Delivering the Lake Essential Climate Variables - an Update from ESA CCI Lakes

Lakes and enclosed inland seas are integrators of environmental and climatic changes occurring within their contributing basins. The factors that drive lake conditions vary widely across space and time, and lakes, in turn, impact their surrounding environments in important and diverse ways. As a result, lakes are important to our understanding of climate change as sentinels of change, as actors in influencing change, and as integrators of their surrounding basins. They constitute essential components of the hydrological and biogeochemical water cycles due to their basic ability to store, retain, clean, and provide water. The geographical distribution of the world's lakes is very irregular, with most lakes located at high latitudes in the Northern Hemisphere and in currently or formerly glaciated areas. Their global distribution is of special interest for large-scale studies of environment, biodiversity, health (spread of water-borne diseases), agricultural suitability, climate change modeling, and for assessments of present and future water resources. Their temperature and thermal structure is strongly influenced by incoming radiation. They have significant chemical variation in terms of nutrients, major ions, and contaminants. Biomass, population numbers, and growth rates are also impacted by climate changes. They vary in terms of water quantity. Lakes also have a broader influence on many facets of terrestrial ecology, biodiversity, economy, and human welfare. It is recognized that lakes also play also a substantial role in greenhouse gases flux exchanges with the atmosphere. Nonetheless, in situ lake observations are scarce in many areas of the globe, especially geographically remote, lake-rich regions such as the Canadian and Siberian Arctic. Existing and forthcoming remote sensing technologies have exhibited intriguing potential to address this deficiency in several areas. Current altimeters provide dense time series of water surface elevation measurements for large lakes, and multispectral optical and thermal sensors can be used to measure lake area, water quality, temperature, and ice cover (e.g., Woolway and Merchant, 2019; Wang et al., 2018; Duguay and Lafleur P. M., 2003; O'Reilly, C. M., et al. 2015, Pekel et al., 2016. In the future, wide-swath altimeters like the upcoming SWOT (Surface Water and Ocean Topography) mission can provide more robust measurements of height on much smaller lakes. Hyperspectral satellite imagers such as the recently launched PRISMA (Precursore Iperspettrale della Missione Applicativa) mission will have the potential to provide more nuanced views of sediment, carbon, phytoplankton and other constituents of lake water. High resolution thermal imagery will allow to better quantify and extensively map lake water temperature. Much work remains, however, to realize a robust, global dataset of lake water quantity and quality that can be used to understand how lakes are changing in response to climate and other human impacts, and how lakes influence the climate. The GCOS (Global Climate Observing System), in response to an invitation from the UNFCCC (United Nations Framework Convention on Climate Change), have identified "required actions to reduce gaps in knowledge to improve monitoring and prediction, to support mitigation, and to help meet increasingly urgent needs for information on impacts, adaptation and vulnerability" (implementation plan document of the GCOS). To reach this goal, GCOS has defined a set of variables representative of the different components of the Earth system and its climate. These comprise the Essential Climate Variables (ECVs), long-term observations of the atmosphere, the continental surface and sub surface, and the ocean. On each of these compartments GCOS has ident