Proxies are indirect clues to past climate. Scientists date and compare these records to reconstruct changes before thermometer measurements began.
Earth has passed through warmer and cooler periods, including glacial periods with more ice and warmer interglacial periods. Thermometer records cover only a small part of this history. Scientists study proxies: physical or biological evidence linked to past climate.
Seafloor sediments build up in layers. A sediment core contains material deposited at different times, including the shells of tiny organisms. The species present, the sediment and the chemistry of shells reveal past ocean conditions. Scientists must date the layers and check for gaps or disturbance.
Oxygen isotopes are forms of oxygen with different masses. The balance of lighter oxygen-16 and heavier oxygen-18 in marine shells depends on water temperature and seawater composition. During major glacial periods, land ice stores more of the lighter oxygen, leaving seawater richer in heavier oxygen. Shell records therefore help identify colder, ice-rich periods. Scientists must calibrate this proxy; the ratio is not a direct thermometer reading.
Climate zones also move. During cooling, cold conditions and associated ice or vegetation zones can spread towards lower latitudes. During warming, they can retreat towards the poles or higher ground. Rainfall, oceans and terrain affect where the boundaries lie, so they do not move as straight lines. A global conclusion needs evidence from several places.
Step by step
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Sediment layers contain shells and other evidence deposited at different times.
Date and interpret it
Shell chemistry reflects ocean conditions, including temperature and seawater composition.
Compare records
Dated evidence from other places helps test the interpretation.
Worked example: Evidence is not attribution
A marine core may record alternating warmer and colder periods. It shows that climate changed. To explain why a particular period changed, scientists need more evidence about possible causes, such as orbital cycles or volcanism.
Watch out for this
An oxygen-isotope value always means the same temperature in every archive.
Marine shells and ice cores record isotope changes differently. Temperature, seawater composition, ice volume and the type of archive must be considered.
Check your understanding
Why compare a marine isotope record with other dated evidence?
- Because one shell directly measures global temperature.
- Because the proxy has several influences and independent records can test the inference.
- Because sediment layers cannot contain climate information.