In ice sheets and high mountain glaciers, old snowfall can survive beneath newer layers. Pressure gradually turns the buried snow into ice, preserving particles, chemicals and bubbles of ancient air. Scientists recover this record by drilling cylindrical samples called ice cores.
A timeline with several kinds of evidence
Depth generally increases with age, but an ice core is not a perfectly uniform calendar. Researchers date sections by counting seasonal layers where possible and by using volcanic deposits, ice-flow models and other markers.
Air bubbles are especially valuable because they contain samples of past atmospheres. Researchers can directly measure gases such as carbon dioxide and methane in them. Other findings are indirect indicators, or proxies. Ratios of oxygen and hydrogen isotopes in the ice, for example, reflect conditions associated with the snowfall and help reconstruct past temperature patterns.
Dust can indicate changes in wind, dryness or distant source regions. Sulfate and ash can record major eruptions. The chemistry and thickness of layers may also reveal snowfall, sea salt, wildfire products and industrial pollution.
What the record can—and cannot—show
Ice cores allow scientists to compare atmospheric composition and climate across long periods, including glacial cycles and abrupt regional changes. They are most powerful when combined with tree rings, marine sediments, corals and climate models.
Interpretation still requires care. Ice can deform as it flows, gases may become sealed later than the surrounding snowfall, and local conditions do not automatically represent the whole planet. Teams therefore compare cores from multiple sites and report uncertainty.
The result is not a frozen weather diary with every detail preserved. It is a layered archive containing both direct atmospheric samples and physical or chemical clues. Together, those records extend climate knowledge far beyond the era of thermometers and modern instruments.
Sources
- NOAA NCEI, Ice Core: https://www.ncei.noaa.gov/products/paleoclimatology/ice-core
- NOAA NCEI, How Can Ice Teach Us About Climate?: https://www.ncei.noaa.gov/news/how-can-ice-teach-us-about-climate