Ice cores and tree rings provide evidence of past climate change by preserving measurable signals of earlier atmospheric and growing conditions. They are proxy records used to reconstruct climate before direct instrumental measurements existed.
The Reasoning and Mechanism
This is IB ESS subtopic 6.2, Climate change: causes and impacts. Comparing past carbon dioxide concentrations with temperature is required at SL and HL; detailed isotope measurements and dendrochronology are HL only.
| Evidence source | How the record forms | What scientists infer |
|---|---|---|
| Ice cores | Successive snowfall is compressed into ice layers. Trapped air bubbles preserve samples of the past atmosphere. | Bubble composition shows past greenhouse gas concentrations, including carbon dioxide and methane. Oxygen or hydrogen isotope ratios act as temperature proxies; dust and ash may indicate drought or eruptions. |
| Tree rings | Trees normally form one ring annually. Ring width and wood density respond to conditions during growth. | Wider or denser rings can indicate favourable temperature or precipitation; narrow rings may indicate cold or drought. Matching patterns creates an annually dated chronology. |
Scientists compare multiple records rather than trusting one sample. The EPICA Dome C Antarctic ice core extends about 800,000 years, while long-lived bristlecone pines support reconstructions over thousands of years. Agreement among ice cores, tree rings, sediments and modern measurements increases confidence.
A common misconception is that every narrow ring proves a cold year. Drought, disease, soil conditions or competition can also restrict growth, so chronologies are replicated across sites and calibrated against modern climate data. Trapped gases directly sample former atmospheres, but isotope-based temperature estimates remain indirect.
Exam Technique
For an explain question, describe how each record forms, identify what is measured, and link it to the inferred climate variable. For evaluation, address limitations, use several proxies and give a named example. Do not claim that correlation alone proves causation.