What Can Ancient Ice Tell Us About the Air?

Share with your friends

Deep inside a thick sheet of ice, a tiny bubble holds a sample of old air. Scientists can study that bubble to discover something about the atmosphere long before anyone built a modern weather station.

—

This is one way researchers learn how the amount of carbon dioxide has changed. Carbon dioxide, or CO2, is a gas in the air. It is also a greenhouse gas, helping slow the escape of heat from Earth to space and thus contributing to global warming.

How Does Air Get Trapped in Ice?

In very cold places, snow can build up year after year. New snow presses down on the snow below. Over time, the buried snow becomes denser and eventually forms ice.

Air in the spaces between snow grains becomes enclosed in bubbles as those spaces close. A bubble is therefore a small sample of past atmosphere. It is not necessarily the same age as the surrounding ice, because trapping the air takes time.

Scientists drill long cylinders called ice cores from ice sheets. Different depths provide evidence from different periods. Working out those ages carefully is part of understanding the record.

What Does Carbon Dioxide Tell Us?

Researchers measure how much CO2 is present in the trapped air. They can compare those results with measurements taken directly from the atmosphere today. The amount is often given in parts per million, or ppm. Imagine a million air molecules. A concentration of 280 ppm means about 280 of them are CO2 molecules. The number describes a proportion, not the total amount of air.

Before the Industrial Revolution, atmospheric CO2 was around 280 ppm. Modern measurements are well above 400 ppm. Burning coal, oil and gas adds carbon dioxide, and changes in land use also affect the amount in the atmosphere.

Check this out next: What are Greenhouse Gases?

Why Compare the Past and Present?

Ice cores show that CO2 changed naturally during past ice-age cycles. Those changes help scientists understand how climate works. They also provide a background against which to examine today’s rapid increase.

CO2 is not the only influence on climate. Sunlight, other gases, ice and oceans matter too. Researchers combine several kinds of evidence rather than asking one bubble to explain everything.

For much older times, scientists use other clues, such as information preserved in rocks and sediments. Each method has strengths and uncertainties.

A bubble cannot speak, but careful measurements can turn it into evidence. That is the interesting part: something very small can help answer a question about the whole planet.

Resources and attribution

Adapted for Kids.Earth.Org with additional factual sources and original learning activities. Original Earth.Org source: A Graphical History of Atmospheric CO2 Levels Over Time, written by Owen Mulhern (2020; updated 2025). This adaptation has not been reviewed by the original author.

• NOAA: atmospheric carbon dioxide
• NOAA: Annual Greenhouse Gas Index

Keep exploring

What If Your Street Helped Keep You Cool?

Empowering Future Climate Leaders: Westminster Students Host Mock COP

How This 17-Year-Old Created a Sensor to Detect Wildfires Early

Back to Climate Change →