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

Final Report Summary - CLASS (Cold season climate reconstructions from Lakes in the Alpine regions of Switzerland and Sweden)

This research project aimed to provide winter temperature reconstructions from two study sites: Lake Silvaplana, (Swiss Alpine region) and Lake Nylandssjon (central Sweden). For the Swiss site the initial aim was to focus on three time windows during the past 1 000 years, comparing particularly warm phases (e.g. Medieval Warm Anomaly) with known cool phases (Little Ice Age). For the Swedish site the objective was to do an annual-resolution reconstruction covering recent time, to allow for detailed comparison to meteorological data.

The primary tool to generate these reconstructions were Chrysophyte stomatocysts. Chrysophyte algae produce silicious cysts, which are retained in lake sediments and can be analysed in detail using a scanning electron microscope. Previous work demonstrated the sensitivity of these algae to the timing of lake ice break-up, which is in turn strongly correlated with late-winter to early-spring air temperature. The methodology is novel and thus required careful testing, as well as methodological studies to optimise laboratory procedures.

The objectives for this project were achieved and even exceeded. The first study objective was the detailed comparison of reconstructed temperatures with measured mean monthly temperatures from the Swiss lake. This comparison showed that chrysophyte stomatocyst assemblages were significantly correlated to mean October - April temperatures, thus confirming the strength of the proxy for cool season temperature reconstructions. Next, cold-season temperatures were reconstructed from AD 1000 - 1500. The results obtained in this study were of such high quality that the reconstruction period was expanded to cover nearly the entire past 1 000 years. Three time periods (AD 1610 - 1620, 1680 - 1710, 1770 - 1870) could not be analysed to shortcomings of the material (insufficient number of stomatocysts present). Thus, a near-continuous, high-resolution, quantitative reconstruction of mean October - April temperatures was produced for the Swiss Alpine region.

Analysis of these data is ongoing; detailed comparisons to forcing factors, analysis of seasonal contrasts by comparison to summer temperature reconstructions, comparison to documentary data etc. At the Swedish site the analysis period was limited to the period AD 1946 - 2005, due to shortcomings of the sediment prior to that (interruption of annual layer formation). Analysis of these data is ongoing; a detailed comparison of chrysophyte stomatocysts to meteorological data is currently in progress.

Conclusions and impact
This study has clearly demonstrated the high skill of the novel methodology as a proxy for cold-season climate reconstructions. Currently, this is the only known natural proxy reflecting conditions during the cold season. A number of methodological improvements were made:
1. speeding up laboratory procedures,
2. documentation of previously unknown chrysophyte stomatocysts types,
3. the application of innovative analytical techniques.
The most important result of the study so far is the development of the 1 000 year long cold season temperature reconstruction, which forms a unique record and provides insight in a multitude of climatic parameters which could not be studied for this timescale before; seasonality, climatic forcings during winter, long- and short-term temperature variability.

The current impact of this study is primarily scientific. However, from a socio-economic perspective, the analysis of forcing factors could be highly relevant. A strong linkage to e.g. North Atlantic circulation patterns, solar forcing or greenhouse gas emissions could provide insight in future variations of winter temperature in the alpine region. This information would be highly valuable for the local winter tourism sector, as well as the energy sector (hydro-electric power plants). The analysis of forcing factors is currently ongoing.

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