For more than 20 years the paleoclimate modeling community has sought the mechanism for generating a warm, equable early Paleogene climate. Increased greenhouse gas concentrations and warm sea-surface temperatures (SSTs) do much to reproduce the climate indicated by proxy data; however, discrepancies between model output and data remain, and a term in the equable climate equation is clearly missing. Based on proxy data indications of reduced early Paleogene seasonality, we test the climate impact of a reduction in Earth's obliquity to 18°. Obliquity is one of the few forcing factors that can directly affect seasonality, and our hypothesis of changing obliquity is supported by large planetary mass redistributions in the past 120 m.y. Such mass redistributions have the potential to change the tilt of Earth's axis. In our experiment the combination of reduced obliquity, increased early Paleogene greenhouse gas concentrations, and conservatively warm SSTs reproduces early Paleogene climate with greater accuracy than any previous modeling efforts and provides a plausible solution to the equable climate challenge.

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