How Do Air Bubbles Trapped In Antarctic Ice Preserve Ancient Air?

Snow that falls on Antarctica takes decades to thousands of years to pack down into solid ice, and the air between the grains keeps mixing with the sky until the pores pinch shut somewhere between 40 and 120 meters down. From that depth on, about a tenth of the ice by volume is sealed air, which is why a 10-gram chip of ice is enough to measure the carbon dioxide of a vanished atmosphere to better than one part per million. The deepest Antarctic cores hold a continuous 800,000-year record of that air, and a core drilled to bedrock in 2025 is expected to stretch it past 1.2 million years.

In a freezer lab in Bern, Switzerland, a machine shatters a chunk of Antarctic ice. The machine is called a cracker, and it works inside a sealed chamber where the ice never melts. What comes out is a small puff of air. That air last touched the sky hundreds of thousands of years ago.

You have seen the photo: a cylinder of ice, cloudy with tiny bubbles. They look like a flaw, the way bubbles do in a freezer ice cube. They are the opposite of a flaw. Those bubbles are why anyone bothers to drill 2,800 meters into Antarctica, about 9,200 feet.

How does a snowflake turn into a sealed jar of air? Why is the air in a layer of ice younger than the ice itself, by 30 years at one site and 3,000 at another? And what happens deep down, where the bubbles vanish but the air stays put?

How Does Falling Snow Turn Into Solid Ice?

Scoop up fresh snow and squeeze it. The handful shrinks to a fraction of its size, because most of what you held was air. The National Snow and Ice Data Center puts the water content of most new snowfall in the United States at 4 to 10 percent. The rest of the volume is air between the crystals. (We cover why snow falls elsewhere.)

On the Antarctic plateau that snow never melts. It is buried by the next fall, and the next. Under that weight the grains settle, round off and bond to each other. After a year or more, the layer packs to more than 550 kilograms per cubic meter. That is about 55 percent of the density of solid ice, and glaciologists now call it firn. Firn is the halfway house between snow and ice. One property of it matters for everything that follows: firn is porous. The gaps between the grains connect, all the way up to the surface.

So the firn breathes. Air drifts up and down through the gaps by diffusion. That is the slow spreading that carries the smell of coffee across a kitchen. The Law Dome notes at NOAA name diffusion as the main way air moves from the surface down to the sealing depth. The air inside the firn is, for practical purposes, today's air, however old the grains around it are.

In Antarctica the firn is thick. Christo Buizert's 2021 analysis in Geophysical Research Letters puts it at 50 to 120 meters (160 to 390 feet). The snowier and colder the site, the thicker it grows.

Where Does The Atmosphere Get Locked In?

Keep going down and the firn keeps squeezing. The pores narrow, then pinch off into separate pockets. The depth where the last pockets seal is called the close-off depth. From that moment the trapped air stops trading with the sky. A 2024 review by Thomas Bauska of the British Antarctic Survey gives the range. Close-off happens anywhere from 40 to 120 meters (130 to 390 feet) down, depending on how cold and how snowy the site is.

One site shows how sharp the line is. Law Dome is a snowy dome near the East Antarctic coast. Its DE08 core seals at 72 meters (236 feet), and the nearby DSS core at 66 meters (217 feet).

What gets sealed is not a trace. At close-off, about 10 percent of the ice by volume is air, roughly 10 milliliters in every 100 milliliters of ice. By weight it is only 0.01 percent, because air is light. It is still plenty. A 10-gram chip of ice, two or three sugar cubes' worth, gives a carbon dioxide reading good to better than 1 part per million.

So yes, there are air pockets in ice. In glacier ice they are not accidents. They are a dated sample set, and the date comes with the depth.

What Do Air Bubbles Trapped In Ice Tell Us?

Almost everything we know about ancient climate comes from proxies. A tree ring, a shell, an oxygen isotope ratio: each is a clue that a scientist has to turn into a temperature. The bubbles are different, because nobody interprets them. They are the atmosphere, held in a glass case.

Getting the air out takes care. Bernhard Bereiter and colleagues described the method in 2015. The ice must stay dry, so the air is freed by shattering the ice, or by evaporating it away without ever melting it. Meltwater would dissolve some of the carbon dioxide and spoil the reading.

