CLASS · Cold season climate reconstructions from Lakes in the Alpine regions of Switzerland and Sweden

Past variations in interannual climatic variability are currently not well understood, mainly due to a lack of temperature reconstructions for the winter season. This proposal aims to provide quantitative data on winter-spring temperatures for two lake sites in the Alpine regions of Switzerland and northern Sweden, since these are likely to contain sensitive archives of past climate change. The reconstruction of winter temperatures in these two regions is highly relevant, since local winter temperatures and precipitation are controlled by the variability and strength of the North Atlantic Oscillation (NAO). Increased knowledge on the impact of NAO in these sensitive mountain regions is crucial for a better understanding of future impacts on e.g. lake ecology, water-and energy supplies and winter tourism. In addition, obtained data can be compared to existing summer temperature reconstructions to study changes in interannual variability. A novel methodology developed at the host institute is proposed, using Chrysophyte stomatocysts as a proxy for winter-spring temperatures. However, to distinguish between climate-induced changes in stomatocyst assemblages and the impact of other factors affecting lake ecology, such as human influence, a multi-proxy approach is required including the analysis of pollen, C and N ratios, total biogenic silica and sediment grainsize. At the Swiss site the proposal targets three time periods; I) modern time (AD 1864-present), for validation with measurement data, II) the most intense phase of the Little Ice Age, including the Maunder Minimum (AD 1650-1800) and III) the warmest phase of the Mediaeval Warm Anomaly (a 100 years window, possibly AD 850-950). In Sweden the proposed method has not been applied before, and therefore the method will be tested her for the calibration period, AD 1913-present.