Scientists chart Earth's ancient rotation slowdown

A joint study of geological data by Chinese and foreign researchers has revealed that the Earth's rotation decelerated in a step-like pattern 700 million to 200 million years ago.

Throughout the Earth's history, its rotation has been gradually slowing down due to factors such as tidal dissipation, glacier melting and tectonic plate movements. However, the variation of the rate of deceleration through time has not been established.

In a paper published last week in the journal Proceedings of the National Academy of Sciences, researchers said there were two intervals — from 650 million to 500 million years ago and 350 million to 280 million years ago — when the deceleration of rotation was high, separated by an interval of stalled deceleration from 500 to 350 million years ago.

Ma Chao, the corresponding author and a professor at the Chengdu University of Technology, said university geologists — along with a team led by French astronomer Jacques Laskar, and teams of geologists from Germany and Ireland — reconstructed the evolution of the Earth's rotational deceleration during the period from 700 million to 200 million years ago, based on new analysis of selected high-quality cyclic sedimentary records.

The study found that during that period, the distance between the Earth and the moon increased by approximately 20,000 kilometers, and the length of a day increased by some 2.2 hours. "This time period has significant research value due to its relatively complete geological records and its involvement in major events of the Earth's evolution, including the Cambrian explosion of life around 540 million years ago, and the Permian-Triassic extinction event about 250 million years ago," Ma said.

Modeling indicates that the development of major glaciations during the Paleozoic Era (roughly 540 million to 250 million years ago) had a negligible impact on Earth's rotation, according to the study.

"Tidal dissipation had been the main driver for decelerating the Earth's rotation, before it slowed down to the point where a day lasted over 23 hours and 45 minutes," Ma said, noting that this occurred approximately 100 million years ago during the Cretaceous period, the era of the dinosaurs.

After that, the Earth's rotation was mainly influenced by factors including mantle convection, tectonic plate movements and the redistribution of ice sheets, he said.

Two studies published earlier this year in Nature and PNAS revealed that the melting of polar ice sheets caused by global warming is significantly decelerating the Earth's rotation, attracting wide attention from academia and the public.

The deceleration rate has increased to 1.33 milliseconds per century since 2000, up from approximately 0.3 to 1 millisecond per century, according to the studies.