A proxy-year analysis shows reduced soil temperatures with climate warming in boreal forest

Scientists unequivocally agree that winter air temperature (T_A) in northern high latitudes will increase sharply with anthropogenic climate change, and that such increases are already pervasive. However, contrasting hypotheses and results exist regarding the magnitude and even direction of changes in winter soil temperature (T_S). Here we use field and satellite data to examine the 'cold soil in a warm world' hypothesis for the first time in the boreal forest using a proxy year approach. In a proxy warm year with a mean annual temperature similar to that predicted for ~2080, average winter T_S was reduced relative to the baseline year by 0.43 to 1.22 ^°C in open to forested sites. Similarly, average minimum and maximum winter T_S declined, and the number of freeze-thaw events increased in the proxy warm year, corresponding to a reduction in the number of snow-covered days relative to the baseline year. Our findings indicate that early soil freezing as a result of delayed snowfall and reduced snow insulation from cold winter air are the main drivers of reduced winter active-layer T_S (at ~2-cm depth) under warming conditions in boreal forest, and we also show that these drivers interact strongly with forest stand structure.