Drilling the Oldest Ice on Earth
Within the ice sheets of Antarctica and Greenland lies one of the most complete natural archives of Earth's climate history. Each year's snowfall, compressing over centuries into dense glacier ice, preserves a record of the atmosphere as it existed at the moment of deposition: the composition of the air itself, trapped as tiny bubbles; the temperature signal embedded in the ratio of water isotopes; the volcanic signature of eruptions deposited as sulphur layers; the dust carried by ancient winds; the sea salt from oceanic storms; the pollen from distant vegetation. Ice core science has read this archive back more than 800,000 years, revealing eight complete glacial-interglacial cycles in a single drill core, and the hunt is now on for ice that is one million years old or more.
The oldest ice ever recovered was obtained from the East Antarctic Ice Sheet, where ice accumulates slowly and immense age is concentrated in relatively thin layers at the base of the ice sheet. The record-holder as of the mid-2020s is a core from the Beyond EPICA project at Little Dome C (also known as Dome C or Concordia Station), which the European consortium aims to push beyond 1.5 million years. Understanding what the oldest ice will reveal — and how scientists retrieve it from ice sheets kilometres thick at the bottom of the world — is one of the most compelling stories in contemporary Earth science.
How Ice Cores Are Drilled
Ice core drilling combines engineering, glaciology, and analytical chemistry. The drill itself is a long, hollow cylinder — typically ten centimetres in diameter — that cuts a continuous cylindrical core from the ice. The cutting is done by a rotating ring of sharp blades at the lower end of the drill barrel. As the barrel fills with a metre or so of core, it is raised to the surface, the core is removed, catalogued, and stored in a cold laboratory, and the drill is sent back down.
Preventing the borehole from collapsing under the pressure of the surrounding ice is the central engineering challenge. Deep ice drilling — below a few hundred metres — requires filling the borehole with a drilling fluid that matches the pressure of the surrounding ice. Various fluids have been used over the decades: jet fuel, ethanol mixtures, and synthetic hydrocarbon compounds. Managing the fluid, keeping the borehole straight, and maintaining the drill in working order at the base of a hole that may be three or four kilometres deep, in temperatures below minus 30 degrees Celsius, over a drilling campaign that may span several years, requires a combination of mechanical robustness and operational patience that defines the culture of the ice core drilling community.
Key Drilling Sites
Vostok Station in East Antarctica holds a place in ice core history as the site of the first core to exceed 400,000 years of record. The Russian-led Vostok project, extending over several decades, produced a core reaching 3,623 metres in depth, capturing four complete glacial cycles. The temperature and atmospheric CO2 data from Vostok provided the first clear evidence that atmospheric carbon dioxide concentrations had varied in concert with temperature over glacial-interglacial cycles, a finding that fundamentally shaped the modern understanding of climate forcing.
EPICA Dome C (European Project for Ice Coring in Antarctica), also in East Antarctica, extended the record to approximately 800,000 years at a depth of 3,270 metres. The core captured eight glacial cycles and revealed that the spacing between glacial periods shifted approximately 900,000 years ago from a 41,000-year cycle to the dominant 100,000-year cycle. This transition, known as the Mid-Pleistocene Transition, remains an active subject of research; the oldest ice expected from the Beyond EPICA project at Little Dome C may hold the record of what the climate was doing before the transition.
WAIS Divide in West Antarctica produced the highest-resolution record of climate variability from the southern hemisphere for the last 68,000 years. The very high accumulation rate at WAIS Divide means that individual annual layers can be distinguished and counted in the ice, providing year-by-year resolution in the record. This allows comparison with tree ring and other annual proxy records and has resolved questions about the relative timing of climate events in the northern and southern hemispheres.
The Greenland Ice Core Project (GRIP) and the Greenland Ice Sheet Project 2 (GISP2) drilled adjacent sites near the Greenland summit through the early 1990s, producing records extending back approximately 110,000 years. The Greenland cores documented the extraordinary climate variability of the last ice age — the Dansgaard-Oeschger events, rapid warm oscillations of up to 10 degrees Celsius in decades, which have no modern analogue and whose cause remains debated.
What Ancient Ice Reveals
The power of ice core records lies in the simultaneous measurement of multiple climate proxies from the same material. The ratio of oxygen isotopes (oxygen-18 to oxygen-16, expressed as delta-O18) in the ice is a temperature proxy — warmer temperatures at the time of precipitation produce ice with a higher ratio. The carbon dioxide and methane concentrations in the trapped air bubbles record the atmospheric greenhouse gas composition directly — the ice preserves actual samples of ancient air at the moment of pore closure, without biological alteration. Dust concentrations indicate wind strength and the extent of arid, unglaciated land surfaces. Sodium and other sea salts trace storm tracks and sea ice extent.
The correlation between atmospheric CO2 and temperature in the ice core records — where warmer periods correspond to higher CO2 and cooler periods to lower CO2 over the entire 800,000-year record — is one of the most powerful empirical demonstrations of the relationship between greenhouse gas concentrations and global temperature. Current atmospheric CO2 levels, measured at approximately 420 parts per million, are higher than at any point captured in the existing 800,000-year ice core record.
The Hunt for Oldest Ice
The site selection process for the Beyond EPICA core at Little Dome C involved extensive modelling and radar surveys to identify a location where ice of the target age exists at a drillable depth, the bed is not disturbed by basal melting, and the annual layers have not been disrupted by ice flow. Getting the site wrong — arriving at million-year depth only to find that the ice is isothermal, folded, or melted at the base — would be a costly failure. The confidence from radar data about the stratigraphy below Dome C is substantial, but drilling remains the only definitive test.
The glacier map shows the major ice sheet regions where ice core science has been conducted. For anyone interested in the intersection of fieldwork, analytical chemistry, and climate science, the ice core record from Antarctica and Greenland represents one of the most remarkable scientific achievements of the past century.