How Do You Read the Air From Eighty Thousand Years Ago? Bubbles in Ice

Snow falling on an ice sheet does not melt, so each year's fall buries the last and the pile compacts under its own weight. Air fills the spaces between snow crystals near the surface, and as compaction proceeds those spaces close off into sealed bubbles at a depth of several tens of metres, trapping a sample of the atmosphere at that moment. Everything the snow carried is trapped with it, including dust, sea salt, volcanic ash and the chemistry of the water itself. Deeper ice is older, and the layers thin with depth as the weight above compresses them and as the ice spreads outward, which is why a core hundreds of metres long covers a period far longer than its length suggests.

Several independent records are extracted from the same column:

• •Carbon dioxide and methane measured directly from the trapped air, which is unique among proxies
• •Temperature inferred from the ratio of oxygen and hydrogen isotopes in the ice itself
• •Dust content, which indicates how arid and windy the source regions were
• •Volcanic sulphate layers, which date eruptions and synchronise cores against each other
• •Sea salt, which indicates sea ice extent and wind
• •Annual layers visible physically in some cores, which allow counting like tree rings

Assigning an age to each depth is the foundation of everything else and uses several methods together. Counting annual layers works where accumulation is high enough for each year to remain distinguishable, which is the case in Greenland for tens of thousands of years and fails in the low-accumulation interior of Antarctica. Flow modelling calculates how layers thin with depth and gives ages where counting fails. Known volcanic eruptions provide fixed points. Matching features between cores from different sites synchronises them. Orbital variations in the earth's motion, which are calculable, appear in the records and anchor the long timescales. The result is a chronology with stated uncertainty that grows with depth, and the oldest continuous cores reach back around eight hundred thousand years.

The engineering is substantial and constrains where cores can be taken. A drill cuts an annular groove and brings up a cylinder of ice a few metres at a time, which is raised, logged, cut and packed, and the process repeats for years to reach several kilometres. The hole must be filled with fluid of matched density below a few hundred metres, since ice at depth flows and would close the hole otherwise, and choosing a fluid that does not contaminate the ice or freeze is a real problem. Samples must be kept frozen from the ice sheet to the laboratory, requiring a cold chain across continents. Sites are chosen where ice is thick, flow is slow and the layering is undisturbed, which means domes and divides rather than anywhere convenient.

The findings from this work changed several fields. Carbon dioxide and methane concentrations track temperature closely through the glacial cycles, which established a relationship that had been inferred and not demonstrated. Concentrations of both remained within a defined range throughout that period and are now far above it, which is the single most cited result from this research. Transitions between climate states occurred far faster than anybody expected, with Greenland records showing warmings of many degrees within decades, which overturned the assumption that climate changes gradually. Volcanic and dust records supply an independent history of eruptions and of aridity. And the method itself is being extended, with projects drilling for ice older than a million years.

Snow buries snow, and air trapped between crystals is sealed into bubbles at depth, preserving a direct sample of the atmosphere rather than a proxy for it. Isotopes give temperature, dust gives aridity and volcanic layers date the sequence. The records show greenhouse gases tracking temperature through glacial cycles and staying within a range that is now far exceeded.