Why did Earth experience drastic climate swings without ice sheets? Scientists reveal slow orbital wobbles as the hidden driver

When audiences watched The Day After Tomorrow, they saw a Hollywood portrayal of dramatic, sudden climate upheaval. While the movie’s timescale is exaggerated, the idea that Earth’s climate can shift abruptly is firmly rooted in real science. During the last Ice Age, for example, temperatures in Greenland jumped by as much as 16°C within decades, and massive iceberg surges repeatedly disrupted the North Atlantic; they are the so-called Dansgaard–Oeschger and Heinrich events. Such abrupt shifts—known as millennial-scale climate events—demonstrate that Earth’s climate system can reorganize far more rapidly than slow orbital cycles alone would suggest.

Such rapid climate swings have traditionally been linked to ice-sheet dynamics, raising a long-standing question: how could similar millennial-scale variability occur during warm house world when large ice sheets were absent? This question has persistently puzzled scientists.

Now, an international research team led by Professor Chengshan Wang at the China University of Geosciences (Beijing) provides a compelling new clue. Working with collaborators from Belgium, Austria, and China, the team shows that Earth’s precession cycles—slow wobbles in its rotational axis—can naturally generate abrupt millennial-scale climate fluctuations even under ice-free conditions. Their findings were published in the Nature Communications journal on 27 November 2025.

This study is based on sediment cores recovered from China’s Songliao Basin, deposited about 83 million years ago during Late Cretaceous times—a classic greenhouse interval marked by high atmospheric CO₂ levels and an absence of ice sheets. These cores were obtained through the Cretaceous Continental Scientific Drilling Project, an international initiative launched in 2006 by Prof. Wang.

In astronomical terms, Earth’s rotation axis slowly wobbles like a spinning top—a motion known as axial precession, which completes one full cycle roughly every 26,000 years. When this axial motion interacts with the gradual rotation of Earth’s elliptical orbit, it produces two climatic precession cycles of about 19,000 and 23,000 years. These cycles control how sunlight is distributed seasonally between the hemispheres and are among the key drivers of long-term climate change.

Because of the tilt of Earth’s rotation axis relative to its orbit plane (Earth’s obliquity), regions outside the tropics experience only a single annual maximum in solar radiation, occurring near the summer solstice in each hemisphere. In contrast, at tropical latitudes, this geometric configuration causes solar radiation to reach two maxima each year near the equinoxes and two minima near the solstices. Consequently, the double-maximum structure that characterizes daily insolation in the tropics leads to four maxima in the interseasonal insolation contrast within a single year. Over a full precessional cycle, this results in four distinct climatic responses to precession-driven insolation forcing, giving rise to a characteristic quarter-precession periodicity of approximately 5 kyr.

This theoretical framework is borne out by the new data. By combining geochemical datasets, mineralogical evidence, and bioturbation simulations, the researchers found that the Late Cretaceous climates were characterized by alternating humid–arid cycles with pronounced 4–5 kyr periodicities. Moreover, the amplitude of these oscillations was modulated by ~100-kyr cycles, corresponding to variations in Earth’s eccentricity.

The team’s Late Cretaceous data align remarkably well with the theoretical pattern of equatorial insolation described above. This finding indicates that equatorial insolation can indeed exert a strong influence on climate, spontaneously triggering millennial-scale climate cycles. The team’s spectral analyses further reveal that these ~5,000-year insolation cycles can give rise to even faster climate swings, lasting 1.8–4 kyr, through nonlinear climate processes.

Together, Cretaceous climate reconstructions and the theoretical calculation demonstrate that even under warm, ice-free conditions, Earth’s climate was far from stable, repeatedly oscillating between arid and humid states driven mainly by precession-related solar forcing.

“During the Late Cretaceous, atmospheric CO₂ levels reached about 1,000 parts per million—comparable to projections for the end of this century,” says Prof. Michael Wagreich, a paleoclimatologist at the University of Vienna. “This makes the Cretaceous greenhouse climate a meaningful analogue for understanding Earth’s future.”

“Because Earth’s orbital configuration will remain stable for billions of years, the unveiled close link we identified between astronomical precession and millennial-scale climate cycles implies that high-frequency climate oscillations, like those seen in the Cretaceous, could also emerge in a warmer future—potentially in ways that are more predictable than previously thought,” concludes the study’s first author, Zhifeng Zhang.

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Reference
DOI: https://doi.org/10.1038/s41467-025-66219-4

About China University of Geosciences (Beijing)
The China University of Geosciences (Beijing) (CUGB) is a public university in Beijing, China. It is a national key university of the People’s Republic of China, administered directly by the Ministry of Education. CUGB carries forward the fine tradition of combining geological education with scientific research and practice. Its core values are “loving the motherland, enduring hardship, being a pioneer, and daring to explore.” CUGB hosts 16 schools, 41 undergraduate programs, 16 doctoral degree conferring spots of first-level disciplines, 33 master’s degree conferring spots of first-level disciplines, and 14 master’s degree conferring spots in professional fields.

Website: https://en.cugb.edu.cn/

About Professor Chengshan Wang from CUGB
Chengshan Wang is a Professor at the School of Earth Sciences and Resources at China University of Geosciences (Beijing). He is a member of the Chinese Academy of Sciences and the President of the Executive Committee of the Deep-time Digital Earth (DDE) Big Science Program. His research interests include the Cretaceous paleoenvironment and paleoclimate, tectonic uplift and sedimentary response, and analysis of petroliferous basins. With over 25000 citations, he is a leader in the field of uplift of mountain ranges and has extensively studied the Tibetan Plateau and the Himalayan range. He has received the Li Siguang Geological Science Award, the National Award of Natural Sciences, and the Ho Leung Ho Lee Prize for Scientific and Technological Progress.

Funding information
This work was funded by the Deep Earth Probe and Mineral Resources Exploration - National Science and Technology Major Project of China (No. 2024ZD1001105), National Natural Science Foundation of China (No. 42272134 to Y.H., 42488201 to C.W., 42502020 to Z.Z., 42172137 to C.M.), National Key Research and Development Program of China (No. 2023YFF0804000 to C.M.), “Deep-time Digital Earth” Science and Technology Leading Talents Team Funds for the Central Universities for the Frontiers Science Center for Deep-time Digital Earth, China University of Geosciences (Beijing) (Fundamental Research Funds for the Central Universities) (No. 2652023001 to C.W.), and the Postdoctoral Fellowship Programof CPSF (No. GZC20241605 to Z.Z.). Q.Y. is a Senior Research Associate of the Fonds de la Recherche Scientifique-FNRS (F.R.S.-FNRS) and acknowledges the support of the F.R.S.-FNRS grant n° T.0246.23. Z.Z. gratefully acknowledges the fellowship from the China Postdoctoral Science Foundation (No. 2025M770431). ACDS thanks the FNRS support WarmAnoxia (grant T.0037.22).