INTRODUCTION:Vector-borne diseases (VBDs) pose a growing threat to human and animal health, driven by complex interactions among and between vectors, hosts, pathogens, and the environment. The Nordic region is warming at twice the global average rate, creating novel ecological conditions that reshape VBD transmission dynamics. OBJECTIVES AND METHODS:This narrative review synthesises current knowledge regarding major tick- and mosquito-borne diseases in the Nordic region, with a focus on Finland, based primarily on outputs from the VECLIMIT project (2020-2023), supported by other Nordic studies. RESULTS:The most prevalent human VBDs are Lyme borreliosis (LB), tick-borne encephalitis (TBE), tularaemia, and Sindbis virus (SINV) infections. Over the past decade, LB and particularly TBE incidence have increased, whereas SINV and tularaemia show cyclical patterns. The principal tick vectors Ixodes ricinus and Ixodes persulcatus are expanding geographically, while key mosquito vectors, Aedes cinereus/geminus and Culex pipiens/torrentium, are widespread. Rising temperatures and extended growing seasons promote vector activity and pathogen development, whereas heterogenous environments, such as wetlands and urban green spaces, support vector and host populations. While small mammals serve as key reservoirs for tick-borne pathogens, cervids drive tick propagation, and avian hosts maintain vector-borne enzootic cycles. Substantial gaps remain in prevention and awareness across public, medical, and veterinary sectors. CONCLUSIONS:Climate warming is likely to increase the spread of many endemic and invasive vectors and pathogens, yet surveillance remains limited and understanding of VBD drivers incomplete. Addressing these gaps requires coordinated One Health frameworks leveraging citizen science and cross-border Nordic collaboration.

Abstract. Effective climate services require professionals who possess the competencies to interpret complex climate data, engage meaningfully with users, and support informed decision-making. This paper presents the development and pilot implementation of the graduate-level course Living with Changing Climate, designed to foster these competencies through transdisciplinary and practice-oriented climate change education. Developed collaboratively by experts in climate science, impact modelling, climate services, and educational sciences, the course aims to strengthen climate action competencies relevant to professional contexts. It is offered as part of the Nordic Master in Environmental Changes at Higher Latitudes (EnCHiL), the University of Helsinki’s Master’s Programme in Atmospheric Sciences, and the Climate University network. The course integrates climate change science, the use of climate data, and the principles of climate services, with a strong emphasis on real-world application. Using a design-based research approach, this study explores the competencies essential for climate-informed decision-making, the challenges of developing an online course for diverse learners, and the key elements of effective course design and implementation. Insights gained from the pilot phase and student feedback provided valuable guidance and highlighted critical issues, the resolution of which substantially improved the final version of the course. This work highlights the role of higher education in advancing user-centred climate services by equipping learners with the knowledge and skills necessary for proactive climate adaptation and mitigation across sectors.

The circulation of tick-borne pathogens is influenced by the availability of ticks, the hosts of ticks and pathogens, and the environmental conditions that affect both the ticks and their hosts. Lyme borreliosis (LB), caused by Borrelia burgdorferi sensu lato and transmitted by Ixodes spp. ticks, is the most common tick-borne disease in the Northern Hemisphere. Understanding the spatio-temporal dynamics of human LB incidence regarding abundance of ticks and hosts and environmental factors is essential for effective disease risk management. We analyzed long-term (1997–2018) and spatially extensive (277 municipalities covering 230,000 km2) data on human LB incidence in Finland. Using dynamic species distribution models, we assessed the effects of (i) the abundance of pathogen reservoir hosts used by immature ticks (voles and squirrels), (ii) abundance of the key reproductive hosts for adult ticks (moose and deer), (iii) landscape characteristics, and (iv) climatic variables on the risk of LB. LB presence and incidence varied across the study area and exhibited a clear increasing trend. While host species showed temporal and regional variation in abundance, their relationships with LB risk were inconsistent. In contrast, environmental variables showed more consistent patterns: increased forest fragmentation, longer growing seasons, and higher humidity were generally associated with elevated LB risk. Our study suggests that the factors explaining LB epidemiology cannot be generalized spatially but depend on local climate, landscape, and host community. Given the available data, environmental conditions seem to play a more predictable role in LB epidemiology than the estimated abundances of hosts at the municipality level, yet we cannot exclude host abundance effects. Hence, the key to enhancing our understanding of the complex mechanisms underlying the epidemiology of LB and other tick-borne infections is to clarify how tick distribution and abundance respond to alterations in the host community, habitat features, and local climate.

