At the northern and southern tips of our planet are tiny bubbles of air trapped for millions of years within polar ice. These microscopic time capsules hold a record of Earth’s atmosphere — and thus its climate history.

Over the course of his career, Princeton climate scientist John Higgins has repeatedly set and then broken world records for ancient ice. Most recently, his postdoctoral researcher Sarah Shackleton — a distant cousin of legendary Antarctic explorer Ernest Shackleton — announced at the annual meeting of American Geophysical Union in December that their team had found 4-million-year-old ice in the blue ice region near Antarctica’s Allan Hills. For context, 4 million years ago our hominid ancestors were just considering coming down from trees, and megalodons still terrorized the oceans.

Thermometer measurements have only been made for a century or two. So climate modelers depend on the air, water and trace chemicals preserved in ice and fossils to understand climate change over the eons and anticipate the future, says Shackleton.

"If you want to ask the questions for what the climate will be like in 50 or 100 years, you have to go outside the bounds of instrumental records, and that’s where Earth history comes in," says John Higgins, a Princeton climate scientist.

Why it works: Every snowfall traps air bubbles. Where it is too cold for snow to melt, that snow and air will be compressed into ice by subsequent years' snowpack, so the oldest ice is usually found at the bottom of a very tall stack of ice.

Drilling ever-deeper ice cores has resulted in a continuous climate record, but it has failed to find any ice more than about 800,000 years old.

Higgins' team takes a different approach for their record-breaking ancient ice cores, drilling shallow cores from ice that has been pushed up from great depths by geologic processes. Their samples don't make a continuous record, but instead provide snapshots of older eras that researchers can then build into their climate models.