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Contributed by Robert M. May, December 17, 2014 (sent for review August 5, 2014; reviewed by Brian John Hoskins and Tim N. Palmer)
Significance
Here we use newly available methods to examine the dynamical association between cosmic rays (CR) and global temperature (GT) in the 20th-century observational record. We find no measurable evidence of a causal effect linking CR to the overall 20th-century warming trend; however, on short interannual timescales, we find a significant, although modest, causal effect of CR on short-term, year-to-year variability in GT. Thus, although CR clearly do not contribute measurably to the 20th-century global warming trend, they do appear as a nontraditional forcing in the climate system on short interannual timescales, providing another interesting piece of the puzzle in our understanding of factors influencing climate variability.
Abstract
As early as 1959, it was hypothesized that an indirect link between solar activity and climate could be mediated by mechanisms controlling the flux of galactic cosmic rays (CR) [Ney ER (1959) Nature 183:451–452]. Although the connection between CR and climate remains controversial, a significant body of laboratory evidence has emerged at the European Organization for Nuclear Research [Duplissy J, et al. (2010) Atmos Chem Phys 10:1635–1647; Kirkby J, et al. (2011) Nature 476(7361):429–433] and elsewhere [Svensmark H, Pedersen JOP, Marsh ND, Enghoff MB, Uggerhøj UI (2007) Proc R Soc A 463:385–396; Enghoff MB, Pedersen JOP, Uggerhoj UI, Paling SM, Svensmark H (2011) Geophys Res Lett 38:L09805], demonstrating the theoretical mechanism of this link. In this article, we present an analysis based on convergent cross mapping, which uses observational time series data to directly examine the causal link between CR and year-to-year changes in global temperature. Despite a gross correlation, we find no measurable evidence of a causal effect linking CR to the overall 20th-century warming trend. However, on short interannual timescales, we find a significant, although modest, causal effect between CR and short-term, year-to-year variability in global temperature that is consistent with the presence of nonlinearities internal to the system. Thus, although CR do not contribute measurably to the 20th-century global warming trend, they do appear as a nontraditional forcing in the climate system on short interannual timescales.
Footnotes
- ↵1To whom correspondence may be addressed. Email: aatsonis{at}uwm.edu, robert.may{at}zoo.ox.ac.uk, or gsugihara{at}ucsd.edu.
Author contributions: A.A.T., R.M.M., and G.S. designed research; A.A.T., E.R.D., G.S., K.S., J.D.V., and G.W. performed research; G.S. contributed new reagents/analytic tools; A.A.T., E.R.D., G.S., K.S., J.D.V., and G.W. analyzed data; and A.A.T., R.M.M., and G.S. wrote the paper.
Reviewers: B.J.H., Imperial College London; and T.N.P., University of Oxford.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1420291112/-/DCSupplemental.
Freely available online through the PNAS open access option.
The causal influence of cosmic rays on temperature
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