Changes in temperature, rainfall and snow cover during the reproductive season of Arctic-breeding birds

Arctic warming is rapidly altering hydroclimatic conditions, yet its implications for the timing and reliability of terrestrial breeding environments remain poorly quantified. This study assesses how changes in temperature, snow dynamics, and rainfall extremes affect the snow-free reproductive window across Arctic tundra systems. We combined ERA5 reanalysis (1940–2024), daily output from ten CMIP6 Earth System Models, and long-term equilibrium simulations (400 years) from EC-Earth3 under present-day and doubled CO₂ conditions. Analyses focused on lowland tundra regions and quantified changes in snow cover, timing and duration of the productive period, and the frequency and magnitude of extreme rainfall events. Results show strong winter warming (up to + 15°C under SSP5-8.5), widespread reductions in snowfall, and declining snow cover across the Arctic. Under doubled CO₂, the snow-free period lengthens substantially due to earlier snowmelt (–28 days) and delayed snow onset (+ 36 days). However, rainfall extremes intensify sharply. The frequency of extremely wet days increases up to sixteen-fold, and present-day 100-year rainfall events recur every 2–7 years, particularly in historically dry regions. These changes indicate that although the snow-free season is expanding, increasingly frequent and intense rainfall events may reduce the suitability of breeding conditions. This highlights the growing importance of hydroclimatic extremes in shaping ecological responses to climate change in Arctic systems.