A framework for ensemble modelling of climate change impacts on lakes worldwide : the ISIMIP Lake Sector

Empirical evidence demonstrates that lakes and reservoirs are warming across the globe. Consequently, there is an increased need to project future changes in lake thermal structure and resulting changes in lake biogeochemistry in order to plan for the likely impacts. Previous studies of the impacts of climate change on lakes have often relied on a single model forced with limited scenario-driven projections of future climate for a relatively small number of lakes. As a result, our understanding of the effects of climate change on lakes is fragmentary, based on scattered studies using different data sources and modelling protocols, and mainly focused on individual lakes or lake regions. This has precluded identification of the main impacts of climate change on lakes at global and regional scales and has likely contributed to the lack of lake water quality considerations in policy-relevant documents, such as the Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC). Here, we describe a simulation protocol developed by the Lake Sector of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) for simulating climate change impacts on lakes using an ensemble of lake models and climate change scenarios for ISIMIP phases 2 and 3. The protocol prescribes lake simulations driven by climate forcing from gridded observations and different Earth system models under various representative greenhouse gas concentration pathways (RCPs), all consistently bias-corrected on a 0.5 degrees x 0.5 degrees global grid. In ISIMIP phase 2, 11 lake models were forced with these data to project the thermal structure of 62 well-studied lakes where data were available for calibration under historical conditions, and using uncalibrated models for 17 500 lakes defined for all global grid cells containing lakes. In ISIMIP phase 3, this approach was expanded to consider more lakes, more models, and more processes. The ISIMIP Lake Sector is the largest international effort to project future water temperature, thermal structure, and ice phenology of lakes at local and global scales and paves the way for future simulations of the impacts of climate change on water quality and biogeochemistry in lakes.Peer reviewe

Physical background of the development of oxygen depletion in ice-covered lakes

The effect of the heat interaction between a water column and sediments on the formation, development, and duration of existence of anaerobic zones in ice-covered lakes is estimated based on observational data from five frozen lakes located in northwestern Russia and North America. A simple one-dimensional model that describes the formation and development of the dissolved oxygen deficit in shallow ice-covered lakes is suggested. The model reproduces the main features of dissolved oxygen dynamics during the ice-covered period; that is, the vertical structure, the thickness, and the rate of increase of the anaerobic zone in bottom layers. The model was verified against observational data. The results from the verification show that the model adequately describes the dissolved oxygen dynamics in winter. The consumption rates of DO by bacterial plankton and by bottom sediments, which depend on the heat transfer through the water-sediment interface, are calculated. The results obtained allow the appearance of potentially dangerous anaerobic zones in shallow lakes and in separate lake areas, which result from thermal regime changes, to be predicted

OPTICAL PROPERTIES OF LAKE VENDYURSKOE

We conducted a field study on light conditions in a small boreal Karelian Lake Vendyurskoe over two years. Albedo of ice-covered lake varied from 0.9 to 0.1, and the euphotic zone depth exceeded 3.5 m during the melting stage. The Secchi disc depth changed from 2.5 m after ice-break to 3.7 m at the stage of early summer. The vertical distribution of the photosynthetically active solar radiation (PAR) attenuation coefficient for water Kw was characterized by high spatial (vertical) and temporal (seasonal and interannual) variabilitywhich can be connected with the dynamics of plankton cells. The highest values of Kw  eached 2–2.8 m–1 in the upper 0.5 m layer of a water column, and decreased to 0.5–1.5 m–1 with increasing depth. The highest values of Kw were marked in the end of ice-covered period

Towards critical white ice conditions in lakes under global warming

Many studies report on a shrinking cryosphere with a rapid lake ice cover loss in a warmer world, but very little information is available on ice quality changes. Here we show for the first time spatial and seasonal variation in the thickness and proportion of white ice on lakes across the Northern Hemisphere during one of the warmest winters since 1880. Based on 167 field observations during 2020/2021, we found white ice conditions in 77 % of the sampling occasions with a white ice layer varying between 0 and 44 cm, contributing between 0 and 100 % to the total ice thickness. We noted that white ice built up over the winter season, being thickest and taking up the largest proportion towards the end of the ice cover season when fatal winter drownings occur most often and light limits the growth and reproduction of primary producers. We attribute the dominance of white ice before ice-off to air temperatures varying around the freezing point, causing the upper ice layer and snow on ice to melt and refreeze to a white ice layer. During warmer winters, the seasonal cycle of freezing air temperatures flattens, resulting in an increased number of days when air temperatures vary around the freezing point. Thus, under continued global warming, the prevalence of white ice is likely to increase during the critical period before ice-off, for which we adjusted commonly used equations for ice safety and light transmittance through ice