An ice core is a cylindrical column of ice drilled from a glacier or ice sheet, used as a frozen archive of Earth’s past climate. Each layer of ice corresponds to a period of snowfall, trapping air bubbles, dust, volcanic ash, and chemical signatures that scientists can read like a timeline stretching back hundreds of thousands of years. The longest continuous ice core records reach back over 800,000 years, and a recently completed European drilling project in Antarctica aims to push that to 1.5 million years.
How Snow Becomes a Climate Record
Ice cores begin as snowfall. Fresh snow is about 90 percent air. As more snow accumulates on top, the weight compresses the lower layers, packing snowflakes into coarser, denser grains. Snow that survives at least one summer without melting into ice is called firn, an intermediate stage between loose snow and solid ice. Over time, the continued pressure squeezes out the air passages between grains until they seal off completely, locking air into isolated bubbles within solid glacier ice.
Those trapped bubbles are the key to why ice cores matter. Each bubble is a tiny sample of the atmosphere from the time it was sealed, preserving the exact concentrations of gases like carbon dioxide and methane. By analyzing bubbles at different depths, researchers can reconstruct how the atmosphere has changed over millennia. Antarctic ice cores have shown, for example, that carbon dioxide levels rose by 80 to 100 parts per million during the warming periods that ended the last three ice ages.
What Gets Trapped Beyond Air
Gas bubbles are just one layer of information. Ice cores also capture dust blown in from deserts, sea salt carried by storms, pollen, soot from wildfires, and chemical traces from volcanic eruptions. Volcanic layers are particularly useful because they act as time markers. When a major eruption sends sulfur compounds into the upper atmosphere, that sulfate eventually falls onto ice sheets and shows up as a distinct chemical spike in the core. Tiny fragments of volcanic ash in the same layer can be matched to a specific volcano through their chemical fingerprint. One striking example: ultrafine sampling of sulfur isotopes and ash in a Greenland ice layer precisely dated to 1628 BCE conclusively identified a colossal eruption of Alaska’s Aniakchak II volcano as the source.
These volcanic markers also help scientists calibrate their dating of ice layers and understand how eruptions affected global temperatures. Sulfur-rich aerosols from large eruptions redistribute solar energy in the atmosphere and temporarily cool Earth’s surface, and ice core chemistry can reveal the scale of that cooling effect.
How Cores Are Drilled and Handled
Extracting an ice core requires specialized drilling rigs operated in some of the most remote places on the planet. Two main types of drills do the work. Electromechanical drills cut through the upper 150 to 200 meters of firn and shallow ice. Below that, thermal electric drills take over, using heat to melt through denser ice. For deep boreholes exceeding 1,000 meters, the hole is typically filled with an antifreeze fluid to prevent it from collapsing under the immense pressure of the surrounding ice sheet. At Russia’s Vostok Station in Antarctica, drilling reached 2,755 meters deep using this approach.
The cores themselves are typically about 10 centimeters in diameter and cut into segments roughly one meter long. Once extracted, they need to stay frozen. Storage temperatures should match the conditions deep in the ice sheet, ideally around minus 20 degrees Celsius or colder, to prevent physical, chemical, and biological changes that would corrupt the record.
Reading the Ice in the Lab
Analyzing an ice core without contaminating it is a major challenge. One widely used technique is continuous flow analysis, where a section of core is placed on a heated plate that melts it steadily from one end. Only the meltwater from the inner part of the core is collected for testing, since the outer surface may have picked up contamination during drilling or handling. This inner stream flows directly into instruments that measure its chemical content in real time, providing extremely detailed records. Air bubbles are also extracted continuously from the meltwater for separate gas measurements.
This method gives researchers remarkably fine resolution. In Greenland, where snowfall is relatively heavy, the GISP2 ice core was sampled at a resolution of about one week near the surface and one month at greater depths. That means scientists could detect seasonal changes in atmospheric chemistry from thousands of years ago.
Greenland vs. Antarctica
Not all ice cores are created equal. The two primary drilling locations, Greenland and Antarctica, offer different strengths. Greenland receives more snowfall, which means each year of climate history is represented by a thicker layer of ice. This gives Greenland cores their extraordinary time resolution, sometimes down to individual weeks. The tradeoff is that Greenland’s ice doesn’t go back as far in time.
Antarctica, with its much lower snowfall, compresses each year into a thinner layer. At the South Pole, resolution drops to about one year per layer, and at the inland Vostok station it falls to roughly two years. But because the ice sheet is so thick and accumulates so slowly, Antarctic cores preserve far older ice. The current oldest continuous ice core record comes from Antarctica, and the Beyond EPICA project recently completed its first deep drilling campaign at the Little Dome C site, targeting ice up to 1.5 million years old.
The Search for Even Older Ice
Continuous cores, where each layer sits in chronological order from top to bottom, are the gold standard. But they aren’t the only source of ancient ice. In certain areas of Antarctica, particularly blue ice zones where old ice is pushed to the surface by glacial flow, researchers have found fragments of ice far older than any continuous core. A team working in the Allan Hills Blue Ice Area of East Antarctica has dated samples to 4 million years old. This ice is “mixed up,” meaning the layers are no longer in neat chronological order, but the trapped gases still hold valuable information about atmospheric conditions from a time when Earth’s climate was dramatically different from today.
Between the continuous records being extended by projects like Beyond EPICA and the ancient fragments surfacing in blue ice zones, ice cores continue to push the boundaries of how far back scientists can directly sample Earth’s atmosphere. A 1.5-million-year continuous record, if successfully recovered, would cover a critical period when Earth’s ice age cycles shifted from occurring every 41,000 years to every 100,000 years, a transition that remains one of the biggest open questions in climate science.