Soil Hot Extremes are Increasing Faster than Air Hot Extremes Regionally

Hot temperature extremes are changing in intensity and frequency. Quantifying these changes is key for developing adaptation and mitigation strategies. The conventional approach to study changes in hot extremes is based on air temperatures. However, many biogeochemical processes, i.e. decomposition of organic material and release of CO2, are triggered by soil temperature and it remains unclear whether it changes as does air temperature. Here, we demonstrate that soil hot extremes are intensifying and becoming even more frequent faster than air hot extremes over central eastern and western Europe. Based on existing model simulations, we also show that the increase in hot soil extremes could amplify or spread future heat waves by releasing sensible heat during hot days. We find an increase of 3 (7) % in the number of hot days with a contribution of heat from the soil under a warming level of 2.0 (3.0) °C than under a warming level of 1.5 °C. Furthermore, defining intensity and frequency extreme indices based on soil and air temperatures leads to a difference of more than 1 °C in intensity and 10% in frequency regionally during the last decades of the 21st century under the SPP5 8.5 emission scenario. In light of these results, maximum soil temperatures should be included in ecological risk studies as a complementary perspective to the conventional approach using extreme indices based on air temperatures.