The climate of the past millennium was marked by substantial decadal and centennial scale variability in the Northern Hemisphere1. Low solar activity has been linked to cooling during the Little Ice Age (AD 1450–1850; ref. 1) and there may have been solar forcing of regional warmth during the Medieval Climate Anomaly2,3,4,5 (AD 950–1250; ref. 1). The amplitude of the associated changes is, however, poorly constrained5,6, with estimates of solar forcing spanning almost an order of magnitude7,8,9. Numerical simulations tentatively indicate that a small amplitude best agrees with available temperature reconstructions10,11,12,13. Here we compare the climatic fingerprints of high and low solar forcing derived from model simulations with an ensemble of surface air temperature reconstructions14 for the past millennium. Our methodology15 also accounts for internal climate variability and other external drivers such as volcanic eruptions, as well as uncertainties in the proxy reconstructions and model output. We find that neither a high magnitude of solar forcing nor a strong climate effect of that forcing agree with the temperature reconstructions. We instead conclude that solar forcing probably had a minor effect on Northern Hemisphere climate over the past 1,000 years, while, volcanic eruptions and changes in greenhouse gas concentrations seem to be the most important influence over this period.
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This work was financially supported by NERC grant NE/G019819/1. We acknowledge CMIP5 and PMIP3 and we thank the climate modelling groups (listed in Supplementary Section 5) for producing and making available their model output, G. Jones for making a long HadCM3 control simulation available for our use, D. Frank for making his reconstructions available, for providing code to carry out the cubic spine smoothing and for several helpful comments and M. Williams, J. Pongratz and T. Crowley for advice in forcing implementation.
The authors declare no competing financial interests.
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Schurer, A., Tett, S. & Hegerl, G. Small influence of solar variability on climate over the past millennium. Nature Geosci 7, 104–108 (2014). https://doi.org/10.1038/ngeo2040
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