1. Ocean Sciences Department, University of California, Santa Cruz, California 95064, USA
2. Earth Sciences Department, University of California, Santa Cruz, California 95064, USA
3. Center for Geophysical Investigation of the Shallow Subsurface, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA
4. Present address: Rutgers University, Institute of Marine and Coastal Sciences, 71 Dudley Road New Brunswick, New Jersey 08904, USA

Correspondence to: Ana Christina Ravelo¹ Email: acr@es.ucsc.edu

Abstract

The Earth's climate has undergone a global transition over the past four million years, from warm conditions with global surface temperatures about 3 °C warmer than today, smaller ice sheets and higher sea levels to the current cooler conditions. Tectonic changes and their influence on ocean heat transport have been suggested as forcing factors for that transition, including the onset of significant Northern Hemisphere glaciation 2.75 million years ago, but the ultimate causes for the climatic changes are still under debate. Here we compare climate records from high latitudes, subtropical regions and the tropics, indicating that the onset of large glacial/interglacial cycles did not coincide with a specific climate reorganization event at lower latitudes. The regional differences in the timing of cooling imply that global cooling was a gradual process, rather than the response to a single threshold or episodic event as previously suggested. We also find that high-latitude climate sensitivity to variations in solar heating increased gradually, culminating after cool tropical and subtropical upwelling conditions were established two million years ago. Our results suggest that mean low-latitude climate conditions can significantly influence global climate feedbacks.

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