The current acceleration in climate change is largely due to the greenhouse gases emitted by human activities. However, ancient Antarctic ice cores reveal that for the last three million years, such emissions weren’t always the dominant factor in climate shifts.
Recent studies published in Nature suggest that during key transitional periods, ocean temperatures may have played a more significant role in influencing Earth’s climate than greenhouse gases.
Two research teams have examined ice cores collected from the Allan Hills, a blue ice area in Antarctica. These Allan Hills cores include some of the oldest ice on Earth, with specimens as ancient as 6 million years.
Blue ice regions like Allan Hills cover only about 1 percent of Antarctica’s ice sheet. These areas are characterized by strong winds that remove new snowfall, leaving ancient glacial ice visible on the surface.
The Allan Hills site remains relatively stable in location, making it ideal for extracting cores of extremely old ice. Ice cores offer some of the best natural records of Earth’s long-term climate history.
Although not always providing a continuous chronological record, the Allan Hills cores contain layers that reveal insights into the climate conditions at different points in time. Various methods can decode these layers’ secrets.
Isotopes in the ice provide clues about ocean temperatures, while impurities like volcanic ash indicate historical air pollution sources. Crucially for climate scientists, the ice traps small air bubbles, allowing researchers to study atmospheric gas compositions from millions of years ago.
Sarah Shackleton, a paleoclimatologist with the Woods Hole Oceanographic Institute, led a team examining global ocean temperatures over the past 3 million years. Xenon and krypton, gases that dissolve in seawater and vary with temperature, were used to estimate ocean heat levels.
The findings suggest a significant cooling of the ocean around 2.7 million years ago, aligning with the Plio-Pleistocene Transition. During this period, Earth transitioned from a warmer to a cooler climate, resulting in glacier growth in the Northern Hemisphere.
The ice core analysis also indicates that average ocean temperatures remained relatively consistent during the Mid-Pleistocene Transition, which occurred between 1.2 and 0.8 million years ago and marked another change in glacial cycles.
A separate team led by Julia Marks-Peterson of Oregon State University studied the same ice cores and found that atmospheric carbon dioxide and methane levels were “broadly stable” over the last 3 million years.
The researchers note, “Although paleoclimate archives from Antarctic blue ice areas are complex, our records show that measurements of greenhouse gases in ice cores can be extended to the late Pliocene epoch, providing snapshots of Earth’s climate system over a time of global cooling and falling sea level.”
Eric Wolff, a climatologist from Cambridge, comments in a related article that this evidence suggests either ice-sheet dynamics were highly sensitive to minor carbon dioxide changes or other factors influenced past climate shifts.
The studies led by Shackleton and colleagues may offer more clarity, as they observed a disconnect between changes in sea surface and average ocean temperatures.
Understanding the pre-human impact on Earth’s climate is crucial for efforts to restore stability to our environment.
Shackleton noted in a Science Sessions podcast that interpreting these ice cores is challenging. “These records are still quite new, and they’re more complicated to interpret than the continuous ice cores that we’re used to working with,” she said.
She explained that due to the high compression of the ice, especially in the oldest layers, the data likely averages over glacial and interglacial phases, limiting the ability to study climate evolution in detail during these periods.
“Exactly what these records capture in terms of exactly how smooth or exactly how much we’re averaging over a glacial versus interglacial conditions is still an outstanding question.”
Related: A Record-Breaking Drill Beneath Antarctic Ice Revealed a Big Surprise
Both papers were published in Nature, here and here.
