Why Does Antarctic Ice Hold Air From 800,000 Years Ago?
― When snow turns to ice, the air in its gaps is sealed into bubbles
Drill 3,000 metres down into Antarctic ice, and the ice near the bottom still holds air from 800,000 years ago, trapped in tiny bubbles. Researchers can pull out air from a time before humans could use fire at will, and measure it. The key is the moment the pores close as snow turns into ice, together with cold so extreme that the ice has not melted for hundreds of thousands of years.
Look closely at ice made in your freezer. Some parts are cloudy white. Move closer and you will see lots of tiny bubbles.
Those bubbles are filled with air that was trapped when the water froze. In other words, ice can "preserve" a little of the air around it.
Now suppose the ice never melted and stayed put for hundreds of thousands of years. In Antarctica, that is exactly what has happened.
Just two reasons
Fresh snow is full of gaps. As more snow piles on top, it is pressed together, and at a depth of about 100 metres the gaps no longer connect to the outside. The air inside is sealed in as bubbles.
Inland Antarctica stays below freezing even in summer, so the ice almost never melts. Snowfall is also light: a year's worth adds only a few centimetres of ice. Thin layers have stacked up for hundreds of thousands of years.
With both in place, the ice becomes "a shelf of canned air, newer on top and older at the bottom." Let's take it step by step.
How does snow trap air?
Freshly fallen snow is mostly empty space by volume. At this stage, the air in the gaps mixes freely with the air above the surface.
As snow keeps piling up, the snow below is squeezed by the weight above. The grains stick together and slowly pack tighter. This in-between state, part snow and part ice, is called firn.
At a depth of a few dozen metres to about 100 metres, the packing passes a certain limit. The passages between the gaps are pinched shut, and the air can no longer escape. Look at the upper half of Figure 1. This is where air turns into bubbles and the air of that era is sealed in.
Note that the air in the bubbles is a little younger than the ice around it. Even when snow fell thousands of years ago, new air kept seeping in until the pores closed. Researchers correct for this age gap by calculation.
Why can 800,000 years be stacked up?
The first reason is that the ice does not melt. In much of inland Antarctica the average temperature is below minus 50°C, and it stays below freezing even in summer. A single melt would wipe out the bubbles, so this cold protects the record.
The second reason is that very little snow falls. Inland Antarctica is as dry as a desert, and a year's snowfall is said to add only a few centimetres of ice. Even in thick ice, each year takes up so little room that a great many years are packed in.
Here is one more surprise. If the layers kept their surface thickness of 3 centimetres all the way down, 3,200 metres of ice would hold only about 110,000 years. Yet the real bottom is about 800,000 years old. The deep layers are stretched thin, down to a few millimetres or less, as the ice slowly flows sideways and outward (the right side of Figure 1).
A column of ice pulled out this way is called an "ice core." The core drilled at Dome C in Antarctica is reported to record about 800,000 years of air. Japan's Dome Fuji Station also drilled a core about 3,000 metres long, said to cover about 720,000 years.
Below a few hundred metres, strong pressure crushes the bubbles into clear crystals made of air and water locked together. When the ice is brought to the surface, the crystals slowly turn back into bubbles. For that reason, drilled ice is sometimes left to rest before measuring so that it does not crack.
The carbon dioxide in the bubbles swung between about 180 in cold ice ages and about 280 to 300 in warm periods (parts per million of air). In 800,000 years, it is said never to have gone above 300. Today's atmosphere is above 420.
Summary
Old air survives in Antarctic ice because the pores close as snow turns to ice, forming bubbles, and that ice has barely melted while stacking up for hundreds of thousands of years. The deeper layers are stretched and thinned, so a 3,000-metre column holds 800,000 years.
Antarctic ice is a tin of air sealed by snow.
Drilling down 3,000 metres is travelling back 800,000 years.
How ice flows sideways is explained in "Why does a glacier flow like a river even though ice is solid?", and what happens at the base of the same Antarctic ice is covered in "Why is there an unfrozen lake under the coldest place on Earth?". For why Antarctica is so much colder, see also "Why is Antarctica so much colder than the Arctic?".
- Fill two cups with water. Leave one as tap water, and use boiled-then-cooled water in the other.
- Freeze both, then take them out and hold them up to the light.
- The tap-water ice should have white bubbles gathered in the middle. As the water freezes from the outside in, the air that was dissolved in it is pushed toward the centre and trapped there.
Antarctic ice is not frozen water but snow pressed solid, so its bubbles form a little differently. Still, the point is the same: it hardens with air sealed inside.
For those who want more ― terms, formulas, and links to textbooksEach part is labelled by level, from junior high science to university-level courses
- Junior highCovered in junior high school science
- High schoolCovered in high school earth science
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityUniversity-level specialist courses (glaciology, paleoclimatology) not taught in high school
- ResearchOpen questions that researchers are still investigating, not taught as settled even at university
Junior highTerms: this phenomenon has names
- Ice sheet: a wide, thick mass of ice covering a continent. Today they exist in Antarctica and Greenland.
- Ice core: a cylinder of ice drilled out of an ice sheet. Depth corresponds to age.
