science

Earth’s Orbital Shifts May Have Triggered Ancient Global Warming

A new study combining astronomical and geologic data hints at an extraterrestrial cause for extreme climate change 56 million years ago

Some 56 million years ago, during the transition between the Paleocene and Eocene epochs, Earth caught a fever. 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. The planet was transformed. Crocodiles basked on Arctic beaches lined with palm trees, and steamy swamps and jungles stretched across much of the midlatitudes. Such “hyperthermal” events periodically come and go throughout Earth’s history, but this one was particularly intense for unclear reasons. 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. A surge in volcanic eruptions likely played a role, perhaps aided by a comet impact. But a new study suggests the PETM may have been instigated by subtle shifts in Earth’s orbit around the sun.

Determining 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. Astronomers’ best models of planetary motions cannot go further back in time than about 50 million years. 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. 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.

“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.

Some 56 million years ago, during the transition between the Paleocene and Eocene epochs, Earth caught a fever. 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. The planet was transformed. Crocodiles basked on Arctic beaches lined with palm trees, and steamy swamps and jungles stretched across much of the midlatitudes. Such “hyperthermal” events periodically come and go throughout Earth’s history, but this one was particularly intense for unclear reasons. 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. A surge in volcanic eruptions likely played a role, perhaps aided by a comet impact. But a new study suggests the PETM may have been instigated by subtle shifts in Earth’s orbit around the sun.

Determining 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. Astronomers’ best models of planetary motions cannot go further back in time than about 50 million years. 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. 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.

“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.