Abstract. This study investigates how increasing greenhouse gas concentrations may affect environmental conditions favourable for thunderstorms over Fennoscandia. The future scenarios are based on data produced with the HARMONIE-Climate convection-permitting regional climate model forced by two global climate models at the lateral boundaries. The distribution of thunderstorm-favourable days, defined as days with co-occurring conditional instability and precipitation, in model data compares well with observed thunder days in May–September 2002–2018 across the study domain. By 2081–2100, the area-average frequency of favourable days is projected to increase by approximately 40–185 % in Finland, 30–185 % in Sweden, and 15–210 % in Norway relative to 1986–2005 under the RCP4.5 and RCP8.5 emission scenarios. Projected increases are largest over the northern Fennoscandia and in simulations exhibiting stronger warming. Thunderstorm-favourable conditions occurring simultaneously with strong vertical wind shear are also simulated to become more common, suggesting more frequent potential for severe organized convection in a future climate. More research is needed to understand how the changes in general storm-favourable environmental conditions may affect actual hazard occurrence. The results support the use of the HARMONIE-Climate model for convective storm scenario studies and offer valuable context for climate change adaptation in Fennoscandia.

Effective climate change adaptation requires climate information that is not only scientifically robust but also aligned with the diverse needs of end users operating at regional, national, and local scales. These needs vary widely across sectors such as public authorities, municipalities, emergency services, critical infrastructure operators, and insurance providers, creating challenges related to spatial and temporal resolution, uncertainty communication, data formats, and interpretability. In this contribution, we synthesise insights from several ongoing projects to examine how different types of climate information can be tailored to support efficient climate risk assessment and adaptation decision-making.ILMOS Uusimaa project focuses on municipal-scale adaptation, producing high-resolution climate projections, economic impact assessments of adaptation measures, and storyline-based Climate Digital Twin simulations of extreme events to support practical decision-making. These results demonstrate the importance of kilometre-scale data, impact-oriented indicators, and clear narratives when engaging local actors.Sector-specific requirements are further illustrated by projects addressing critical infrastructure and financial risk. MAWECLI and WIND‑IMPACT investigate single and compound weather hazards relevant for nuclear safety and electricity networks, respectively, combining physical and statistical modelling, extreme value analysis, and advanced uncertainty quantification. These projects emphasize the need for rare-event statistics, physically consistent simulations, and explicit treatment of compound and cascading risks. Meanwhile, PIISA focuses on co-developing climate-resilient insurance solutions, where probabilistic risk information, loss data, and comparability across regions and hazards are essential.Finally, the CLAIMS and Climate Digital Twin Storyline analyses of extremes showcase how event-based attribution and counterfactual simulations can make climate change impacts tangible by quantifying how specific events differ between past, present, and warming scenarios. Across all projects, common challenges emerge in balancing resolution and computational cost, integrating uncertainties into decision processes, and translating complex climate data into actionable knowledge. Our synthesis highlights pathways for bridging global climate information with end-user-relevant, locally actionable climate risk assessments.

Summer 2024 was exceptionally warm in northern Fennoscandia, with June-August mean temperatures at several long-term weather stations surpassing the long-standing record set in 1937. In this region, summer mean temperatures have been reconstructed from tree-ring proxies, which provide annually resolved and millennium-long records of past climate. Here we show, using in-situ observations and two different tree-ring reconstructions, that summer 2024 was the warmest summer in 2000 years in northern Fennoscandia. Employing an attribution method based on Coupled Model Intercomparison Project Phase 6 climate models, we further estimate that climate change increased the likelihood of this extreme season by a factor of 93 (5–95% uncertainty range 19–881) and increased the temperature an additional 2.1 °C (1.4–2.8 °C). Atmospheric circulation patterns influencing both summers 1937 and 2024 were largely similar, suggesting a comparable large-scale circulation influence. Our findings highlight the impact of climate change for the contemporary heat extremes in Fennoscandia, indicating that the warming of summer climate is emerging from its range of natural climate variability over the last two millennia.

