Antarctic Ice Study Questions Greenhouse Gases’ Role in Prehistoric Cooling

Study findings

A study of ancient ice samples from Antarctica suggests greenhouse gases were not the main driver of global cooling around three million years ago.

Earlier research had suggested carbon dioxide and methane helped drive falling temperatures as the warm Pliocene epoch gave way to the ice ages of the Pleistocene. But the new study, published in *Nature*, found that levels of both gases remained relatively stable even as temperatures fell.

The researchers said factors other than greenhouse gases may have played a key role in the cooling, pointing to ocean circulation, ice cover and the Earth’s reflectivity as possible influences.

Ice core evidence

The samples were taken from the Allan Hills in Antarctica’s Victoria Land, where strong winds have eroded the terrain and brought older “blue” ice closer to the surface.

The ice cores confirmed that temperatures fell as the epochs changed, but found greenhouse gases were unlikely to have been the cause. Carbon dioxide levels began at about 250 parts per million, with only around a 20ppm shift over a three million-year period, while methane levels remained constant.

Researchers had previously predicted carbon dioxide would have needed to fall from 400ppm in the warm Pliocene to 250ppm in the later epoch to account for the cooling.

Joint author Julia Marks-Peterson of Oregon State University said: “At face value, our results indicate that the contribution of changes in carbon dioxide and methane changes in direct radiative forcing during long-term Pleistocene cooling was small.” She added that the modest change in carbon dioxide, and no change in methane, contrasted with evidence of significant cooling from the same ice core samples.

Marks-Peterson said the findings suggested the climate system is more complex than previously thought and would help refine understanding of how different parts of the Earth system interact.

Scientific reaction

Ed Brook, director of the project, said the discoveries were pushing climate records further back than ever before and raising new questions about Earth’s climate evolution and how far ice core data might eventually reach.

Steven Koonin, a senior fellow at Stanford University’s Hoover Institution and former US Department of Energy undersecretary for science under President Obama, said the results showed that forces beyond carbon dioxide affected the planet and highlighted the complexity of climate science.

Other scientists said the findings on prehistoric cooling do not change the evidence on present-day warming. Carrie Lear of Cardiff University said the research showed how sensitive the climate system is to small changes, but added that modern carbon dioxide levels are rising much faster, and to far higher values, than those seen in the ancient records.

Richard Allan of the University of Reading said improved measurements showed greenhouse gas changes were not the main factor in this ancient cooling, but said multiple lines of evidence still show that increasing greenhouse gases are heating the climate today.

Toby Young said the findings raised questions about accepted climate wisdom, arguing that they challenged the idea that variations in atmospheric carbon dioxide alone explain climate change.