There is one honest limit. The firn breathes for decades before it seals, so each bubble holds a blend of air from a span of years, not a single season. Bauska calls the effect a low-pass filter. Anything faster than the trapping time is smoothed away. At snowy sites that is a few decades. At the driest plateau sites it is a few hundred years. A plateau core cannot show a ten-year spike from half a million years ago.

Why Is The Air Younger Than The Ice Around It?

Here is the step that trips up almost everyone. Take a layer of ice from 100 meters down. The grains in it fell as snow long ago, but the air in its bubbles kept mixing with the surface until the day the pores closed. So at any depth, the air is younger than the ice that holds it. The NOAA Law Dome notes spell this out: the enclosed air has a mean age younger than its host ice layer.

Glaciologists call the gap delta-age, written Δage. Its size depends on how long the snow takes to reach the close-off depth.

At Law Dome, snow piles up fast, so the trip is quick. The DE08 core seals at 72 meters, where the ice is just 40 years old. In the published table, a sample from 83 meters holds ice from 1939 and air with a mean age of 1969, a 30-year gap. High on the plateau at Dome C, snowfall is scarce, and the same journey takes thousands of years. The Antarctic Ice Core Chronology 2023 ties all the deep cores to one timescale. It puts the Dome C gap at about 3,000 years on average, and about 4,000 years in the coldest stages.

Why does anyone care about a 3,000-year bookkeeping error? Ice core science has one big question: did carbon dioxide move before temperature, or after it? Temperature is read from the ice itself, through its water isotopes. Carbon dioxide is read from the air. Get Δage wrong and you slide one record against the other by thousands of years, and the answer changes.

The gap is so sensitive to conditions that Buizert turned it around. Measure Δage at a site, he showed, and you can estimate how cold the surface was when the snow fell.

What Happens To The Bubbles Deeper Down?

The bubbles do not survive the whole trip. Below about 600 meters at Dome C, the weight of the ice above starts to crush each bubble out of existence. The air stays put, forced molecule by molecule into cages inside the ice crystal itself. The result is a solid called air clathrate hydrate. A 2025 study in The Cryosphere describes the cages. Water molecules form the box, and one air molecule sits inside as the guest.

The change is gradual. At Dome C it runs from roughly 600 to 1,200 meters; at the EDML core across the continent, from 700 to 1,225 meters. Then the bubbles are gone and the ice turns clear. The air is packed tight. At ordinary pressure, one volume of hydrate can hold up to 150 volumes of air.

This matters in the lab. A cracker recovers only about half the air from clathrate ice, Bereiter's group found. Evaporating the ice recovers all of it. That difference hid an error of about 10 parts per million in the oldest part of the Dome C record. It took 107 re-measurements to find and correct it. The IPCC says so on its own charts: the gases were measured "in air bubbles and clathrate crystals".

What Are The CO2 Levels In Antarctic Ice Cores?

The deepest continuous record comes from the EPICA Dome C core. It reached 3,260 meters, a few meters above bedrock. Dieter Lüthi and colleagues measured its lowest 200 meters in 2008. That took the record back to 800,000 years.

Over those 800,000 years the planet swung through eight cold-warm cycles. Carbon dioxide swung with it, between about 172 and 300 parts per million. The lowest value ever measured in an ice core sits at 3,062 meters, about 667,000 years ago. Bereiter's 2015 re-measurement nudged it to 173.7 ppm. The highest value before the industrial era is 300 ppm. Through all eight cycles, carbon dioxide tracks Antarctic temperature closely.

Then there is the dot at the far right of the chart. In August 2026 the Mauna Loa Observatory in Hawaii recorded a monthly average of 427.55 ppm. Nothing in the bubbles comes within 120 ppm of that. The British Antarctic Survey review lists it as a key point: today's level has no match in at least the last 800,000 years. For what that does to the planet's heat budget, see why carbon dioxide is a greenhouse gas.

Are We Technically Still Living In An Ice Age?

Yes, by the geologist's definition. The sawtooth on the chart is the rhythm of the Quaternary, the period that began about 2.6 million years ago. Long cold glacial stages alternate with shorter warm ones called interglacials. Britannica dates the Pleistocene glaciations to between 2.6 million and 11,700 years ago. The warm spell since then, the Holocene, is the most recent interglacial of the Quaternary. Ice sheets still cover Antarctica and Greenland, which is the giveaway. An ice age is any period when thick ice sheets cover vast areas of land, and we are living in a warm pause inside one. (For a far more extreme version, see whether Earth was ever a giant snowball.)

