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 substantial Arctic warming under SSP2-4.5 (approximately 4.9 °C by 2081-2100 relative to 1995-2014), reductions in snowfall, and declining snow cover across the Arctic. Separately, the idealized EC-Earth equilibrium experiment, representing approximately 6 °C of global-mean warming relative to pre-industrial conditions in the 2×CO₂ state, shows how reproductive-season hydroclimate responds under strong sustained warming. Between the 1×CO₂ and 2×CO₂ equilibrium states, the snow-free period lengthens substantially due to earlier snowmelt (-28 days) and delayed snow onset (+36 days), while rainfall extremes intensify sharply. Extremely wet days become up to sixteen times more frequent, and rainfall events with a 100-year return period in the 1×CO₂ climate recur every 2-9 years in the 2×CO₂ climate, with particularly large relative changes in historically dry regions. These results show that a longer snow-free season under strong warming can coincide with substantially greater exposure to rainfall extremes, highlighting the importance of hydroclimatic extremes when assessing ecological responses to Arctic climate change.