New paper finds Medieval Warming Period was ~1°C warmer than current temperatures

A paper published today in Palaeogeography, Palaeoclimatology, Palaeoecology finds that the Medieval Warming Period "was warmer than the late 20th century by ~1°C." The paper adds to the peer-reviewed publications of over 1000 scientists showing that the global Medieval Warming Period was warmer than the current warming period. 

Marine climatic seasonality during early medieval times (10th to 12th centuries) based on isotopic records in Viking Age shells from Orkney, Scotland

• Donna Surge^a, , , 
• James H. Barrett^b, 

• ^a University of North Carolina, Department of Geological Sciences, 104 South Road, CB #3315, Chapel Hill, North Carolina, USA
• ^b McDonald Institute for Archaeological Research, University of Cambridge, Downing Street, Cambridge, CB2 3ER, UK

• Received 1 March 2012. Revised 2 July 2012. Accepted 8 July 2012. Available online 15 July 2012.

• http://dx.doi.org/10.1016/j.palaeo.2012.07.003, How to Cite or Link Using DOI

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Abstract

Seasonal sea-surface temperature (SST) variability during the Medieval Climate Anomaly (MCA), which corresponds to the height of Viking exploration (800–1200 AD), was estimated using oxygen isotope ratios (δ¹⁸O) obtained from high-resolution samples micromilled from archaeological shells of the European limpet,Patella vulgata. Our findings illustrate the advantage of targeting SST archives from fast-growing, short-lived molluscs that capture summer and winter seasons simultaneously. Shells from the 10th to 12th centuries (early MCA) were collected from well-stratified horizons, which accumulated in Viking shell and fish middens at Quoygrew on Westray in the archipelago of Orkney, Scotland. Their ages were constrained based on artifacts and radiocarbon dating of bone, charred cereal grain, and the shells used in this study. We used measured δ¹⁸O_WATER values taken from nearby Rack Wick Bay (average 0.31 ± 0.17‰ VSMOW, n = 11) to estimate SST from δ¹⁸O_SHELL values. The standard deviation of δ¹⁸O_WATER values resulted in an error in SST estimates of ± 0.7 °C. The coldest winter months recorded in the shells averaged 6.0 ± 0.6 °C and the warmest summer months averaged 14.1 ± 0.7 °C. Winter and summer SST during the late 20th century (1961–1990) was 7.77 ± 0.40 °C and 12.42 ± 0.41 °C, respectively. Thus, during the 10th to 12th centuries winters were colder and summers were warmer by ~ 2 °C and seasonality was higher relative to the late 20th century. Without the benefit of seasonal resolution, SST averaged from shell time series would be weighted toward the fast-growing summer season, resulting in the conclusion that the early MCA was warmer than the late 20th century by ~ 1 °C. This conclusion is broadly true for the summer season, but not true for the winter season. Higher seasonality and cooler winters during early medieval times may result from a weakened North Atlantic Oscillation index.

Highlights

► We investigated oxygen isotope ratios of Viking Age limpet shells. ► Seasonal SST was reconstructed for the early MCA (10th-12th centuries). ► Early MCA winters were cooler and summers were warmer than late 20th century. ► MCA seasonality was almost twice that of the late 20th century.