The terrestrial CO2 exchange in the Arctic plays an important role in the global carbon (C) cycle. The Arctic ecosystems, containing a large amount of organic carbon (C), are experiencing ongoing warming in recent decades, which is affecting the C cycling and the feedback interactions between its different components. To improve our understanding of the atmosphere-ecosystem interactions, the Greenland Ecosystem Monitoring (GEM) program measures ecosystem CO2 exchange and links it to biogeochemical processes. However, this task remains challenging in northern latitudes due to an insufficient number of measurement sites, particularly covering full annual cycles, but also the frequent gaps in data affected by extreme conditions and remoteness. Combining ecosystem models and field observations we are able to study the underlying processes of Arctic CO2 exchange in changing environments. The overall aim of the research is to use data-model approaches to analyse the patterns of C exchange and their links to biological processes in Arctic ecosystems, studied in detail both from a measurement and a modelling perspective, but also from a local to a pan-arctic scale. In Paper I we found a compensatory response of photosynthesis (GPP) and ecosystem respiration (Reco), both highly sensitive to the meteorological drivers (i.e. temperatures and radiation) in Kobbefjord, West Greenland tundra. This tight relationship led to a relatively insensitive net ecosystem exchange (NEE) to the meteorology, despite the large variability in temperature and precipitations across growing seasons. This tundra ecosystem acted as a consistent sink of C (-30 g C m-2), except in 2011 (41 g C m-2), which was associated with a major pest outbreak. In Paper II we estimated this decrease of C sink strength of 118-144 g C m-2 in the anomalous year (2011), corresponding to 1210-1470 tonnes C at the Kobbefjord catchment scale. We concluded that the meteorological sensitivity of photosynthesis and respiration were similar, and hence compensatory, but we could not explain the causes. Therefore, in Paper III we used a calibrated and validated version of the Soil-Plant-Atmosphere model to explore full annual C cycles and detail the coupling between GPP and Reco. From this study we found two key results. First, similar metrological buffering to growing season reduced the full annual C sink strength by 60%. Second, plant traits control the compensatory effect observed (and estimated) between gross primary production and ecosystem respiration. Because a site-specific location is not representative of the entire Arctic, we further evaluated the pan-Arctic terrestrial C cycling using the CARDAMOM data assimilation system in Paper IV. Our estimates of C fluxes, pools and transit times are in good agreement with different sources of assimilated and independent data, both at pan-Arctic and local scale. Our benchmarking analysis with extensively used Global Vegetation Models (GVM) highlights that GVM modellers need to focus on the vegetation C dynamics, but also the respiratory losses, to improve our understanding of internal C cycle dynamics in the Arctic. Data-model approaches generate novel outputs, allowing us to explore C cycling mechanisms and controls that otherwise would not have been possible to address individually. Also, discrepancies between data and models can provide information about knowledge gaps and ecological indicators not previously detected from field observations, emphasizing the unique synergy that models and data are capable of bringing together.

En växande befolkning i städer bidrar till ökat fokus av att främja hållbara transporter. I enlighet med Sveriges nationella mål för transportpolitik och Förenta Nationernas globala mål 11 ökar behovet av att säkerställa ett väl utformat cykelnät. Allt fler kommuner strävar efter att främja cykling i städer. Cykelvänliga stadsområden med väl utformade sammankopplingar bidrar till enklare orientering i staden, vilket skapar möjlighet att välja cykel som transportmedel. Cykling har påvisat fördelaktiga effekter för människors välbefinnande, vilket förstärker den sociala hållbarheten för samhället. Ökat fokus på klimatfrågor kräver förbättrade insatser för att möta mobilitetsbehoven. Det förutsätter nya planeringsstrategier och innovationer i städer. Nederländerna, Danmark och Tyskland anses vara föregångsländer inom cykelinfrastrukturen. Länderna arbetar med att implementera åtgärder i cykelnätet för att öka antalet cyklister. Studiens syfte var att öka förståelsen för hur befolkningen upplever och använder cykelnätet. Det för att föreslå förbättringar och åtgärder Gävle kommun kan implementera för att utveckla en hållbarare cykelstad. Fallstudien genomfördes inom stadsdelen Brynäs. Kombinerade metoder användes för att samla in kvalitativ och kvantitativ data. Observationen syftade till iakttagelser av befintlig utformning av tre huvudcykelstråk. En webbaserad enkät riktades till avsedd population i Gävle kommun för att undersöka respondenternas upplevelse och användning av cykelnätet. Intervjuer genomfördes med tjänstemän inom Gävle kommun för att förbättra kunskapen om planeringsprocessen inom kommunen. Tidigare forskning gav förslag på effektiva åtgärder från andra länder och städer som implementerats i cykelnät. Dessa åtgärder studerades för att uppnå studiens syfte. Förbättrade sammankopplingar, separering mellan fotgängare- och cyklister samt tydliga cykelpassager är åtgärder som kräver kontinuerligt arbete för att möjliggöra ökad cykelanvändning. Resultatet av studien visar att Gävle kommun behöver implementera förbättringsåtgärder i cykelnätet för att uppnå sin vision om att bli en hållbar cykelstad. / A growing population in cities contributes to an increased focus on promoting sustainable transports. In accordance with Sweden's national goals for transport policy and the United Nations’ Sustainable Development Goal 11, there is an increasing need to ensure a well-designed cycling network. Several municipalities strive to promote cycling in cities. Urban areas with bicycle-friendly infrastructure and well-connected routes facilitate easier navigation in the city, which creates the opportunity to choose cycling as transportation. Cycling has demonstrated beneficial effects on people's well-being, which reinforces social sustainability for society. Increased focus on climate issues requires improved efforts to meet mobility needs. It requires new planning strategies and innovations in cities. The Netherlands, Denmark, and Germany are considered successful countries in cycling infrastructure. These countries are working to implement measures in the cycling network to increase the number of cyclists. The purpose of the study was to increase understanding of how the population experiences and utilizes the cycling network. It aims to propose improvements and measures that Gävle municipality can implement to create a more sustainable cycling city. The case study was conducted in the Brynäs district. A combined methods were used to collect qualitative and quantitative data. The observation aimed to observe the existing design of three main cycle routes. A web-based survey was directed to the intended population to investigate respondents' experiences and use of the cycling network. Interviews were conducted with officials from Gävle municipality to enhance understanding of the planning process within the municipality. Previous research provided suggestions for effective measures from other countries and cities that have been implemented in cycling networks. These measures were studied to achieve the purpose of the study. Improved connections, separation between pedestrians and cyclists and clear cycling passages are measures that require continuous work to enable increased cycling usage. The study’s results indicate that Gävle municipality needs to implement improvement measures in the cycling network in order to achieve its vision of becoming a sustainable cycling city.