Ancient Ice Reveals Clues to Earth’s Cooling Trend

For over a century, scientists have understood that Earth wasn’t always the ice-covered planet it is today. Evidence from as recently as three million years ago—found in temperate forest fossils in Alaska and Greenland, and ancient beach lines along the U.S. Atlantic coast—shows warmer temperatures and higher sea levels. However, pinpointing the reasons behind this long-term cooling has remained a challenge. Now, two new studies analyzing ancient Antarctic ice and trapped air are offering a rare, direct glimpse into this mystery.

Ocean Cooling and Greenhouse Gases

Researchers have discovered that Earth’s oceans cooled by approximately 2 to 2.5°C (3.6 to 4.5°F) over the last three million years. Interestingly, this cooling occurred alongside only a modest decline in heat-trapping greenhouse gases like carbon dioxide and methane. This combination is prompting scientists to re-evaluate the factors driving the planet’s shift towards colder conditions.

The COLDEX Project and Ancient Ice

The research was led by scientists associated with the National Science Foundation Center for Oldest Ice Exploration (COLDEX), based at Oregon State University. COLDEX focuses on locating and analyzing the oldest preserved ice on Earth. The ice used in these studies comes from the Allan Hills region of East Antarctica, a unique location where ancient ice becomes stranded and deformed, providing “snapshots” of past climate conditions.

Tracking Ocean Temperatures Through Noble Gases

One study, published in Nature, analyzed ratios of different noble gases trapped within the ancient ice. This method provides a global view of ocean temperature changes, unlike many traditional methods that focus on single locations. The researchers found that the mean ocean temperature declined by 2 to 2.5°C (3.6 to 4.5°F) over the past three million years, a gradual but significant shift.

Timing of Ocean Cooling: A Discrepancy

The timing of ocean cooling wasn’t uniform. While surface temperature records suggest a steady cooling trend until around one million years ago, the noble gas data indicates a significant portion of the overall cooling began much earlier—around three million years ago—coinciding with the formation of major ice sheets in the Northern Hemisphere. This suggests changes in ocean circulation and heat transfer played a crucial role in how energy flowed between surface and deep waters.

Ancient Greenhouse Gas Levels

The second Nature study analyzed air bubbles trapped in the ice to directly measure carbon dioxide and methane levels dating back millions of years. The results show that carbon dioxide levels remained below 300 parts per million throughout the entire three-million-year period, starting near 250 ppm around 2.7 million years ago and declining slightly to around 280 ppm by one million years ago. Methane levels remained consistently around 500 parts per billion.

Modern Greenhouse Gas Levels: A Stark Contrast

In comparison, current atmospheric levels, as of 2025, are significantly higher. NOAA data shows carbon dioxide averaging around 425 ppm and methane averaging around 1,935 ppb. These levels are dramatically higher than those of the past three million years and have risen rapidly in the last two centuries.

Beyond Greenhouse Gases: Other Cooling Factors

While acknowledging the importance of carbon dioxide and methane, researchers emphasize that other factors contributed to Earth’s cooling. These include Earth’s reflectivity (albedo), changes in vegetation and ice cover, and ocean circulation patterns. Increased ice cover reflecting more sunlight and shifts in ocean currents altering heat distribution likely played significant roles.

Future Research and Deeper Insights

COLDEX researchers have already found ice as old as six million years and are working to analyze it. New drilling projects are underway to recover additional ancient ice, and methods for verifying carbon dioxide reconstructions are being refined. The goal is to extend the ice-core record further back in time and gain a more comprehensive understanding of Earth’s climate evolution.

As scientists delve deeper into the past, they are discovering that Earth’s climate history is more complex than previously thought. The cooling trend of the last three million years may not be solely attributable to greenhouse gases, but rather a result of the interplay between oceans, ice sheets, reflectivity, ecosystems, and circulation patterns.