On the Determination of GCOS ECV Product Requirements for Climate Applications

Abstract The essential climate variables (ECVs) defined by the Global Climate Observing System (GCOS) of the World Meteorological Organization (WMO) are the baseline for establishing and maintaining a global observing system that critically contributes to characterizing Earth’s climate. The ECVs describe parts of the physical, chemical, and biological Earth system. Originally, scientists developed and used individual ECV products for the analysis of the climate system and its changes. In recent years, the focus has shifted to using combined (i.e., multiple) and in-filled (i.e., without gap) ECV products, e.g., to analyze the energy, water, and carbon cycles. Requirements for ECV products have been established by the GCOS. Although the requirements include spatial and temporal resolution and product, they often lack a detailed breakdown per application beyond a few broad areas. This paper presents a universal and statistically rigorous procedure to determine—for any ECV product at any location on Earth—the observational requirements of different climate applications while considering variability due to natural and anthropogenic sources and due to observational limitations. We provide a categorization of known types of climate applications. Furthermore, we discuss the sources of natural and anthropogenic variability of ECVs and of uncertainties in observational ECV products, such as carbon dioxide mole fraction, sea surface temperature, and accumulated precipitation, and show how to quantify these. Finally, the presented procedure is applied to illustrate how observational requirements of a sea surface temperature product can be determined for a regional and global climate application. The procedure, however, may apply to observational ECV products from any source including from in situ measurements. Significance Statement The paper addresses the request of the Global Climate Observing System (GCOS) for transparent and quantitative information on the quality of its essential climate variable (ECV) products and verifying their suitability for different climate applications. Beyond describing its international embedding, this position paper proposes a universal and statistically rigorous procedure to assess the feasibility of an ECV product for any climate application at any location on Earth. The procedure—which is based on metrological principles and assumes Gaussian distributed anomalies over time—considers both natural and anthropogenic variability of the ECV as well as observational limitations of the ECV product. Although the procedure is demonstrated for satellite-based ECV products, it can also be applied to other measurement datasets.