Terrestrial Essential Climate Variables, known as terrestrial ECVs, are key sources of information for both application- and scientific- oriented research. A large number of global terrestrial ECV products have been derived from satellite observations, and more are forthcoming. To unlock the full potential of these products, end-users need to know their uncertainties and error magnitudes. Due to the lack of conformity among validation strategies, a wide range of validation approaches have been employed to assess the quality of these products, and have resulted in reduced comparability even for the same terrestrial ECV. Addressing this challenge in validation practices requires the use of unified, standard, publicly available, traceable and objective validation procedures that are operational for all products of a specific terrestrial ECV, and preferably also applicable for all ECVs at the global scale. This can allow end-users to perform comparative assessments. To this end, the current study aims to investigate the readiness status of a selected group of seven global long-term satellite-based terrestrial ECVs for operational validation. Selected variables are Leaf Area Index (LAI), Land Surface Temperature (LST), Evapotranspiration (ET), Soil Moisture (SM), Albedo, the fraction of Absorbed Photosynthetically Active Radiation (fAPAR), and Land Cover (LC). For each of these terrestrial ECVs, we reviewed key prerequisites and primary tools [notably, long term global product availability, globally distributed in situ measurement availability, a validation good practice protocol, and an online validation platform] required for developing an operational validation system. With respect to the “readiness level”, the investigation results demonstrate that LAI, SM, and LC are at the highest level of readiness for moving toward a full operational validation at the global scale. However, ET is at the lowest level of readiness, mainly due to the lack of standard validation good practice protocol and lack of a pilot online validation platform. The remainder of the selected terrestrial ECVs are identified to be at mid-level readiness, mainly because either a validation platform (i.e., LST and albedo) or good practice protocol (i.e., fAPAR) still needs to be developed. This review can pave the way for open-access, traceable, transparent, and operational validation procedures of satellite-based global terrestrial ECVs.

陆地基本气候变量，即陆地ECV，是面向应用和面向科学研究的重要信息来源。全球大量的地面ECV产品已经从卫星观测中获得，并且还会有更多的产品。为了充分发挥这些产品的潜力，最终用户需要了解其不确定性和误差幅度。由于验证策略之间缺乏一致性，因此已采用多种验证方法来评估这些产品的质量，即使对于相同的地面ECV，其可比性也会降低。要应对验证实践中的这一挑战，需要使用统一的，标准的，可公开获得的，可追溯的和客观的验证程序，这些程序适用于特定地面ECV的所有产品，并且最好也适用于全球范围内的所有ECV。这可以使最终用户执行比较评估。为此，本研究旨在调查选定的七个全球长期基于卫星的地面ECV的选定状态，以进行操作验证。选择的变量是叶面积指数（LAI），地表温度（LST），蒸散量（ET），土壤水分（SM），反照率，吸收的光合有效辐射分数（fAPAR）和土地覆盖（LC）。对于每个这些地面ECV，我们都审查了开发运营验证系统所需的关键先决条件和主要工具（尤其是长期的全球产品可用性，全球分布的现场测量可用性，验证良好实践协议和在线验证平台）。关于“准备水平”，调查结果表明，LAI，SM和LC处于最高水平，可以在全球范围内进行全面的运营验证。但是，ET处于最低的准备水平，这主要是由于缺乏标准的验证良好实践协议以及缺乏试验性的在线验证平台。选定的其余地面ECV被确定为处于中级就绪状态，这主要是因为仍然需要开发验证平台（即LST和反照率）或良好实践协议（即fAPAR）。这项审查可以为基于卫星的全球陆地ECV的开放获取，可追溯，透明和可操作的验证程序铺平道路。和LC处于为在全球范围内进行全面运营验证的最高准备水平。但是，ET处于最低的准备水平，这主要是由于缺乏标准的验证良好实践协议以及缺乏试验性的在线验证平台。选定的其余地面ECV被确定为处于中级就绪状态，这主要是因为仍然需要开发验证平台（即LST和反照率）或良好实践协议（即fAPAR）。这项审查可以为基于卫星的全球陆地ECV的开放获取，可追溯，透明和可操作的验证程序铺平道路。和LC处于为在全球范围内进行全面运营验证的最高准备水平。但是，ET处于最低的准备水平，这主要是由于缺乏标准的验证良好实践协议以及缺乏试验性的在线验证平台。选定的其余地面ECV被确定为处于中级就绪状态，这主要是因为仍然需要开发验证平台（即LST和反照率）或良好实践协议（即fAPAR）。这项审查可以为基于卫星的全球陆地ECV的开放获取，可追溯，透明和可操作的验证程序铺平道路。主要原因是缺乏标准的验证良好实践协议，也缺少试点在线验证平台。选定的其余地面ECV被确定为处于中级就绪状态，这主要是因为仍然需要开发验证平台（即LST和反照率）或良好实践协议（即fAPAR）。这项审查可以为基于卫星的全球陆地ECV的开放获取，可追溯，透明和可操作的验证程序铺平道路。主要原因是缺乏标准的验证良好实践协议，也缺少试点在线验证平台。选定的其余地面ECV被确定为处于中级就绪状态，这主要是因为仍然需要开发验证平台（即LST和反照率）或良好实践协议（即fAPAR）。这项审查可以为基于卫星的全球陆地ECV的开放获取，可追溯，透明和可操作的验证程序铺平道路。