What are climate indices and indicators?

The terminology concerning climate indices and indicators differs, with no consistent definitions in the literature. Climate indices generally refer to the diagnostic quantity of one or several meteorological and/or hydrological variables including possible thresholds, for different temporal and spatial aggregations (cf. Kjellström et al., 2016), whereas indicators refer to the function of a variable to represent a phenomenon (cf. Hinkel, 2011), such as climate change hazards, impacts or vulnerabilities. Climate indices can be used as indicators to understand phenomena and to support adaptation planning and decision making (Dubois et al., 2016, WMO, 2014), and in such cases their definitions are the same. However, a climate index is not intuitively or necessarily an indicator.