This paper is dedicated to Professor Raymond Hide on the occasion of his 80th birthday; we thank Ray for his mentoring and advice and for igniting our interest in Earth’s core. We gratefully acknowledge Professor George Philander for insightful discussions, Jim Hansen and colleagues for the GISS data set, Tim Johns for providing the HadCM3 data set, and Peter Stott and Gareth Jones for providing the HadGEM1 data set. The authors also thank the editor Anthony J. Broccoli and three anonymous reviewers whose comments greatly improved this paper. This paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, sponsored by the National Aeronautics and Space Administration (NASA). The contribution of author OdV to this work is IPGP contribution 3063.
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