1. Quick Summary
Snow compresses into ice, and as it does so it seals off small bubbles of air. Those bubbles are samples of the atmosphere at the time the ice closed.
Measuring gases in the bubbles, together with the chemistry of the ice itself, gives both atmospheric composition and a proxy for temperature going back hundreds of thousands of years.
2. What It Means
The ice records temperature indirectly through isotopes: the ratio of heavy to light water molecules depends on conditions at the time the snow formed.
The bubbles record composition directly, which is why ice cores are uniquely valuable for greenhouse gas concentrations.
Dating comes from counting layers where possible and from matching patterns to other records, with uncertainty growing deeper in the core.
3. Why It Happens
Bubbles are not sealed instantly. Air is trapped gradually over a depth range, so the gas record is slightly younger than the surrounding ice, and that offset must be corrected.
Layer counting works well near the surface and becomes difficult at depth, where layers are thin and deformed.
The records show that greenhouse gas concentrations and temperature have moved together across many natural cycles, which is central evidence about how the climate system responds.
They also reveal abrupt changes in the past, showing that the system can shift quickly rather than only gradually.
Volcanic layers and dust provide additional markers, giving independent ways to date and cross-check the record.
The records have limits: resolution decreases with depth, and very old continuous ice is rare, which is why extending the record is a major logistical effort.
4. Real Examples
Gas measurements: carbon dioxide and methane extracted from trapped bubbles.
Isotopic temperature proxies: ratios in the ice that reflect formation conditions.
Volcanic sulphate layers: time markers from known and unknown eruptions.
Deep drilling programmes: multi-year projects retrieving cores from the thickest ice sheets.
Abrupt climate events: rapid transitions visible in high-resolution sections.
5. How It Affects Us
Climate science: ice cores underpin understanding of how greenhouse gases and temperature co-vary.
Model validation: past climates are used to test whether models reproduce real behaviour.
Public understanding: the record is direct evidence rather than inference.
Observation: preserving and continuing these records depends on sustained, expensive fieldwork.
6. Key Takeaways
- Trapped bubbles are direct samples of past atmosphere.
- Temperature comes from isotopes; composition comes from the gas.
- Gas age and ice age differ and must be corrected for.
- The record shows tight coupling between greenhouse gases and temperature.