Tree-ring chronologies underpin the majority of annually-resolved reconstructions of Common Era climate. However, they are derived using different datasets and techniques, the ramifications of which have hitherto been little explored. Here, we report the results of a double-blind experiment that yielded 15 Northern Hemisphere summer temperature reconstructions from a common network of regional tree-ring width datasets. Taken together as an ensemble, the Common Era reconstruction mean correlates with instrumental temperatures from 1794-2016 CE at 0.79 (p < 0.001), reveals summer cooling in the years following large volcanic eruptions, and exhibits strong warming since the 1980s. Differing in their mean, variance, amplitude, sensitivity, and persistence, the ensemble members demonstrate the influence of subjectivity in the reconstruction process. We therefore recommend the routine use of ensemble reconstruction approaches to provide a more consensual picture of past climate variability.

Aix Marseille University CNRS IRD INRA Coll France CEREGE Aix en Provence France

ARC Centre of Excellence for Australian Biodiversity and Heritage University of NSW Sydney Australia

CAS Centre for Excellence in Tibetan Plateau Earth Sciences Chinese Academy of Sciences Beijing China

Centre d'Études Nordiques Université Laval Québec QC Canada

Department F A Forel for Environmental and Aquatic Sciences University of Geneva Geneva Switzerland

Department of Atmospheric and Environmental Sciences University at Albany Albany NY USA

Department of Biology Chemistry and Geography University of Quebec in Rimouski Rimouski QC Canada

Department of Earth and Climate Sciences San Francisco State University San Francisco CA USA

Department of Earth and Planetary Sciences Harvard University Cambridge MA USA

Department of Earth Sciences Goteborg University Goteborg Sweden

Department of Earth Sciences University of Geneva Geneva Switzerland

Department of Geography Environment and Society University of Minnesota Minneapolis MN USA

Department of Geography Faculty of Science Masaryk University Brno Czech Republic

Department of Geography Johannes Gutenberg University Mainz Germany

Department of Geography Université du Québec à Montréal Montréal QC Canada

Department of Geography University of Cambridge Cambridge UK

Department of Geography University of Innsbruck Innsbruck Austria

Department of Physical Geography Bolin Centre for Climate Research Stockholm University Stockholm Sweden

GEOTOP Université du Québec à Montréal Montréal QC Canada

Global Change Research Centre Brno Czech Republic

GREMA and Forest Research Institute Université du Québec en Abitibi Témiscamingue Amos Canada

Institute for Environmental Sciences University of Geneva Geneva Switzerland

Institute of Ecology and Geography Siberian Federal University Krasnoyarsk Russia

Institute of Geography Friedrich Alexander University of Erlangen Nürnberg Erlangen Germany

Institute of Humanities Siberian Federal University Krasnoyarsk Russia

Key Laboratory of Desert and Desertification Northwest Institute of Eco Environment and Resources Chinese Academy of Sciences Lanzhou China

Laboratory of Tree Ring Research University of Arizona Tucson AZ USA

Lamont Doherty Earth Observatory of Columbia University Palisades NY USA

McDonald Institute for Archaeological Research Cambridge UK

Natural Resources Institute Finland Rovaniemi Finland

Potsdam Institute for Climate Impact Research Potsdam Germany

Qinghai Research Centre of Qilian Mountain National Park Academy of Plateau Science and Sustainability and Qinghai Normal University Xining China

School of Earth and Environmental Sciences University of St Andrews Scotland UK

School of Ecosystem and Forest Sciences University of Melbourne Richmond Australia

School of Geography Development and Environment and Laboratory of Tree Ring Research University of Arizona Tucson AZ USA

School of Statistics University of Minnesota Minneapolis MN USA

State Key Laboratory of Cryospheric Sciences Northwest Institute of Eco Environment and Resources Chinese Academy of Sciences Lanzhou China

Sukachev Institute of Forest SB RAS Krasnoyarsk Russia

Swiss Federal Research Institute Birmensdorf Switzerland

Université Clermont Auvergne Geolab UMR 6042 CNRS Clermont Ferrand France

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