This study investigates the impacts of extratropical cyclones on Finland's electricity grids, focusing on 92 significant windstorms from 2005 to 2018. We present a classification method for extratropical cyclones based on the arrival location and direction. Rather than using meteorological criteria to identify windstorms, we select them based on their impacts, namely the number of power outages, to reach a more targeted understanding of windstorm impacts compared to traditional approaches. Key findings indicate that south-west-originating windstorms cause the most damage in total, while north-westerly windstorms individually lead to the highest average outages. The largest impacts occur when a windstorm moves across the northern part of the country, from the north-west to east, with the strongest wind gusts concentrated on the southern side of the low-pressure centre, in highly populated regions. Of the meteorological characteristics of windstorms, the most relevant for grid damage besides the wind gust speed are the extent and spatial distribution of wind gusts. The seasonal analysis shows that windstorms are more frequent and damaging in autumn and winter, but even weaker wind speeds during summer can cause significant damage. Factors such as soil frost influence the severity of windstorm damage, highlighting the importance of expanding research to include environmental and geographical aspects.

IntroductionThe Specialisation programme in climate expertise is an education programme that aims to provide professionals from different fields with the know-how to drive systemic change towards a climate-resilient future. This two-year-long programme is meant to be carried out alongside work, teaching important and previously identified competencies necessary for effective climate action1, including both subject knowledge and general skills, like argumentation, problem-solving, critical thinking, collaboration, and effective communication2. Through education we hope to provide the basis for future collaboration, innovation, and understanding necessary to tackle climate change and related environmental as well as socio-economic problems.ResearchThe programme is offered by the University of Helsinki, University of Eastern Finland, Finnish Meteorological Institute and was designed together with the Climate University network and Climate Leadership Coalition. The programme launches in spring 2024 with students from both public, and private sectors. The program's duration (2 years), scope (60 ECTS), as well as the large number of students (up to 50) from different backgrounds provide a unique opportunity for the students to network and exchange ideas, as well as providing a platform for us to explore the following questions: (1) how education shapes the agency and professional identities of climate experts, (2) how education meets the multidisciplinary needs of various stakeholders, and (3) how education translates to concrete climate actions?ConclusionsThe Specialisation programme in climate expertise aspires to find effective ways to address the needs of different stakeholders facing the current climate crisis, and to empower professionals with the necessary know-how to lead transformative climate actions within their respective fields. The programme is currently offered only in Finnish, but we hope to grow the programme in the future and to include international students, therefore expanding our multidisciplinary network of climate experts across national borders as well as societal sectors. More information: https://www.helsinki.fi/en/faculty-science/teaching-and-studying/continuous-learning-and-web-based-studies/specialisation-programme-climate-expertiseReferences1. Siponen, J., M. Santala, J. Salovaara, V.-M. Vesterinen, S. Tolppanen, A. Lauri, J. Lavonen and L. Riuttanen. Climate Competence – a view of professionals in the field (submitted).2. Riuttanen, L., Ruuskanen, T., Äijälä, M. and Lauri, A., 2021. Society needs experts with climate change competencies–what is the role of higher education in atmospheric and Earth system sciences?. Tellus B: Chemical and Physical Meteorology, 73(1), pp.1-14.

