Decadal changes in seasonal snow cover of the Northern Hemisphere

Seasonal snow cover of the Northern Hemisphere (NH) is an important part of the global climate system. Snow cover greatly influences surface albedo and thus the Earth’s energy balance, which in turn makes snow cover an important variable in climate models. Snow cover also greatly influences the hydrological cycle in the high latitudes and mountainous regions, and affects many human activities, such as road traffic, tourism, and agriculture. Due to the sparse measurement network, monitoring snow cover at a continental scale is only possible from satellites or using reanalysis data. The aim of this thesis is to investigate satellite-based timeseries and trends in snow cover during the satellite era (1980s-present) and to analyze how well the most recent reanalyses (ERA5 and ERA5-Land) and climate models (CMIP6) can capture the observed changes. The results show that statistically significant trends exist toward earlier melt onset in the NH. Also, snow cover extent and the NH total seasonal snow mass have been decreasing, but considerable spatial and temporal variability exists. The trends intensify as spring progresses. The analysis revealed that CMIP6 climate models still struggle to describe snow cover accurately. The CMIP6 models generally overestimate snow mass, but large variability exists between models. However, the results also suggest that climate models may be able to simulate snow mass trends in a warming climate correctly, even if snow mass itself is not accurately reproduced. Also, both ERA5 and ERA5-Land reanalyses show inaccuracies in snow cover properties and the multidecadal trends in ERA5 are not fully reliable due to temporal instability. However, the results also indicate that both ERA5 and ERA5-Land can capture the annual variability of seasonal snow cover quite well. Based on the results of this thesis, it is evident that work is still needed to accurately describe snow cover in both reanalyses and climate models.