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Warm Mediterranean mid-Holocene summers inferred from fossil midge assemblages

Abstract

Understanding past climate trends is key for reliable projections of global warming and associated risks and hazards. Uncomfortably large discrepancies between vegetation-based summer temperature reconstructions (mainly based on pollen) and climate model results have been reported for the current interglacial, the Holocene. For the Mediterranean region these reconstructions indicate cooler-than-present mid-Holocene summers, in contrast with expectations based on climate models and long-term changes in summer insolation. We present new quantitative and replicated Holocene summer temperature reconstructions based on fossil chironomid midges from the northern central Mediterranean region. The Holocene thermal maximum is reconstructed 9,000–5,000 years ago and estimated to have been 1–2 °C warmer in mean July temperature than the recent pre-industrial period, consistent with glacier and marine records, and with transient climate model runs. This combined evidence implies that widely used pollen-based summer temperature reconstructions in the Mediterranean area are significantly biased by precipitation or other forcings such as early land use. Our interpretation can resolve the previous discrepancy between climate models and quantitative palaeotemperature records for millennial-scale Holocene summer temperature trends in the Mediterranean region. It also suggests that pollen-based evidence for cool mid-Holocene summers in other semi-arid to arid regions of the Northern Hemisphere may have to be reconsidered, with potential implications for global-scale reconstructions.

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Figure 1: Location of the Mediterranean proxy records discussed in the text.
Figure 2: Fossil chironomid assemblages and chironomid-inferred mean July air temperatures from the Gemini and Verdarolo sediment records.
Figure 3: Chironomid-inferred mean July air temperature for Gemini and Verdarolo compared with other palaeotemperature records.

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Acknowledgements

Gemini has been cored with permission of the Parco Nazionale Appenino Tosco-Emiliano and Verdarolo with permission of the Parco Regionale Valli del Cedra e del Parma. This study has been supported by the Swiss National Science Foundation (SNF projects PP00P2-114886, 200021_ 134616) and O.H. by the European Research Council (ERC) under the European Union’s Seventh Framework Programme (FP/2007–2013) (Starting Grant Project no. 239858).

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S.S., O.H. and W.T. conceived the research; S.S. analysed the microfossil data; S.S., O.H., J.F., F.J., H.R. and S.B. designed numerical analyses and S.S., O.H. and F.J. implemented them; all authors contributed to the final interpretation and writing of the manuscript with major contributions by S.S., O.H. and W.T.

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Correspondence to Oliver Heiri.

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Samartin, S., Heiri, O., Joos, F. et al. Warm Mediterranean mid-Holocene summers inferred from fossil midge assemblages. Nature Geosci 10, 207–212 (2017). https://doi.org/10.1038/ngeo2891

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