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Summary
After the demise of the dinosaurs some 65 million years ago, Earth experienced irregular global cooling because of processes associated with the drifting of continents (1). About 3 million years ago, glaciers formed in high northern latitudes, surface waters cooled in parts of the equatorial Atlantic and Pacific Oceans (2), and climate sensitivity to variations in obliquity (the tilt of Earth's axis) increased substantially. Since that time, changes in sunlight associated with obliquity variations at a period of 41,000 years induced variations in global ice volume and equatorial sea surface temperatures (SST). In general, variations at the equator occurred a few thousand years before those in high latitudes and thus could not have been a direct consequence of the waxing and waning of glaciers. How did the changes in sunlight, which were large mainly near the poles, affect the tropics? Two papers in this issue, on pages 1550 and 1530 (3, 4) shed light on the matter.