Thunderstorms are common in Finland, however, high-impact damaging cases do not occur every summer. On 21-23 June 2021, three damaging thunderstorms, named Ahti, Paula and Aatu, occurred in Finland. We provide an overview of the thunderstorms and their environment and impacts. The thunderstorms led to heavy rain, hail, intense lightning and, most notably, extensive forest damage due to strong surface wind gusts. Thunderstorms are common in Finland, however, high-impact damaging cases do not occur every summer. On 21-23 June 2021, three damaging thunderstorms, named Ahti, Paula and Aatu, occurred in Finland. We provide an overview of the thunderstorms and their environment and impacts. The thunderstorms led to heavy rain, hail, intense lightning and, most notably, extensive forest damage due to strong surface wind gusts.image

Fine-scale temperatures are important drivers of ecosystem functions and biodiversity in boreal forests. However, accounting for large thermal variability has been difficult due to the coarse spatiotemporal resolution of climate data that is commonly applied in studies of biodiversity and forest health. Here, we use a mechanistic microclimate model and geospatial environmental and weather data to reveal microclimate temperature variability in a broad macroclimatic gradient in boreal forest environments. We modelled hourly near-surface temperatures (0.15 m above ground) in May-August 2020 over three focus areas located in hemiboreal, southern boreal and northern boreal forest zone in Finland at a spatial resolution of 10 m x 10 m. A comparison against data from 150 microclimate stations showed reasonable agreement (root mean square error [RMSE] 2.9 °C) between the measured and modelled temperatures. RMSE for the three focus areas ranged 2.2 –3.2 °C, and the difference was found to be generally smaller under dense canopies compared to open areas. The modelling revealed substantial thermal variability over the landscapes; for example, seasonal near-surface temperature ranges varied 26.5 °C – 42.9 °C, with the variation being smallest in the hemiboreal landscape with multiple large waterbodies, and largest in southern boreal landscape with large wetland areas. These results demonstrate the great potential of mechanistic microclimate modelling to increase our understanding of the thermal characteristics of various boreal forest environments. Ultimately, high-resolution spatiotemporal microclimate data will permit better understanding of e.g., boreal species distribution under climate and land use change and fine-scale variability in disturbances, including insect pests and forest fires.

We investigate the evolution of the large-scale weather patterns prior to the 40 most intense thunderstorm days in Finland in 2002-2021 by using lightning observations and the ERA5 reanalysis data.Our results show that intense thunderstorm days are typically associated with a surface high pressure to east and low pressure to west of Finland.An upper-level ridge is co-located with a warm and moist airmass over Fennoscandia.The large-scale weather pattern is largely similar for the three preceding days, however, large case-to-case variability occurs three days before the intense thunderstorm day.

The West Nile Virus (WNV) and Sindbis virus (SINV) are avian-hosted mosquito-borne zoonotic viruses that co-circulate in some geographical areas and share vector species such as Culex pipiens and Culex torrentium. These are widespread in Europe, including northern parts and Finland, where SINV is endemic, but WNV is currently not. As WNV is spreading northwards in Europe, we wanted to assess the experimental vector competence of Finnish Culex pipiens and Culex torrentium mosquitoes to WNV and SINV in different temperature profiles. Both mosquito species were found susceptible to both viruses and got infected via infectious blood meal at a mean temperature of 18 °C. WNV-positive saliva was detected at a mean temperature of 24 °C, whereas SINV-positive saliva was detected already at a mean temperature of 18 °C. Cx. torrentium was found to be a more efficient vector for WNV and SINV over Cx. pipiens. Overall, the results were in line with the previous studies performed with more southern vector populations. The current climate does not seem optimal for WNV circulation in Finland, but temporary summertime transmission could occur in the future if all other essential factors are in place. More field data would be needed for monitoring and understanding the northward spreading of WNV in Europe.

