Comprehensive Assessment of Seasonally Frozen Ground Changes in the Northern Hemisphere Based on Observations

Abstract Seasonally frozen ground (SFG) in the Northern Hemisphere (NH) plays a significant role in the earth system via changes in the freeze‐thaw cycle. Previous studies primarily focus on permafrost; however, the SFG response to climate change on a hemispheric scale is uncertain due to a lack of observations. We rectify this with a newly assembled comprehensive database of 1,220 stations with daily observations. To quantify the spatiotemporal characteristics of SFG in response to climate change, we calculate eight variables with these observations: the first date of soil freeze (FFD), freezing duration (FDR), maximum freeze depth (MFD), the date of maximum freeze depth (MFDD), the last date of soil thaw (TLD), thawing duration (TDR), freeze‐thaw duration (FTDR), and actual number of freezing days (AD). During the variables' common 1986–2005 period, MFD decreased 8.9 cm (9% change). FFD was later by 5.3 days (2% change), MFDD and TLD were earlier by 14.5 days (27% change) and 24.7 days (22% change), respectively, and FDR and TDR decreased by 9 days (11% change) and 4.6 days (10% change). FTDR and AD decreased 18.1 days (14% change) and 12.1 days (10% change), respectively. The spatial pattern of freeze‐thaw variables depends on latitude and elevation, and varies by climatic zone: FTDR increases, going from the warm temperate climate, to the arid climate, and the snow and polar climates. The variability in freeze‐thaw changes is mainly driven by air temperature and latitude, while precipitation, soil moisture, snow depth, and elevation are relatively insignificant at the hemispheric scale.