“We’ve been waiting for someone to do something like this for some time,” says Linda Hinnov, a paleoclimatologist at George Mason University, who was not involved in the study. “The geologic record and our calculation seem to be very much in agreement until 58 million years ago.” Most significantly, Zeebe and Lourens’s calculations show the PETM also began around one of the 405,000-year cycles, which tracks with past hyperthermal events, suggesting planetary dynamics initiated it. “We came up with a remarkable match,” Zeebe says. In their study, Zeebe and Lourens examined geologic records to identify orbital eccentricity cycles, calculating a new astronomical solution for the positions and velocities of the planets in the past and checking it against the sediments from Atlantic Ocean seafloors. One cycle in particular, with a duration of 405,000 years, helps geologists calibrate planetary dynamics using sediment records: like clockwork, when this cycle brought Earth closer to the sun, the climate warmed, leaving behind evidence laid down in rock. And its orbit slips between circular and elliptical paths in complex cycles across millennia. ![]() Nudged by the gravitation of Jupiter, Mars, Venus and other planets, our world’s axial tilt and precession are always slowly shifting. ![]() A 405,000-Year ClockĮarth’s orbit is eccentric, meaning it has changed repeatedly over time. “Given an orbital trigger for the PETM and the strong evidence for orbital pacing of the subsequent hyperthermals, no other trigger is necessary,” Zeebe says. In the new study, published in Science, Richard Zeebe, a paleoceanographer at the University of Hawaii at Manoa, and Lucas Lourens, a geoscientist at Utrecht University in the Netherlands, combined astronomical and geologic data to push our detailed knowledge of Earth’s position back another eight million years, linking the PETM’s onset with a larger cycle of orbital change. Geologists can help by seeking clues about Earth’s paleoclimate in ancient seafloor sediments and using those data to extrapolate information about the planet’s past position, relative to the sun. Astronomers’ best models of planetary motions cannot go further back in time than about 50 million years. But a new study suggests the PETM may have been instigated by subtle shifts in Earth’s orbit around the sun.ĭetermining where Earth was tens of millions of years ago is a surprisingly thorny problem, because the sun and its planets form a chaotic system, in which minuscule orbital changes can, over time, magnify into enormous effects. A surge in volcanic eruptions likely played a role, perhaps aided by a comet impact. For decades, researchers have puzzled over what triggered this Paleocene-Eocene Thermal Maximum (PETM), peering through the lens of the past to better understand our planet’s present-day warming. Such “hyperthermal” events periodically come and go throughout Earth’s history, but this one was particularly intense for unclear reasons. ![]() Crocodiles basked on Arctic beaches lined with palm trees, and steamy swamps and jungles stretched across much of the midlatitudes. In a span of scarcely 20,000 years-not even a rounding error in most measures of geologic time-massive amounts of carbon dioxide flowed into the atmosphere, and average temperatures rose by five to eight degrees Celsius. Some 56 million years ago, during the transition between the Paleocene and Eocene epochs, Earth caught a fever.
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