Retrieval Methods for Microwave Remote Sensing of Soil Moisture, Above-Ground Biomass, and Freeze-Thaw Dynamics: A review

Tangible climate changes are experienced by Earth. They refer to long-term changes in, e.g., temperature and precipitation as well as other aspects of the Earth system. Essential climate variables (ECVs) are used to monitor climate changes, validate climate models, and ultimately, inform policy decisions; they are, therefore, essential markers that help scientists understand the Earth system dynamics. Fundamental contributions to the understanding and characterization of ECV values can be given by the analysis of data produced by current and next-generation Earth observation (EO) satellites, which, among other options, can be based on microwave remote sensing (RS). EO sensors can directly characterize bio-geophysical variables connected to ECVs, such as, e.g., the soil moisture (SM), the above-ground biomass (AGB), and the freeze-thaw (FT) dynamics. While FT dynamics are primarily relevant in northern latitudes, SM and AGB are globally significant parameters. SM, AGB, and FT are also key variables for the European Space Agency (ESA) EO science strategy and the ESA Climate Change Initiative program, mainly due to their strong relevance in understanding the global hydrological and carbon cycles. This work aims to be a comprehensive review of retrieval techniques for SM, AGB, and FT, considering data collected with passive and active microwave RS systems. An overview of the methods proposed in the last decades is reported and discussed, and then future perspectives and current challenges are outlined. This article targets a very broad scientific community, not necessarily engineers but rather any scientist with an interest in microwave EO and satellite RS. For that reason, some basic concepts are briefly reported and discussed throughout the manuscript.