The bubbles added a twist. Over the past 800,000 years the cycles ran about 100,000 years long. Before about 1.2 million years ago they were shorter, around 40,000 years, and milder. Yan and colleagues noted this in 2019. Why the planet changed gear, the Mid-Pleistocene Transition, is one of the big open questions. It is also the reason for the next section.

How Far Back Do Ice Core Records Go?

Antarctica's ice is nearly 4.9 kilometers (3 miles) thick at its thickest point. Deeper is older. But the deepest ice is also the most squashed, and the most likely to have melted or mixed near the rock below. The Beyond EPICA project used radar surveys and ice-flow models to pick its spot. They needed a place where 1.2-million-year-old ice survives in order. The spot is Little Dome C, on the plateau at 3,200 meters (10,500 feet) above sea level. The average summer temperature there is −35 °C (−31 °F). (Yes, people live in Antarctica year-round. Not many.)

There, in the 2024/2025 season, the team reached bedrock at 2,800 meters. The drilling took more than 200 days, spread across four seasons. Twelve institutions from ten European nations took part. Field results gave the chief scientist "a strong indication" about the top 2,480 meters. They hold, he said, a continuous record back to 1.2 million years. The 800,000-to-1.2-million-year stretch sits between 2,426 and 2,490 meters. There, up to 13,000 years are packed into a single meter of ice. Project coordinator Carlo Barbante called it the longest continuous climate record from an ice core.

What is under that? The lowest 210 meters, right above the rock, is old ice that is heavily deformed. It may be mixed or refrozen, its origin is unknown, and it is not a climate record. The team also drilled into the bedrock itself. Dating those rocks should reveal when this part of Antarctica was last ice-free.

The cores crossed the ocean in a cold chain held at −50 °C (−58 °F). As of April 2026, labs across Europe are analysing them, bubbles included. No carbon dioxide value for the 1.2-million-year ice has been published yet.

Is There Even Older Air In Antarctica?

There is, but in pieces. The Allan Hills is a blue ice area of East Antarctica. There, the flow of the ice sheet brings ancient layers close to the surface, out of order. In 2019, Yuzhen Yan and colleagues reported ice there more than two million years old. They call it "stratigraphically discontinuous": snapshots, not a continuous film. Samples older than two million years had been altered by respiration. Something alive had been breathing in the ice. Some younger samples were pristine. A 2025 study in Nature pushed the snapshots back to 3.1 million years. It found carbon dioxide broadly stable, with a drop of only about 20 ppm between 2.9 and 1.2 million years ago.

Treat those numbers with more care than the continuous record. The Bauska review warns of organic carbon contamination in the oldest Allan Hills ice. And snapshots cannot say what happened between the frames. That gap is exactly why Beyond EPICA drilled a continuous core. (Why the ice moves at all is its own story; see how glaciers move.)

So, How Do Air Bubbles In Antarctic Ice Preserve Ancient Air?

Put the pieces in order and the whole thing is physics you could follow with a shovel. Fresh snow is mostly air. On the Antarctic plateau it never melts, so it is buried, squeezed into firn, and squeezed again. For the first 40 to 120 meters the pores stay connected, and the air inside keeps trading with the sky. Then the pores pinch shut. From that depth down, a tenth of the ice by volume is sealed air from the day of closing.

That one fact explains the paradox. The air stayed in touch with the surface until the firn sealed. So the ice at any depth is older than its air. At snowy Law Dome the gap is 30 years; at Dome C it is about 3,000.

Deeper still, the pressure dissolves the bubbles into cages inside the crystal. The labs crack or evaporate the air back out. What they have read so far is 800,000 years of carbon dioxide. It never left the band between about 172 and 300 parts per million. Mauna Loa read 427.55 ppm this August.

The next chapter is sitting in freezers in Europe. The Beyond EPICA core is expected to add 400,000 years to the record. That covers the switch from 40,000-year cycles to 100,000-year ones. Nobody will have to interpret those bubbles. A 10-gram chip, a sealed chamber, and a count of the molecules will do it.