(Hydro-)meteorological extreme events account for a large portion of immediate exposure to and long-term impact by climate change for humans, ecosystems and infrastructure worldwide. These extremes, especially high-impact events, are often communicated by means of return periods, e.g., a 1-in-100 years flood, a 1-in-50 years drought. Frequency increases are already observed for a range of meteorological extremes, in parts also at temporal scales of merely a few decades (i.e., less than a human lifetime). Distributional shifts of atmospheric variables, e.g., temperature or precipitation, lead to further frequency increases of (current) extremes and the emergence of yet unseen, sometimes compound and cascading events. Therefore, one could think that the (decreasing) rareness of extreme events forms a good basis for illustrating the impacts of climate change among the broad public. However, the common reaction is similar to the headline introduction: Again another once-in-a-lifetime flood? But we just had one last year … In other cases, the new regularity of extreme events often seems to cause habituation or fatalism rather than concern. Apparently, there is a discrepancy between the scientific understanding of extremes on the one side and the public perception and personal experience on the other side. In this contribution, we try to illustrate this discrepancy and explore ways to approximate the different perceptions. We start by showing how prominently return periods are used explicitly and implicitly in grasping extremes (both in science and everyday-talking). A clear scientific definition will follow and hint to potential sources of misunderstanding. We next present examples where return periods are used for communicating (hydro-)meteorological hazards to the general public: 1. teaching the meaning and understanding of meteorological data with a focus on extremes with the “Living with changing climate”-course of the Finnish Climate University initiative (https://climateuniversity.fi/portfolio-items/living_with_changing_climate/), which is targeting people from outside academia as well, and 2. results from the Bavarian-Québec ClimEx project (https://climex-project.org) formatted as a hydrometeorological atlas. Further, we discuss ideas of tailoring return periods to peoples’ life experience e.g. by referring to literature and using 80-years as the averaged life expectation. We conclude by summarizing where we think that these examples succeed or struggle in clarifying the meaning and implications of extremes and provide an outlook on further ideas and narratives to bridge the gap between science and society.

Finnish Meteorological Institute’s Climate Bulletin Research Letters is a recently established research-based publication that features short and easy-to-read research articles on climate and climate services. The issues are published as a supplement to the Climate Bulletin journal by the Finnish Meteorological Institute. Even though the articles may be short, each one is peer-reviewed and citeable with a unique DOI. Research Letters is published in English once or twice a year. All issues and articles are open access. No publication fees are charged either. The editorial and publication processes have been designed to facilitate swift publication. Couple of weeks review and revision times are imposed. Once approved for publication, an article is instantly published as online and preprint versions. After 7-8 articles have been published this way, they will be collected to a single issue and published online at ISSUU digital publication platform. Research Letters is a publication channel for short reports, results of projects or case studies that may not be suitable for a full-scale peer reviewed publication. It was established to satisfy a clear need for short research publications. The themes of the series revolve around climate, climate services, climate change, marine research and results of research projects. From time to time, theme issues present extreme weather phenomena or maritime themes, among other topics. Special emphasis of the publication is in introducing new climate services and tools for different target groups, such as cities and agricultural, forestry, tourism and energy sectors. Since the first issue in spring 2019, four regular issues and one special issue have been published with 39 articles in total. The first issue has now over 400 reads. It is possible to subscribe to receive email reminders when new issues of the Climate Bulletin (in Finnish with a short summary in English) or Research Letters are published. Subscribers will only receive reminders when new issues are published. Subscription data is not used for advertising or other purposes. Research Letters is available on the Climate Bulletin website: https://www.ilmastokatsaus.fi/category/research-letters/. Climate Bulletin editorial team can be reached via email: ilmastokatsaus@fmi.fi. The editor-in-chief of the Climate Bulletin is Hilppa Gregow, Head of Unit, Finnish Meteorological Institute, hilppa.gregow@fmi.fi.

We modelled the impact of selected meteorological factors on the daily number of new cases of the coronavirus disease 2019 (COVID-19) at the Hospital District of Helsinki and Uusimaa in southern Finland from August 2020 until May 2021. We applied a DLNM (distributed lag non-linear model) with and without various environmental and non-environmental confounding factors. The relationship between the daily mean temperature or absolute humidity and COVID-19 morbidity shows a non-linear dependency, with increased incidence of COVID-19 at low temperatures between 0 to −10 °C or at low absolute humidity (AH) values below 6 g/m3. However, the outcomes need to be interpreted with caution, because the associations found may be valid only for the study period in 2020–2021. Longer study periods are needed to investigate whether severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has a seasonal pattern similar such as influenza and other viral respiratory infections. The influence of other non-environmental factors such as various mitigation measures are important to consider in future studies. Knowledge about associations between meteorological factors and COVID-19 can be useful information for policy makers and the education and health sector to predict and prepare for epidemic waves in the coming winters.