- Firn: snow partway to becoming ice after being pressed together. The air in its gaps is still connected to the outside.
Junior highHigh schoolChecking with a formula: how many years are in 3,200 metres of ice?
As with rock strata, the age span of stacked layers can be estimated by "dividing the total thickness by the thickness of one year's layer." First we calculate with the surface thickness, then compare with the real age.
| In symbols | t ＝ z ÷ λ |
| In words | Years recorded ＝ depth of ice ÷ thickness of one year's layer |
| Where it comes from | It builds on the law of superposition, which says lower layers are older: if one layer is added each year, you can count the layers. If the layers thin out along the way, λ changes with depth. |
| t | Years recorded (years) |
| z | Depth of ice (metres) |
| λ | Thickness of one year's layer (metres), as a value in ice equivalent |
| Length of the Dome C ice core | said to be about 3200 m |
| Thickness added per year near the surface (ice equivalent) | said to be about 0.03 m |
| Age of the ice near the bottom | reported as about 800,000 years (800000 years) |
| Depth where pores close and form bubbles | said to be about 100 m |
| Years if stacked at the surface thickness | 3200 ÷ 0.03 ≒ 106667 |
| Average thickness of one year's layer, worked back from the real age (m) | 3200 ÷ 800000 ＝ 0.004 |
| Convert to millimetres | 0.004 × 1000 ＝ 4 |
| How many times thinner than at the surface | 0.03 ÷ 0.004 ＝ 7.5 |
| Years until bubbles close (rough guide) | 100 ÷ 0.03 ≒ 3333 |
At the surface thickness it would be about 110,000 years, but the real figure is about 800,000 years. Even on average, one year's layer has thinned to about 4 millimetres, and near the bottom it must be thinner still. Also, because air keeps being exchanged for the few thousand years before the bubbles close, the air in the bubbles is a few thousand years younger than the ice around it (in practice, because firn is less dense, the estimate is somewhat shorter).
High schoolHigh school+Bubbles and ice can even tell us past temperatures
High schoolIn high school earth science, carbon dioxide levels and temperature changes in ice cores are introduced as the repeating cycle of glacial and interglacial periods. Cold and warm periods alternated about every 100,000 years, and carbon dioxide rose and fell along with them.
High school+Temperature is read not from the bubbles but from the water in the ice itself. Some water molecules contain slightly heavier oxygen or hydrogen than usual. The colder the era, the smaller the share of heavy molecules in the snow, so that share (the oxygen and hydrogen isotope ratios) serves as a guide to past temperature.
UniversityFirn densification and the gas-age offset
The process by which snow becomes ice is studied in glaciology as "firn densification." Bubbles are said to close off once the density exceeds about 0.83 grams per cubic centimetre, and this depth is called the bubble close-off depth. The difference between the age of the air in the bubbles (gas age) and the age of the ice is called "delta-age," and it is larger in cold periods with little snowfall. The relationship between depth and age is estimated with models such as the Nye age model, which assumes a pattern of ice flow, and is pinned down by checking against volcanic ash layers and other records. Deep bubbles turn into air hydrates (clathrate hydrates).
📖 Derivations and further reading: Ice core (Wikipedia, Japanese) ／ Antarctic ice sheet (Wikipedia, Japanese)
ResearchWhat is still not well understood
- Is there ice older than 1 million years? A European research project announced in 2025 that it had reached ice older than 1.2 million years. Analysis is under way as a clue to why the cycle of ice ages changed from 40,000 to 100,000 years around 1 million years ago.
- How disturbed are the bottom layers? Ice near the bedrock can have layers folded or melted by flow and geothermal heat, and it is said to be hard to judge how far the age order is preserved.
- Is the content of the bubbles really unchanged? It has been pointed out that carbon dioxide may rise or fall slightly by reacting with impurities in the ice, and special care is said to be needed with Greenland ice.
In other words, this article too is "an explanation of what is understood so far." The age figures may be rewritten by new drilling and analysis.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: layers of strata / changes of state | Lower layers are older; snow turning to ice |
| High school | Earth science: changes in the Earth's environment | Glacial and interglacial periods, changes in carbon dioxide |
| High school+ | Earth science: past temperatures from isotopes | Reading temperature from the share of heavy water |
| University | Glaciology, paleoclimatology | Firn densification, delta-age, age models |
| Research | The search for the oldest ice | Ice over 1 million years old, disturbed bottom layers |
| ― | Connection to daily life | The evidence that today's carbon dioxide exceeds the range of the past 800,000 years |
- Lüthi, D. et al. (2008) High-resolution carbon dioxide concentration record 650,000–800,000 years before present. Nature 453
- EPICA community members (2004) Eight glacial cycles from an Antarctic ice core. Nature 429
- National Institute of Polar Research (国立極地研究所), explanatory materials on the Dome Fuji ice sheet deep drilling
- Wikipedia, "Ice core" (ウィキペディア「氷床コア」)
- Wikipedia, "Antarctic ice sheet" (ウィキペディア「南極氷床」)
※This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand how things work. Ice core ages and concentrations vary between studies and may be updated by future analysis.