The new findings could help scientists tackle a big question in climate science. About 1 million years ago, Earth's ice ages would wax and wane on a roughly 40,000-year cycle, different than today. After around 800,000 years ago, the planet entered a cooler climate phase and the ice ages shifted to a 100,000-year cycle. Many researchers think lower carbon dioxide levels were a key player in the changeover. Directly measuring levels of the gas could reveal whether a drop in carbon dioxide triggered the flip.
"Gas bubbles are the gold standard for reconstructing climate," said lead study author John Higgins, a geochemist at Princeton University.
The ice analysis reveals there weren't any major swings in greenhouse gas levels while the planet was in its warmer phase, an interglacial period, before 800,000 years ago, the study reported. Also, for the past 1 million years, carbon dioxide levels never rose above about 300 ppm. (Carbon dioxide levels are currently at about 400 ppm and rising due to human activity, such as fossil fuel emissions.)