Future climate warming prolongs spring mixing and increases annual greenhouse gas emissions in a boreal lake

Seasonally ice-covered lakes are significant sources of methane (CH₄) and carbon dioxide (CO₂), with substantial emissions during spring and autumn turnover. During these events, gases accumulated in the hypolimnion are released to the atmosphere, with turnover accounting for 26–59% of CH₄ and 15–30% of CO₂ in some lakes. Climate change is altering ice cover duration and mixing regimes, affecting greenhouse gas (GHG) dynamics: reduced ice cover prolongs the ice-free period, increasing opportunities for GHG production and release, while shifts in turnover timing and duration can modify both the magnitude and seasonality of emissions, potentially generating climate feedbacks. Although the effects of climate change on ice cover and mixing are increasingly studied, the combined impacts on greenhouse gas production and release under future warming scenarios are still not well quantified.
We simulated projected changes in ice cover, turnover periods, and GHG dynamics in Lake Kuivajärvi, a small boreal lake in Finland, under future warming scenarios using outputs from five general circulation models and the LAKE model. LAKE reproduces temperature, horizontal velocities, O₂, CO₂, and CH₄ using a horizontally averaged transport equation, including sediment interactions and a snow-ice module.
Our results show substantial inter-model differences in ice cover length, and turnover timing and duration. Trends in ice cover duration and spring turnover are generally consistent—ice cover is decreasing (-14 ± 6 days per decade) and spring turnover is starting earlier and lasting longer (~3.6 days per decade)—whereas changes in autumn turnover are highly uncertain, with low model agreement and high variability. Lake Kuivajärvi is projected to experience occasional years with a monomictic regime by the late 21st century. Alongside changes in ice cover and turnover timing, CH₄ and CO₂ emissions are increasing during extended ice-free periods and altered mixing events.