How do ice cores provide evidence about Earth’s past climate?

Ice cores act as vital archives of Earth’s past climate by preserving records of ancient snowfalls that have compressed into glacier ice over millennia. Scientists extract these long cylinders of ice to analyze several key indicators of historical environmental conditions:1

• Isotopic Composition: The ratios of oxygen (${}^{18}$O) and hydrogen (${}^2$H) isotopes in the ice reflect the temperature and the source of moisture at the time the snow fell, allowing researchers to reconstruct past temperature variations.12
• Atmospheric Gases: As snow transforms into dense ice, it traps small bubbles of air. By analyzing these bubbles, scientists can measure historical concentrations of greenhouse gases like carbon dioxide and methane, revealing links between gas levels and climate shifts.12
• Particulates and Chemistry: The presence of dust and sulfate concentrations provides evidence of past volcanic eruptions and periods of intense atmospheric circulation, which help map the history of global climate events.12

By counting annual layers or using indirect dating methods, researchers can correlate these findings to create a detailed timeline of climate change spanning hundreds of thousands of years.12