Abstract Background Ticks are responsible for transmitting several notable pathogens worldwide. Finland lies in a zone where two human-biting tick species co-occur: Ixodes ricinus and Ixodes persulcatus. Tick densities have increased in boreal regions worldwide during past decades, and tick-borne pathogens have been identified as one of the major threats to public health in the face of climate change. Methods We used species distribution modelling techniques to predict the distributions of I. ricinus and I. persulcatus, using aggregated historical data from 2014 to 2020 and new tick occurrence data from 2021. By aiming to fill the gaps in tick occurrence data, we created a new sampling strategy across Finland. We also screened for tick-borne encephalitis virus (TBEV) and Borrelia from the newly collected ticks. Climate, land use and vegetation data, and population densities of the tick hosts were used in various combinations on four data sets to estimate tick species’ distributions across mainland Finland with a 1-km resolution. Results In the 2021 survey, 89 new locations were sampled of which 25 new presences and 63 absences were found for I. ricinus and one new presence and 88 absences for I. persulcatus. A total of 502 ticks were collected and analysed; no ticks were positive for TBEV, while 56 (47%) of the 120 pools, including adult, nymph, and larva pools, were positive for Borrelia (minimum infection rate 11.2%, respectively). Our prediction results demonstrate that two combined predictor data sets based on ensemble mean models yielded the highest predictive accuracy for both I. ricinus (AUC = 0.91, 0.94) and I. persulcatus (AUC = 0.93, 0.96). The suitable habitats for I. ricinus were determined by higher relative humidity, air temperature, precipitation sum, and middle-infrared reflectance levels and higher densities of white-tailed deer, European hare, and red fox. For I. persulcatus, locations with greater precipitation and air temperature and higher white-tailed deer, roe deer, and mountain hare densities were associated with higher occurrence probabilities. Suitable habitats for I. ricinus ranged from southern Finland up to Central Ostrobothnia and North Karelia, excluding areas in Ostrobothnia and Pirkanmaa. For I. persulcatus, suitable areas were located along the western coast from Ostrobothnia to southern Lapland, in North Karelia, North Savo, Kainuu, and areas in Pirkanmaa and Päijät-Häme. Conclusions This is the first study conducted in Finland that estimates potential tick species distributions using environmental and host data. Our results can be utilized in vector control strategies, as supporting material in recommendations issued by public health authorities, and as predictor data for modelling the risk for tick-borne diseases.

Microclimate varies greatly over short horizontal and vertical distances, and timescales. This multi-level heterogeneity influences terrestrial biodiversity and ecosystem functions by determining the ambient environment where organisms live in. Fine-scale heterogeneity in microclimate temperatures is driven by local topography, land and water cover, snow, and soil characteristics. However, their relative influence over boreal and tundra biomes and in different seasons, has not been comprehensively quantified. Here, we aim to (1) quantify temperature variations measured at three heights: soil (-6 cm), near-surface (15 cm) and air (150 cm), and (2) determine the relative influence of the environmental variables in driving thermal variability. We measured temperature at 446 sites within seven focus areas covering large macroclimatic, topographic, and ecosystem gradients (tundra, mires, forests) of northern Europe. Our data, consisting of over 60 million temperature readings during the study period of 2019/11-2020/10, reveal substantial thermal variability within and across the focus areas. Near-surface temperatures in the tundra showed the greatest instantaneous differences within a given focus area (32.3 degrees C) while the corresponding differences for soil temperatures ranged from 10.0 degrees C (middle boreal forest) to 27.1 degrees C (tundra). Instantaneous differences in wintertime air temperatures were the largest in the tundra (up to 25.6 degrees C, median 4.2 degrees C), while in summer the differences were largest in the southern boreal forest (13.1 degrees C, median 4.8 degrees C). Statistical analyses indicate that monthly-aggregated temperature variations in boreal forests are closely linked to water bodies, wetlands, and canopy cover, whereas in the tundra, variation was linked to elevation, topographic solar radiation, and snow cover. The results provide new understanding on the magnitude of microclimate temperature variability and its seasonal drivers and will help to project local impacts of climate change on boreal forest and tundra ecosystems.

Preliminary results of outdoor and