Initial analysis of Essential Climate Variables from ESA's Lake_CCI Satellite Data Package

Lakes are key indicators of local and regional water¬shed changes, making them useful for detecting Earth's response to climate change (Adrian et al., 2009). Specifically, variables such as lake surface temperature, water level and extent, ice cover and lake colour are recognized by the Global Climate Observing System (GCOS) as Essential Climate Variables (ECVs) because they contribute critically to the characterization of Earth's climate (Woolway et al., 2020). Lakes are already responding rapidly to climate change. Some of the most pervasive and concerning physical consequences of climate change on lakes are the loss of ice cover (Sharma et al. 2019), changes in evaporation and water budgets (Rodell et al., 2018; Wang et al., 2018), warming surface water temperature (O'Reilly et al., 2015) and alterations in mixing regimes (Woolway and Merchant, 2019). Variations in temperature and precipitation can profoundly affect the hydrological functioning of the lake and its catchment. Together with changes in ice formation, lake level and hydrogeochemistry the effect on lake ecological functioning can be significant (Adger et al., 2007; Cisneros et al., 2014). The thermal structure of the lake can be strengthened by an increase in temperature leading to deoxygenation and an alteration of nutrient cycling that in some case exerts a stronger control than trophic status (Rogora et al., 2018). Lakes are of significant interest to the scientific community, local to national governments, industries and the wider public. A range of scientific disciplines including hydrology, limnology, climatology, biogeochemistry and geodesy are interested in the distribution and functioning of the millions of lakes (from small ponds to inland seas) from the local to the global scale. Future efforts investigating lake responses to climate change need to be grounded in sustainable, systematic, multivariate observations for a consistent set of lakes. One effort in this direction is the ongoing European Space Agency Climate Change Initiative for Lakes (CCI Lakes), which coordinates a range of remote- sensing techniques to address the lake ECVs identified by the GCOS (Woolway et al., 2020). An important aspect of efforts such as CCI Lakes is that they focus on maximizing the benefit of legacy Earth observations made over the past decades, as well as developing better observational capabilities from current and prospective missions (Woolway et al., 2020). The overarching objective of the Lakes_cci project is to produce and validate a consistent data set of the variables grouped under the Lakes ECV. This includes aiming for the longest period of combined satellite observations by designing and operating processing chains, designed to be ultimately feature in a sustainable production system. The Lakes_cci develops products for the following five thematic climate variables: oLake Water Level (LWL): a proxy fundamental to understand the balance between water inputs and water loss and their connection with regional and global climate changes. oLake Water Extent (LWE): a proxy for change in glacial regions (lake expansion) and drought in many arid environments, water extent relates to local climate for the cooling effect that water bodies provide. oLake Surface Water temperature (LSWT): correlated with regional air temperatures and a proxy for mixing regimes, driving biogeochemical cycling and seasonality. oLake Ice Cover (LIC): freeze-up in autumn and advancing break-up in spring are proxies for gradually changing climate patterns and seasonality. oLake Water-Leaving Reflectance (LWLR): a direct indicator of biogeochemical processes and habitats in the visible part of the water column (e.g. seasonal phytoplankton biomass fluctuations), and an indicator of the freque