1. How Snow Piles Up and Turns into Ice in Antarctica
In the interior of the Antarctic continent, it is very cold, so snow that falls rarely melts. Instead, it piles up layer by layer. The snow at the bottom is slowly pressed and compacted by the weight of the snow above it, eventually turning into ice. According to the National Institute of Polar Research, it takes about 3,000 years for snow to become ice in the inland areas [1]. However, this is just a rough guide for the interior. The time it takes varies greatly depending on the amount of snowfall and the temperature.
2. How Air Gets Trapped as Bubbles in Ice
Freshly fallen snow is full of gaps. As this snow is pressed, it becomes a white, hard layer called "firn." The gaps in the firn remain connected to the surface air for a while. When the density of the compressed snow reaches about 800 kilograms per cubic meter, the gaps close up. At this point, air can no longer move in or out, and it is preserved inside the ice as bubbles [2]. It is like sealing a bottle; the air at that moment remains inside.
Bubbles close long after the snow first fell. Because of this delay, the air in the bubbles is younger than the surrounding ice. The size of this age difference between the ice and the bubbles varies from place to place [2].
3. Ice Cores Drilled 3,035 Meters Deep at Dome Fuji
Japan’s Antarctic observations have involved drilling ice columns at "Dome Fuji" in the interior. These extracted ice columns are called ice cores. The second phase of drilling took place from 2003 to 2007. They drilled to a depth of 3,035 meters. The ice at the deepest part is said to be from about 720,000 years ago [3]. It is important to note that this does not mean the entire 3,035-meter column represents 720,000 years. Rather, it means the ice at the very bottom is from that time.
4. What Air Bubbles Reveal About Carbon Dioxide Over 800,000 Years
By extracting air bubbles from ice and measuring their components, scientists can determine the amount of carbon dioxide in the distant past. A 2008 study analyzing cores from the European EPICA project extended this record to about 800,000 years ago [4]. During those 800,000 years, cold ice ages and warm interglacial periods repeated 8 times. The amount of carbon dioxide was low during ice ages and high during interglacial periods, moving closely with Antarctic temperatures [4].
But did carbon dioxide or temperature rise first? According to the National Institute for Environmental Studies, when moving from an ice age to an interglacial period, they rose almost at the same time, or temperature rose slightly first. The change in sunlight is thought to be the trigger, followed by a rise in gas concentration [6].
5. Past Carbon Dioxide Levels Compared with Today's Numbers
Looking at ice records, carbon dioxide concentrations show a cycle of being low in ice ages and high in interglacial periods over 800,000 years. Before the Industrial Revolution, around 1750, levels were below 280 ppm. In the past 800,000 years, it never exceeded 300 ppm [5]. ppm stands for parts per million.
According to the US National Oceanic and Atmospheric Administration (NOAA), the level in 2024 was about 422.8 ppm, which is about 50% higher than before the Industrial Revolution [5]. The speed of the rise is also very different from the past. During the transition from an ice age to an interglacial period, it took about 1,000 years for carbon dioxide to increase by about 20 ppm. Now, an increase of about 20 ppm is happening in about 10 years, according to the National Institute for Environmental Studies [6].
6. Why Scientists Are Seeking Ice Older Than 1.2 Million Years
In the interior of ice sheets, older ice is deeper, but it is not just about depth. Even in the interior, snowfall is low, so old ice is compressed into very thin layers. Ice also flows sideways, eventually forming icebergs at the continent's edge. The deeper and older parts become thinner and more compressed [8].
In January 2025, the "Beyond EPICA" project, involving 10 countries and 12 institutions, finished drilling about 2,800 meters of ice to bedrock at Little Dome C in Antarctica. The ice age is expected to be older than 1.2 million years. However, this is an estimate; precise dating and bubble analysis results are still pending [7].
Japan is also planning a third phase of drilling near Dome Fuji Base II to retrieve ice older than 1 million years. The ice thickness is estimated to be between 2,200 and 2,700 meters, but details on the schedule are not clear from the materials [8].
There is also a record from a different direction. In 2017, a US team reported finding ice from about 2.7 million years ago in the blue ice areas of the Allan Hills in Antarctica. This is a place where strong winds blow snow away, exposing old ice layers. However, this is not a continuous column record; it involved measuring the age of ice blocks and arranging them [9].
7. How to Look for Bubbles in Ice at Home
What can you do today? Look at ice in your refrigerator or ice cube trays. Check for cloudy white parts or small bubbles. If there are bubbles, they are a close example of air being trapped inside ice.
If you are interested in reading more, the question "How will carbon dioxide levels appear in the analysis of ice older than 1.2 million years?" has not been answered yet. The National Institute of Polar Research’s magazine "Kyoku" has articles on Japan’s next drilling plan. Compare them with news reports when the results come out.