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Described is the concept of Essential Climate Variables developed under the Global Climate Observing System for a range of applications, as well as to provide an empirical basis for understanding past, current, and possible future climate variability and change.
Observations are fundamental to advancing scientific under- standing of climate (Doherty et al. 2009; Shapiro et al. 2010) and delivering the vetted, timely, and purposeful climate information needed to support decision making in many sectors. Observations and monitoring are key elements of the emerging Global Framework for Climate Services (WMO 2011a) and more generally support climate research, the assessment of climate change, and the development of policy responses (Fig. 1). For these purposes, observational datasets in general need to be traceable to quality standards, be readily inter- pretable and freely available, and cover sufficiently long periods: for example, the 30 years traditionally used for calculating climate normals (WMO 2011b). Transparency in the generation of climate datasets is essential for ensuring the credibility of the climate record (UN 2012).
In the 1990s, gaps in knowledge of climate and declining core observational networks in many coun- tries (Houghton et al. 2012) led to calls for systematic observation of a limited set of critical variables. To provide guidance, the Global Climate Observing System (GCOS) program developed the concept of "essential climate variables" (ECVs), which has since been broadly adopted in science and policy circles.
In this article, we define the ECV concept and de- scribe its provenance, scientific rationale and uptake. We also discuss challenges and opportunities con- cerning the ECV concept and its possible evolution, in particular with regard to the GCOS-led process of assessment, adequacy, and implementation of global observing systems for climate.
WHAT ARE THE ECVS? An ECV is a physical, chemical, or biological variable or a group of linked variables that critically contributes to the charac- terization of Earth's climate. ECV datasets provide the empirical evidence needed to understand and predict the evolution of climate, to guide mitigation and adaptation measures, to assess risks and enable attribution of climatic events to underlying causes, and to underpin climate services. The current list of ECVs is specified in GCOS (2010a) (all GCOS reports are available at www.wmo.int/pages/prog/gcos/index ,php?name= Publications) and reproduced in Table 1.
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