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The near‐surface zero degree line (ZDL) is a key isotherm in mountain regions worldwide, but a detailed analysis of methods for the ZDL determination, their properties and applicability in a changing climate is missing. We here test different approaches to determine the near‐surface ZDL on a monthly scale in the Swiss Alps. A non‐linear profile yields more robust and more realistic ZDLs than a linear profile throughout the year and especially in the winter‐half year when frequent inversions disqualify a linear assumption. In the period 1871–2019, the Swiss ZDL has risen significantly in every calendar month: In northern Switzerland, the monthly ZDL increases generally amount to 300–400 m with smaller values in April and September (200–250 m) and a larger value in October (almost 500 m). The largest increases of 600–700 m but also very large uncertainties (±400 m, 95% confidence interval) are found in December and January. The increases have accelerated in the last decades, especially in spring and summer. The ZDL is currently increasing by about 160 m·°C−1 warming in the summer‐half year and by up to 340 ± 45 m·°C−1 warming in winter months. In southern Switzerland, ZDL trends and temperature scalings are somewhat smaller, especially in winter. Sensitivity analyses using a simple shift of the non‐linear temperature profile suggest that the winter ZDL‐temperature scalings are at a record high today or will reach it in the near future, and are expected to decrease with a strong future warming. Nevertheless, the cumulative ZDL increase for strong warming is considerably larger in winter than in summer. Based on a few key criteria, we also present best practises to determine the ZDL in mountain regions worldwide. The outlined methods lay a foundation for the analysis of further isotherms and to study the future ZDL evolution based on climate scenario data. [ABSTRACT FROM AUTHOR]

A range of recent studies and national climate scenario initiatives have shown that 21st Century global climate change will also importantly affect the climate of the European Alps. Among others, a continued near-surface warming larger than the global mean warming is consistently projected, with the average warming rate depending on the emission scenario considered. The projected warming pattern is subject to some horizontal variability and also shows elevation dependencies. These factors will influence the future evolution of the regional zero-degree line elevation in the Alps. This elevation, in turn, often serves as an illustrative but also quantitative proxy for climate change impacts on different sectors.We here assess the future evolution of the zero-degree line in the European Alps under different emission scenarios. For this purpose the EURO-CORDEX regional climate projection ensemble is exploited. The zero-degree line evolution is analyzed in the raw climate model output and, focusing on the region of Switzerland, in bias-corrected scenario products. Overall, we find a consistent rise of the zero degree line elevation with some seasonal and regional variation. In Switzerland, for instance, the mean winter zero-degree line is projected to rise from today's ~800m to ~1700m by the end of the Century and for the strong RCP8.5 emission scenario. The rise per degree Celsius warming approximately corresponds to standard atmospheric lapse rates in today's climate. [ABSTRACT FROM AUTHOR]