Within the last decades, the water temperature of several European lakes has risen. It is assumed that these temperature increases are due to a reconfiguration of the heat‐balance components. This study explores the dominant modifications of heat exchange with the atmosphere and their temporal evolutions. The objective is to identify the primary changes in heat fluxes and the sequence of events of the reconfiguration for the period 1984–2011. For this purpose, a model was applied to Lake Constance to estimate the contributions of the individual heat fluxes to the total heat balance. The results show that increasing absorption of solar radiation (+0.21 ± 0.13 W m−2 yr−1) and of longwave radiation (+0.25 ± 0.11 W m−2 yr−1) was responsible for the lake surface warming of 0.046 ± 0.011°C yr−1. Heat losses to the atmosphere by longwave emission (−0.24 ± 0.06 W m−2 yr−1) and by latent heat flux (−0.27 ± 0.12 W m−2 yr−1) have intensified in parallel due to higher lake surface temperatures. The heat budget is in a quasi‐steady state, whereas incoming solar radiation and the warmer atmosphere increased the lake surface temperature; the warmer surface emits more longwave radiation and more water is evaporated. At each level of the slowly increasing water temperature, the heat fluxes are balanced. The overall change of the total heat content, however, is relatively little. Although the cooling effect of inflowing rivers decreased, this contribution is also small.

Twenty to fifty years of annual mean deepwater (hypolimnetic) temperature data from twelve deep lakes spaced across Europe (2u959W to 14u09E, 46u279 to 59u009N) show a high degree of coherence among lakes, particularly within geographic regions. Hypolimnetic temperatures vary between years but increased consistently in all lakes by about 0.1-0.2uC per decade. The observed increase was related to the weather generated by largescale climatic processes over the Atlantic. To be effective, the climatic signal from the North Atlantic Oscillation (NAO) must affect deep lakes in spring before the onset of thermal stratification. The most consistent predictor of hypolimnetic temperature is the mean NAO index for January-May (NAO J-M ), which explains 22-63% of the interannual variation in deepwater temperature in 10 of the 12 lakes. The two exceptions are remote, less windexposed alpine valley lakes. In four of the deepest lakes, the climate signal fades with depth. The projected hypolimnetic temperature increase of approximately 1uC in 100 yr, obtained using a conservative approach, seems small. Effects on mixing conditions, thermal stability, or the replenishment of oxygen to deep waters result in accumulation of nutrients, which in turn will affect the trophic status and the food web.

The implications of climatic change on Lake Constance are studied by long-term hydrodynamic model simulations. In contrast to earlier studies that mostly have applied one-dimensional models, this investigation utilizes a three-dimensional hydrodynamic model and thus profits from the advantages of a spatial representation of lake bathymetry and hydrophysical processes. Model adaptation and validation are based on half a century of vertically resolved temperature recordings (1961 to 2011). Three different horizontal grid layouts are used to test the sensitivity of the thermal stratification and effective vertical turbulent diffusivities (D V eff ) determined with the heat-budget method to grid resolution. D V eff calculated from observations and from simulations with different grid resolutions agree rather well. However, in the deep water simulated D V eff are overestimated if the basin is resolved only by few grid cells. The investigation of the effect of climatic changes on Lake Constance is focused on the effects of altered air temperatures and wind velocities on deep-water renewal. Numerical tracers are used as indicators of the winter vertical transport and mixing. Effects on stratification and mixing strongly depend on the seasonal course of the climatic changes. Warmer winter temperatures result in reduced deep-water exchange and more frequent years with incomplete mixing. Increased air temperatures in summer have almost no effect on deepwater renewal in winter but increase water-column stability during summer and autumn. Increased wind speeds influence vertical mixing and surface heat fluxes and cause higher deep-water temperatures if wind speeds are higher during the summer.The application of numerical models to investigate the consequences of changes in physical forcing for dynamic processes in lakes has become an established methodology in limnology. Models are chosen according to the questions addressed but also with respect to the limitations of computational resources. The representation of the spatial dimension in these models ranges from quite simple onebox models to highly resolved three-dimensional (3D) grids. Modeling climate effect on lakes often requires longterm calculations to account for the interannual variability of meteorological forcing and lake status as well as its persistence, e.g., due to heat storage. Climate effect studies are therefore mostly carried out by rather efficient vertical one-dimensional (1D) models allowing multidecadal simulations on ordinary computers. An overview of different 1D models that have been applied to study the effect of climate warming on stratification in lakes is given in MacKay et al. (2009).The feasibility of continuous long-term hydrodynamic modeling of deep lakes and the importance of heat carryover between years in such lakes has been demonstrated with the application of a 1D model to Lake Zurich comparing simulation results with five decades of data . Danis et al. (2004) applied a 1D model to study the thermal behavior of Lake Ammersee an...

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