ESA Climate Change Initiative Plus - Soil Moisture Climate Assessment Report Deliverable ID: D5.1 Version 2.0

Within the framework of the European Space Agency (ESA) Climate Change Initiative (CCI) soil moisture project, an over 40-year (1978-2022) soil moisture time series (ESA CCI SM v08.1) is developed, which consists of three products: an ACTIVE data set, a PASSIVE data set and a COMBINED data set. It provides daily surface soil moisture with a spatial resolution of 0.25°. The purpose of the Climate Assessment Report (CAR) is the assessment of the ESA CCI SM time series for climate science. We highlight here the ESA CCI SM overview paper published in the CCI special issue in Remote Sensing of Environment: "Earth Observation of Essential Climate Variables". Dorigo et al. (2017) gives a comprehensive overview on the ESA CCI SM products. Not only their specifications and error characteristics, but also the wide range of Earth system applications currently using the data. Furthermore, we update new versions of the CAR against the most recent scientific literature that makes use of the ESA CCI SM dataset for the different applications. For this version of the CAR, more than 200 new studies (as of March 2024) citing the ESA CCI SM key publications since April 2023 have been reviewed. The outcome underlines soil moisture to be an essential climate variable and highlights the value of the established global long-term soil moisture time series based on satellite observations. Furthermore, the document provides a critical view on the potentials and limitations of the developed datasets for climate studies.