In a groundbreaking advancement in paleoclimatology, recent research conducted on ancient ice samples from Antarctica has shed unprecedented light on Earth’s climatic history over the past three million years. This research, led by researchers affiliated with the National Science Foundation’s Center for Oldest Ice Exploration (COLDEX) at Oregon State University, utilizes newly discovered multi-million-year-old ice deposited in the Allan Hills region to unravel long-standing mysteries about the evolution of Earth’s temperature and atmospheric greenhouse gas concentrations. These findings are published in two pivotal papers featured in the journal Nature.
The Allan Hills site, nestled at the periphery of the East Antarctic ice sheet, offers a rare geological archive where ice originating from the Antarctic interior is trapped within mountainous terrain. Unlike traditional ice core extraction sites characterized by undisturbed horizontal stratigraphy, the deformation and complex flow patterns at Allan Hills cause stratigraphic layers to become uneven and disrupted. As a result, the ice at Allan Hills provides discrete snapshots of Earth’s climatic conditions rather than continuous chronological records, presenting a unique yet challenging opportunity to study ancient climate dynamics.
Employing sophisticated isotopic analysis focused on the ratios of noble gases contained within air bubbles trapped in this ancient ice, researchers have inferred significant ocean temperature shifts spanning millions of years. These gases serve as proxies that reflect temperature-dependent solubility changes in ocean waters, offering a window into deep ocean thermal history. Findings reveal a pronounced ocean cooling trend of approximately 2 to 2.5 degrees Celsius over the past three million years. Notably, this cooling occurred predominantly in deep ocean waters rather than at the surface, diverging from previously held positions based solely on surface temperature proxies.
This divergence in ocean temperature cooling patterns between surface and deep waters invites new hypotheses regarding heat transfer mechanisms within the ocean. The evidence suggests that early in this timeframe, during the onset of extensive northern hemisphere glaciation, the deep ocean cooled rapidly over about a million years before more gradual surface cooling ensued. This nuanced temporal offset points to complex ocean circulation and thermohaline processes influencing Earth’s long-term climate regulation, underscoring the intricate interplay between atmospheric, oceanic, and cryospheric systems.
Complementing these oceanographic insights, a concurrent study analyzed the same Allan Hills ice archives for direct measurements of two critical greenhouse gases: carbon dioxide (CO2) and methane (CH4). Results reveal an unexpected stability in long-term atmospheric concentrations, with CO2 levels generally remaining below 300 parts per million (ppm) over this extensive interval. Specifically, CO2 concentrations measured around 2.7 million years ago stood at approximately 250 ppm, with a subtle decline of nearly 20 ppm by one million years ago, while methane concentrations hovered around 500 parts per billion (ppb) without significant fluctuation.
These results challenge previous reconstructions based on sediment chemistry analyses that have posited higher historical CO2 levels, emphasizing the superior reliability of direct ice core measurements in reconstructing ancient atmospheric compositions. The direct greenhouse gas records from Allan Hills provide a refined framework for understanding the pre-industrial baseline of Earth’s atmospheric greenhouse gas concentrations, highlighting relative stability before the profound anthropogenic increases observed during the industrial era.
In vivid contrast, modern measurements underscore the magnitude of human impact, with atmospheric CO2 now averaging 425 ppm and methane ballooning to 1,935 ppb as of 2025 according to NOAA data. This stark difference accentuates the rapid pace of contemporary climate change and the unprecedented levels of greenhouse gases compared to millions of years of natural variability, emphasizing the urgency for robust climate mitigation strategies.
These insights collectively suggest that the gradual cooling trend observed over the past three million years cannot be attributed solely to changes in greenhouse gas concentrations. Instead, the research implicates additional factors such as alterations in Earth’s albedo—reflectivity changes driven by expanding ice sheets and shifts in vegetation cover—as well as evolving ocean circulation patterns. This holistic perspective reinforces the complexity of Earth’s climate system, where multiple interdependent components modulate long-term climate trajectories.
Research leaders Julia Marks-Peterson and Sarah Shackleton express optimism that this expanding chronological window into past greenhouse gas concentrations and ocean temperatures will refine climate models. By integrating these new empirical constraints, models can better simulate past climate states and improve predictions of future climate dynamics, especially in the context of anthropogenic perturbations.
The discovery of ice as ancient as six million years at the bottom of recently drilled cores underlines the vast potential for further extending the paleoclimate record. Ongoing and future campaigns facilitated by COLDEX aim to retrieve these older ice samples, employing innovative field techniques and core preservation methods designed to minimize contamination and stratigraphic disturbance, ensuring data integrity.
Future research directions extend beyond temperature and greenhouse gases to include investigations into other trace gases trapped within the ice, which may reveal insights into biosphere activity, volcanic emissions, and chemical weathering processes across geologic timescales. Moreover, efforts to elucidate the physicochemical conditions fostering the exceptional preservation of ancient ice are underway, vital for identifying optimal drilling locales across Antarctica’s complex glacial landscape.
COLDEX’s multidisciplinary approach harnesses advanced isotopic geochemistry, coupled with cutting-edge drilling technology and climate modeling, positioning it at the forefront of paleoclimate science. These achievements are supported by collaborative funding from the NSF’s Office of Polar Programs, the Science and Technology Center Program, and Oregon State University, alongside logistical support from the U.S. Antarctic Program, the Ice Drilling Program Office, and the Ice Core Facility in Denver.
In sum, the revelations stemming from the Allan Hills ice core studies represent a pivotal leap in our comprehension of Earth’s climate evolution. They underscore the value of integrating multiple temperature proxies with direct greenhouse gas measurements to unravel the complexities of natural climate variability and offer a refined baseline for assessing ongoing and future anthropogenic influences on the global climate system.
Subject of Research: Not applicable
Article Title: Broadly stable atmospheric CO2 and CH4 levels over the past 3 million years
News Publication Date: 19-Mar-2026
Web References:
https://doi.org/10.1038/s41586-025-10032-y
Image Credits: Julia Marks-Peterson
Keywords: Paleoclimate, Antarctic ice cores, noble gases, ocean temperature, greenhouse gases, carbon dioxide, methane, Allan Hills, Earth’s climate history, ice core analysis, deep ocean cooling, climate evolution
