Climate warming may cause Finnish lake water to grow colder under winter ice

A new international study reveals that despite rising air temperatures, water beneath the ice in Finnish lakes can become colder in winter due to delayed freezing, Finnish public broadcaster Yle reports.

Researchers from the University of Eastern Finland, the Finnish Environment Institute, and Canada’s York University found that when lakes freeze later in the season, the water near the bottom remains colder throughout winter compared to years with earlier ice formation. The study analyzed decades of data from lakes across Finland, tracking ice cover and water temperatures since 1971.

The phenomenon occurs because prolonged open-water periods in autumn allow more heat to escape before ice forms. Without the insulating effect of early ice, lakes lose warmth for longer, leading to lower wintertime water temperatures beneath the surface. The study also linked late freezing and early thawing to maximum surface water temperatures the following summer.

Over the monitoring period, Finnish lakes have warmed significantly, with average autumn surface water temperatures rising by 1.85°C. Ice formation now occurs up to 20 days later than in past decades. These temperature shifts affect aquatic organisms, as many species—from plankton to fish—become less active in colder water.