A. The ice sheet that covers Antarctica is, in places, more than three kilometres thick. It has formed over hundreds of thousands of years as snow falling on the surface was gradually buried by later snowfalls and compressed into ice. Because each year's snow is covered by the next, the ice sheet preserves a record of past conditions, layer by layer, like the pages of a book. By drilling deep into the ice and extracting long cylinders known as ice cores, scientists can read this record.
B. The most valuable information comes from tiny bubbles of air trapped in the ice. As snow is compressed, small pockets of the atmosphere become sealed inside, preserving samples of the air as it was when the ice formed. By analysing these bubbles, researchers can measure the concentration of gases such as carbon dioxide and methane in the ancient atmosphere. The ice itself also holds information: the ratio of different forms, or isotopes, of hydrogen and oxygen in the frozen water reflects the temperature at the time the snow fell.
C. Drilling such cores is a major technical challenge. Projects take several years and involve teams from many countries working in some of the coldest conditions on Earth, where winter temperatures can fall below minus 70 degrees Celsius. Drills must cut through ice under enormous pressure, and the cores, each a few metres long, must be carefully labelled, stored at low temperatures and transported to laboratories without melting or cracking. Most drilling therefore takes place during the short Antarctic summer.
D. The results have been remarkable. A core drilled by a European project at a site called Dome C, completed in the early 2000s, provided a continuous record stretching back about 800,000 years. It showed that the Earth's climate has repeatedly swung between long cold glacial periods and shorter warm periods. Crucially, levels of carbon dioxide rose and fell closely in step with temperature throughout this time, ranging roughly between 180 and 300 parts per million. Today's level, which has passed 420 parts per million, is far higher than at any point in that record.
E. Ice cores also contain evidence of individual events. Layers of volcanic ash and sulphur mark major eruptions, which can sometimes be matched to eruptions recorded in historical documents, helping scientists to date the layers precisely. Dust blown from distant deserts indicates periods when the climate was drier and windier. Even traces of lead from ancient mining and smelting have been detected in cores from Greenland, reflecting the rise and fall of economic activity in the Roman world.
F. Scientists are now searching for even older ice. About a million years ago, the rhythm of ice ages changed: cycles that had lasted around 41,000 years became cycles of roughly 100,000 years. The reason for this shift remains one of the great puzzles of climate science, and researchers hope that ice from this period will help explain it. In 2025, a European team reported that it had reached ice believed to be at least 1.2 million years old at a site near Dome C. Analysing it is expected to take several years.