Satellite Microwave Radiometry at L-Band for Monitoring Earth’s Essential Climate Variables: From Fundamental Physics to Sixteen Years of Global Climate Observations and Beyond

L-band satellite radiometry has emerged as an important tool for monitoring Earth’s essential climate variables (ECVs). It relies on spaceborne radiometers operat- ing in a protected band (1.4–1.427 GHz) to measure Earth’s surface thermal microwave emission in brightness temperatures. Ob- servations at this band experience minimal atmospheric attenuation and radio-frequen- cy interference (RFI), and can be acquired continuously from day to night, ensuring a short global revisit time. Moreover, L-band radiation can partially penetrate natural materials, allowing for the assessment of subsurface state parameters. These features make L-band radiometry satellites valuable for continuous global climate monitoring, offering unique advantages in tracking spe- cific ECVs that are difficult to measure us- ing other remote sensing techniques. This article reviews recent advances in satellite microwave radiometry at L-band, with a focus on the con- tributions of key missions—SMOS, Aquarius and SMAP— in retrieving six critical ECVs: 1) surface soil moisture (SM), 2) soil freeze/thaw (FT) status, 3) vegetation aboveground biomass (AGB), 4) sea surface salinity (SSS), 5) sea surface wind (SSW) speed, and 6) sea ice thickness (SIT). It sum- marizes the rationale behind satellite microwave radiom- etry and its role in understanding of the spatiotemporal dynamics of these ECVs on a global scale based on more than 16 years of continuous data. Furthermore, it identi- fies the state-of-the-art status of the discussed ECV prod- ucts, highlights current challenges, and outlines future directions for the application of microwave radiometry in monitoring ECVs.