Abstract. Windstorms are natural disturbance agents in forests playing a role in natural forest regeneration. In Finland, the most severe individual windstorms have commonly damaged 2–4 million m3 of timber. In addition to financial losses caused to forest owners, windthrown trees have in many cases seriously disrupted the functionality of the national power grid. Communicating windstorm risks in duty forecasting is difficult. In this study, we aimed at developing windstorm impact estimates for forest damage in Finland to help the forecaster to improve communication of the risks of windstorms. We have compared the volume of forest damage caused by the most intense windstorms in Finland during the recent decade to the observed maximum inland wind gust speeds associated with the same windstorms. It was found out that the volume of forest damage follows approximately a power relation as a function of wind gust speed with a power of ∼10. This is a tentative estimate because of a short time series and small number of inspected windstorms. Moreover, also wind direction, location of the affected area and soil properties among other factors have an impact to the amount of damage as illustrated in our inspection. Despite the shortness of the time series, we believe that our results demonstrating the steep increase in the impacts of windstorms with an increasing windstorm intensity are valuable. However, more detailed investigations with longer time series are needed in order to more specifically communicate the windstorm risks and their impacts in boreal forests.

Serious disruptions and exceptional circumstances for society, that the society tries to prepare for and act in them are at the center of security of supply. Current examples are the COVID pandemic and the ongoing energy crisis for which Finland's security of supply has also been strongly highlighted. Disturbances can also be caused by weather phenomena: in Finland, such examples are windstorms, severe thunderstorms, floods, and droughts, which can, at least in principle, paralyze the society. It is possible to prepare for the impacts of weather phenomena, but the ongoing rapid climate change makes it more complicated. Some of the weather phenomena that cause impacts are fast and violent (e.g. intense thunderstorms) and some occur more slowly (e.g. long heat waves), and climate change affects the phenomena in different ways. In this work, the estimated impacts of climate change on Finland's security of supply were investigated. The starting point was to gain an understanding of which weather phenomena and weather situations are central to security of supply and which sectors of security of supply are the most vulnerable. The work constituted of workshops and expert interviews organized with the National Emergency Supply Agency. In addition to the interviews, the work covered past significant weather situations in Finland that are known to have had significant societal impacts. Information was also extracted from recent literature, especially regarding the vulnerabilities and adaptability of different sectors in Finland. Estimates of the climate change impacts on the identified phenomena were combined with the collected information, resulting in a first understanding of how climate change affects Finland's security of supply. Based on the results, it can be concluded that the impacts of climate change on security of supply are quite complex, especially due to the wide spectrum of weather phenomena and their different impact mechanisms. In addition, the matter becomes more complicated by the fact that there is no clear distinction of what weather phenomenon actually is critical to security of supply and what is not. For example, could the increasing adverse impacts on health care due to the increasingly common heat conditions reach a serious societal disturbance situation at some point, if it is not sufficiently prepared in advance? Another key result is that in terms of security of supply, the direct effects of climate change are very small in Finland compared to many other countries. Although the climate in Finland has already changed considerably and will continue to change in the future, the biggest impacts to security of supply seem to be reflected from elsewhere: the experts of the National Emergency Supply Agency consider the worst situation to be a lack of food, water and habitable living environment in the world, which would also be reflected to Finland. Among the sectors, food/water and energy supply and logistics are perceived as the most vulnerable. The work mainly focused on the direct effects of climate change, i.e. the effects of climate change on the occurrence of various weather phenomena. However, the work also considers to some extent indirect effects, i.e. those reflected from other parts of the world, and transitional effects that result from climate change mitigation measures, especially from the rapid energy transition.