elevation site. We measured the individual and interactive effects of warming, dominant species removal and elevation on plant species cover, the normalized difference vegetation index (NDVI), leaf area index (LAI), temperature, soil moisture and instantaneous net ecosystem CO2 exchange.We hypothesized that ecosystems would be stronger CO2 sinks at the low elevation site, and that warming and species removal would weaken the CO2 sink because warming should increase ecosystem respiration (ER) and species removal should reduce gross primary productivity (GPP). Furthermore, we hypothesized that warming and species removal would have the greatest impact on processes at the high elevation where site temperature should be most limiting and dominant species may buffer the overall community to environmental stress more compared to the low elevation site where plants are more likely to compete with the dominant species.The instantaneous CO2 flux, which reflected a weak CO2 sink, was similar at both elevations. Neither experimental warming nor dominant species removal significantly changed GPP or instantaneous net ecosystem CO2 exchange even though species removal significantly reduced ER, NDVI and LAI.Our results show that even the loss of dominant plant species may not result in significant landscape‐scale responses of net ecosystem CO2 exchange to warming. They also show that NDVI and LAI may be limited in their ability to predict changes in GPP in these tundra heaths systems. Our study highlights the need for more detailed vegetation analyses and ground‐truthed measurements in order to accurately predict direct and indirect impacts of climatic change on ecosystem C dynamics.

Subarctic peatlands are rich sources of organic carbon for freshwater ecosystems. Where those peatlands are underlain by permafrost, permafrost thaw may cause an initial release of bioavailable dissolved organic carbon (DOC) to surrounding freshwaters. In this study, we measured icebound and potentially leachable (extracted) DOC quantities and indices of DOC quality in active layer and permafrost layers from two subarctic peat mires, Stord-alen and Storflaket. Most of the permafrost layers did not contain more organic matter or exportable DOC (as g kg(-1) dry soil) than the overlying active layer, and there was no difference in aromaticity, molecular weight, or the ratio between labile and recalcitrant DOC extracted from the permafrost and active layer. However, DOC held in segregated ice of the near-surface permafrost had relatively low aromaticity compared to extracted DOC from the same depth. Total icebound and potentially leachable DOC in the Stordalen mire permafrost that is predicted to experience active layer deepening during each of the next 50 years corresponded to about 0.1% of the current annual aquatic export of DOC from the mire. We conclude that the pool of potentially leachable DOC currently stored in permafrost layers is small. We also highlight differences in permafrost organic material between the two studied mire systems, which has an effect on the pool and properties of leachable DOC that is potentially available for export during thaw. Moreover, the geomorphological form of permafrost thaw will influence future hydrological connectedness and DOC production, in turn determining future DOC export from the mires.

In the face of accelerating climate change, poleward and elevational range expansion have been recorded in many species of the subarctic. In recent decades Scots pine (Pinus Sylvestris) has become more abundant in the mountain-birch dominated forests of subarctic Sweden. Monitoring this shift is vital to understanding the effects of climate change on the subarctic ecosystem. Finding a cost effective and time efficient method for mapping and monitoring the Scots pine population over larger areas would improve our knowledge of the Scots pines response to climate change. This study evaluates the feasibility of mapping the state and change of the local Scots pine population in Abisko national park in subarctic Sweden using three different remote sensing techniques. The aim is both to evaluate the methods and to analyse eventual changes in the local Scots pine population. The first task, mapping of the current Scots pine population, was successfully performed utilizing a Sentinel-2 image and a method of estimating the Scots pine crown coverage of each pixel using a natural difference water index (NDWI). The second task, the change analyses were done in two parts, the first one being a repeat aerial photograph analysis, done by comparing an aerial photo from 1959 with an aerial photo from 2018. The second part measured changes in height and was done by calculating the height of 20 trees from measurements of tree-shadows in an aerial photo from 1959. The measurements were compared to LIDAR data from 2015. The results of the first part show an 9- 34% increase in Scots pine abundancy in two 1km2 large study areas. Most of the increase was however confined to already existing stands and no significant range expansion could be detected. The height comparison showed no increase in height between 1959 and 2015.