The impact of snow cover on seasonal ground frost and freeze-thaw processes is not yet fully understood. The authors therefore examined how snow cover affects seasonal ground frost in a coastal setting in northern Sweden. Air and soil temperatures were recorded in a paired-plot experiment, both with and without snow cover, during the frost season 2012-2013. The frequency, duration, and intensity of the freeze-thaw cycles during the frost season were calculated. The results showed that the freeze-thaw frequency was 57% higher at the soil surface and the intensity 10 degrees C colder in the spring of 2013, when the ground lacked snow cover. Furthermore, the duration of the seasonal freeze-thaw cycle was 30 days longer on average in cases where there was natural snow accumulation. The correlation between air and ground surface temperatures weakened with increased snow-cover depth. The authors conclude that continued increases in air temperature and decreases in snow in coastal northern Sweden might alter freeze-thaw cycles and thus affect natural and human systems such as geomorphology, ecology, spatial planning, transport, and forestry.

Observations of active-layer thickness from nine sites with up to 29 years of gridded measurements located in the Tornetrask region, northernmost Sweden, were examined in relation to climatic trends. Mean annual air temperatures in this area have warmed and recently rose above 0 degrees C. Active layers at all sites have become thicker, at rates ranging from 0.7 to 1.3 cm per year. This trend has accelerated in the past decade, especially in the westernmost site where rates have reached 2 cm per year and permafrost has disappeared at 81 per cent of the sampling points. Increased active-layer thicknesses are correlated with increases in mean summer air temperature, thawing degree-days and, in five of the nine sites, with increases in snow depth. Copyright (C) 2008 John Wiley & Sons, Ltd.