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

-band satellite radiometry has emerged as an important tool for monitoring Earth’s essential climate variables (ECVs). It relies on spaceborne radiometers operating in a protected band (1.4–1.427 GHz) to measure Earth’s surface thermal microwave emission in brightness temperatures. Observations at this band experience minimal atmospheric attenuation and radio-frequency 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 specific ECVs that are difficult to measure using other remote sensing techniques. This article reviews recent advances in satellite microwave radiometry at L-band, with a focus on the contributions 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 summarizes the rationale behind satellite microwave radiometry 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 identifies the state-of-the-art status of the discussed ECV products, highlights current challenges, and outlines future directions for the application of microwave radiometry in monitoring ECVs.