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In page Holocene climatic optimum:

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The effect would have had the maximum heating of the Northern Hemisphere 9,000 years ago, when the axial tilt was 24° and the nearest approach to the Sun (perihelion) was during the Northern Hemisphere's summer. The calculated Milankovitch Forcing would have provided 0.2% more solar radiation (+40 W/m2) to the Northern Hemisphere in summer, which tended to cause more heating. There seems to have been the predicted southward shift in the global band of thunderstorms, the Intertropical Convergence Zone.[citation needed]