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Unusual activity of the Sun during recent decades compared to the previous 11,000 years

Abstract

Direct observations of sunspot numbers are available for the past four centuries1,2, but longer time series are required, for example, for the identification of a possible solar influence on climate and for testing models of the solar dynamo. Here we report a reconstruction of the sunspot number covering the past 11,400 years, based on dendrochronologically dated radiocarbon concentrations. We combine physics-based models for each of the processes connecting the radiocarbon concentration with sunspot number. According to our reconstruction, the level of solar activity during the past 70 years is exceptional, and the previous period of equally high activity occurred more than 8,000 years ago. We find that during the past 11,400 years the Sun spent only of the order of 10% of the time at a similarly high level of magnetic activity and almost all of the earlier high-activity periods were shorter than the present episode. Although the rarity of the current episode of high average sunspot numbers may indicate that the Sun has contributed to the unusual climate change during the twentieth century, we point out that solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades3.

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Figure 1: Atmospheric radiocarbon level Δ14C (expressed as deviation, in ‰, from the ad 1950 standard level15) derived from mostly decadal samples of absolutely dated tree-ring chronologies (INTCAL98 data set)16.
Figure 2: Comparison between directly measured sunspot number (SN) and SN reconstructed from different cosmogenic isotopes.
Figure 3: Reconstructed sunspot number and its uncertainty for the whole interval of time considered.
Figure 4: Distribution of the duration of episodes of high solar activity and the probability that the current episode will reach a given duration.

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Correspondence to S. K. Solanki.

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Estimates of errors of sunspot number reconstruction from 14C data. (PDF 171 kb)

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Solanki, S., Usoskin, I., Kromer, B. et al. Unusual activity of the Sun during recent decades compared to the previous 11,000 years. Nature 431, 1084–1087 (2004). https://doi.org/10.1038/nature02995

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