Ice Cores and What They Reveal About Past Climates
In the ice of Greenland and Antarctica, scientists possess one of the most remarkable natural archives on Earth: a continuous record of past climate stretching back hundreds of thousands of years, preserved in compressed snow layers that have accumulated year by year and never melted. The information encoded in this ice covers temperature, precipitation, atmospheric composition, volcanic eruptions, solar activity, dust transport, sea ice extent, and the chemistry of ancient oceans. No other archive provides the same temporal resolution, continuity, and range of climate variables simultaneously. Ice cores have transformed our understanding of how Earth's climate system works, particularly the relationship between greenhouse gas concentrations and temperature, and continue to be one of the most productive research tools in climate science.
The central technology enabling this science is ice core drilling — the extraction of long cylindrical samples of ice from the interior of ice sheets and mountain glaciers, using electromechanical or thermal drilling systems that can operate kilometres below the surface. The ice cores, typically 10 centimetres in diameter and recovered in sections of roughly one metre, are maintained in frozen storage and analysed in specialised laboratories using techniques ranging from visual stratigraphy to mass spectrometry.
Annual Layers and Chronology
In the accumulation zones of ice sheets and high-accumulation glaciers, snow layers deposited in individual years are distinguishable in the ice core by seasonal variations in dust content, crystal texture, stable isotope ratios, and electrical conductivity. The visual alternation of clear summer ice and bubbly or slightly dusty winter ice allows cores to be dated by counting layers, much as tree rings are counted. This annual layer counting has been extended to more than 100,000 years in the Greenland cores, with annual resolution maintained through that entire period.
At greater depths in cores from low-accumulation sites such as the Antarctic interior, annual layers become too thin to distinguish individually — sometimes only millimetres thick after thousands of years of compression — and chronology must be established by other means, including ice flow modelling that predicts how layers compress with depth, matching of distinctive chemical signals (volcanic sulphate spikes, cosmogenic isotope anomalies) between cores from different locations, and correlation with independently-dated marine sediment cores.
Stable Isotopes and Temperature Reconstruction
The ratio of heavy to light isotopes of oxygen (oxygen-18 to oxygen-16) and hydrogen (deuterium to protium) in glacier ice varies systematically with the temperature at which precipitation formed. Colder temperatures during evaporation and transport deplete the precipitation in heavy isotopes; warmer temperatures result in relatively isotopically heavier ice. By measuring the delta oxygen-18 or delta deuterium values along an ice core, researchers reconstruct a continuous temperature proxy record.
The EPICA (European Project for Ice Coring in Antarctica) Dome C core, drilled at the Russian-operated Concordia Station and reaching approximately 3,270 metres of ice, extends this temperature record back approximately 800,000 years. The record encompasses eight complete glacial-interglacial cycles, revealing the regular approximately 100,000-year periodicity of ice ages driven by orbital forcing (Milankovitch cycles) — variations in Earth's orbital eccentricity, axial tilt, and precession that alter the seasonal and geographic distribution of solar energy. Within each glacial cycle, the EPICA record shows the saw-tooth pattern observed in marine sediment records: gradual cooling into glacial maxima punctuated by abrupt warmings into interglacials.
Greenhouse Gases: Direct Measurement from Bubbles
The most consequential finding from Antarctic ice cores is the direct measurement of past atmospheric greenhouse gas concentrations from the air bubbles trapped at bubble close-off depth. When the EPICA and Vostok cores were analysed, the concentration of carbon dioxide and methane in the trapped bubbles was found to covary tightly with the isotopic temperature record through all eight glacial cycles. During glacial maxima, CO2 concentrations were approximately 180 parts per million by volume; during interglacials, they rose to approximately 280 ppm — the pre-industrial Holocene value. The current atmospheric CO2 concentration, approximately 420 ppm and rising, is unprecedented in the 800,000-year ice core record and is approximately 50 percent higher than the maximum natural interglacial value.
This finding — that atmospheric CO2 and temperature have been closely coupled throughout eight glacial cycles — provided powerful evidence for the role of CO2 as a climate driver, while also revealing the complexity of causation (orbital forcing initiates glacial cycles; CO2 and other feedbacks amplify the response) that has been extensively studied since.
Volcanic Signatures
Volcanic eruptions leave unambiguous chemical markers in ice cores: spikes in sulphate concentration, reflecting the deposition of sulphuric acid aerosols from volcanic clouds, and in some cases visible layers of volcanic ash (tephra) if the eruption occurred within the ice core's source region. The timing and magnitude of volcanic sulphate spikes have been used to build a chronology of major eruptions extending back several hundred thousand years, providing context for the role of volcanism in short-term climate perturbations.
The 1815 Tambora eruption in Indonesia, which caused the Year Without a Summer in 1816, is clearly visible as an exceptional sulphate spike in both Greenland and Antarctic ice cores, allowing precise correlation of the core chronologies across hemispheres. The 1258 mystery eruption — a sulphate signal larger than Tambora in both polar ice cores, corresponding to a period of documented crop failures and cold summers in historical records, but not definitively attributed to a specific volcano for decades — was eventually linked to Samalas volcano in Indonesia through tephra analysis.
Mountain Glacier Ice Cores
Ice cores from high-altitude mountain glaciers, including the Quelccaya Ice Cap in the Peruvian Andes, the Guliya Ice Cap on the Tibetan Plateau, the Huascaran core from Peru, and cores from the Kilimanjaro ice fields, provide climate records for lower latitudes that are not covered by the polar ice sheets. Lonnie Thompson of the Byrd Polar and Climate Research Center at Ohio State University has led most of the major tropical and subtropical ice core programs, using specialised lightweight drilling equipment adapted for use at altitude.
The Quelccaya record, extending back some 1,800 years with annual layer resolution, documents El Nino-Southern Oscillation variability, drought cycles, and the history of precipitation and temperature in the tropical Andes. These records have been correlated with historical accounts of pre-Columbian agricultural civilisations and their relationship to climate variability.
Use the map to locate the glacier and ice sheet sites where the most important ice core records have been collected, from the central Antarctic plateau to the mountain glaciers of the Andes and Himalayas.