A method for estimating the effect of climate change on mean, maximum and minimum temperatures on 1-31 day time-scales: Record-breaking heatwave of summer 2025 in Fennoscandia

We present a method for quantifying the effect of climate change on local daily mean, maximum and minimum temperatures as well as their 2–31-day moving averages for locations that have long observational time-series available. The method utilizes CMIP6 (Coupled Model Intercomparison Project Phase 6) model data to estimate climate-change-induced shifts in both the mean and variance of local temperature distributions. As a case study, we apply the method to several weather stations in Fennoscandia for two 14-day periods in July of 2025, when an intense heatwave occurred in Northern Europe. We find that the station-specific average maximum temperatures in 12-25 July were 1.7 to 2.4 C higher, whereas the minimum temperatures in 19 July to 1 August were 1.5 to 2.4 C higher than they would have been in the beginning of the 20th century.  Furthermore, these temperatures were made 3.4–9.8 times (maximum temperature) and 4.2–13.2 times (minimum temperature) more likely by climate change since the year 1900 that is assumed to represent pre-industrial conditions. The presented method enables operational meteorologists to assess the role of climate change in extreme temperatures in near real-time, for example for media reporting.