Understanding the behavior of the global climate system during extremely warm periods is one of the major themes of paleoclimatology. Proxy data demonstrate that the equator-to-pole temperature gradient was much lower during the mid-Cretaceous "supergreenhouse" period than at present, implying larger meridional heat transport by atmospheric and/or oceanic circulation. However, reconstructions of atmospheric circulation during the Cretaceous have been hampered by a lack of appropriate datasets based on reliable proxies. Desert distribution directly reflects the position of the subtropical high-pressure belt, and the prevailing surface-wind pattern preserved in desert deposits reveals the exact position of its divergence axis, which marks the poleward margin of the Hadley circulation. We reconstructed temporal changes in the latitude of the subtropical high-pressure belt and its divergence axis during the Cretaceous based on spatio-temporal changes in the latitudinal distribution of deserts and prevailing surface-wind patterns in the Asian interior. We found a poleward shift in the subtropical high-pressure belt during the early and late Cretaceous, suggesting a poleward expansion of the Hadley circulation. In contrast, an equatorward shift of the belt was found during the mid-Cretaceous "supergreenhouse" period, suggesting drastic shrinking of the Hadley circulation. These results, in conjunction with recent observations, suggest the existence of a threshold in atmospheric CO₂ level and/or global temperature, beyond which the Hadley circulation shrinks drastically.

Major ice sheets were permanently established on Antarctica approximately 34 million years ago^1,2,3, close to the Eocene/Oligocene boundary, at the same time as a permanent deepening of the calcite compensation depth in the world’s oceans⁴. Until recently, it was thought that Northern Hemisphere glaciation began much later, between 11 and 5 million years ago^1,2,3,5. This view has been challenged, however, by records of ice rafting at high northern latitudes during the Eocene epoch^6,7 and by estimates of global ice volume that exceed the storage capacity of Antarctica⁸ at the same time as a temporary deepening of the calcite compensation depth ∼ 41.6 million years ago⁹. Here we test the hypothesis that large ice sheets were present in both hemispheres ∼ 41.6 million years ago using marine sediment records of oxygen and carbon isotope values and of calcium carbonate content from the equatorial Atlantic Ocean. These records allow, at most, an ice budget that can easily be accommodated on Antarctica, indicating that large ice sheets were not present in the Northern Hemisphere. The records also reveal a brief interval shortly before the temporary deepening of the calcite compensation depth during which the calcite compensation depth shoaled, ocean temperatures increased and carbon isotope values decreased in the equatorial Atlantic. The nature of these changes around 41.6 million years ago implies common links, in terms of carbon cycling, with events at the Eocene/Oligocene boundary⁴ and with the ‘hyperthermals’ of the Early Eocene climate optimum^3,10,11. Our findings help to resolve the apparent discrepancy between the geological records of Northern Hemisphere glaciation^6,7,8 and model results^12,13 that indicate that the threshold for continental glaciation was crossed earlier in the Southern Hemisphere than in the Northern Hemisphere.