Ice and ocean cores preserve indirect climate evidence. Comparing dated records reveals repeated warming and cooling, with uncertainty that must be assessed.
The Quaternary extends from about 2.6 million years ago to the present. It includes the Pleistocene, with repeated glacial and interglacial periods, and the Holocene, which began about 11,700 years ago. Glacial periods had more extensive land ice; interglacials were warmer intervals. These labels do not mean that the whole planet was frozen or ice-free.
A proxy is a record used to infer a variable that was not measured directly. In ice cores, the proportions of oxygen and hydrogen isotopes in the ice help reconstruct past temperatures and moisture conditions. Trapped air bubbles preserve samples of past atmospheric gases, including carbon dioxide. Dust and other particles add evidence about the environment.
Ocean cores contain accumulated sediment and tiny fossil shells. Species assemblages indicate the conditions in which organisms lived. Oxygen-isotope ratios in foraminifera, small shell-forming organisms, reflect both water temperature and the global storage of water in ice. Interpreting the signal therefore requires calibration and other evidence.
Layers must be dated before records can be compared. Ice and trapped air can have different ages because air is enclosed after snowfall. Sediment mixing, dating uncertainty and varying accumulation rates affect time resolution. Agreement among independent records strengthens an interpretation; one core is not a direct, complete record of global temperature.
Climate variability means changes in climatic averages and extremes beyond individual weather events; internal processes and natural or human external influences can cause it. These records establish that climate variability predates industrialisation. They do not identify the cause of contemporary warming by themselves. For that, compare the timing, scale and physical effects of present natural and human influences.
Step by step
Recover a dated record
Locate layers and estimate their age and time resolution.
Interpret the proxy
Connect isotope ratios, fossils or gases to the variable they represent.
Compare evidence
Check multiple sites and proxies before drawing a global conclusion.
Worked example: Two measurements from one ice core
Temperature is inferred from isotopes in the ice, while carbon dioxide can be measured in trapped air. A correlation between their histories supports climate-carbon interaction, but dating and other evidence are needed to establish which change led during a particular interval.
Watch out for this
Air bubbles in ice directly record past air temperature.
They preserve past gases. Temperature is commonly inferred from properties such as the isotopic composition of the ice.
Check your understanding
Why should an ocean-core oxygen-isotope record be checked against other evidence?
- It cannot contain any climate information.
- Every layer represents exactly one year.
- Its signal can reflect both water temperature and global ice volume.