Temperature proxies such as trees, ice and corals act like natural thermometers, changing in step with the temperature of their surroundings. Scientists use proxies to look backwards in time – from decades to millennia – and find out what earlier climates were like. But this isn’t straightforward. Many environmental factors affect proxies, including rainfall and air pollution. This sometimes obscures their relationship with temperature. That’s why scientists use many different types of proxy and check data for signs of contamination. A few proxy temperature measurements could be incorrect, but it’s very unlikely that hundreds of trees, ice cores, sediments and other proxy records would all be wrong in the same way.

Ice builds up in layers each year. Scientists extract columns of ice, called ice cores, to find out about conditions when the ice was formed – one of these cores is over 3 km long and contains ice formed over 800,000 years ago. Water vapour in the atmosphere contains different isotopes of oxygen – oxygen-18 and oxygen-16. The less oxygen-18 in the ice, the colder the air was at the time the ice formed. By measuring the amounts of each isotope in different layers of an ice core, scientists can trace changes in past temperature. In some locations, scientists also use pollen particles trapped in the ice to reconstruct past temperature variations from changes in the local vegetation. Ice cores show global temperature rising significantly over the last century.

Tree trunks contain bands called growth rings. Counting these rings reveals a tree’s age – some are 9000 years old. Scientists use the width of tree rings to figure out past temperatures, since trees often grow faster in warmer periods. But growth also depends on other factors, including water, sunlight and nutrients. Tree rings indicate rising temperatures at the beginning of the 20th century, in line with other measurements. But in the second half of the 20th century tree rings from northern forests produce contradictory results. Scientists think that other factors, such as air pollution, have influenced tree growth, stopping trees being a reliable proxy for recent temperature.

Air temperature leaves a thermal ‘imprint’ on the ground which gradually spreads downwards through the soil and rock. By measuring the ground’s temperature at various depths, scientists can calculate how the surface temperature has changed over time. Boreholes are vertical shafts drilled into the ground, sometimes kilometres deep. Scientists have used more than 600 boreholes worldwide as proxies for changes in temperature over the past 500 years. Boreholes cannot give information about short-term temperature changes, but can show trends on timescales of decades to centuries. Temperature data gained from boreholes show a significant recent warming trend.

Corals and some types of algae form skeletal structures using minerals from the surrounding water. The structures form in bands each year, which can be ‘read’ to reveal their age. Scientists extract coral cores to analyse their characteristics. The ratio of different oxygen isotopes (oxygen-18 and oxygen-16) in each coral band enables scientists to estimate the temperature of the surrounding water the year it formed. The amount of magnesium in the structures of red coralline algae also depends on sea temperature. Warmer water means more magnesium, so the magnesium concentration in each band is a good proxy for sea temperature. Most corals indicate significant warming over the 20th century.

Scientists extract sediment cores from the ground, lake beds and ocean floors. These provide vertical timelines of the past – older sediments are compressed as new sediment settles on top. Some cores contain sediment from up to 200 million years ago. Scientists can determine the age of different layers using radiocarbon dating. The ratio of different oxygen isotopes (oxygen-18 and oxygen-16) in each sediment layer enables scientists to estimate the water temperature when the layer was formed. Some sediments also contain fossils. This gives further information about past climates, since different species flourish in different conditions and some types of plankton grow different-shaped shells. Sediment core data indicate significant warming in the 20th century.

Eric uses ancient ice to learn about climates past. He and his team spend ten weeks in chilly Antarctica, drilling deep into the polar ice. Once they’ve extracted the cores, they transport them to the laboratory in Cambridge for analysis. The content of trapped air bubbles, the amount of salt in the ice and other measurements reveal clues about the climate when the ice was formed. Eric works with experts around the world to unravel these clues and build up a picture of past climate changes. ‘Working out how our planet worked in the past gives us more confidence in predicting how it will behave in future,’ says Eric.

• NOAA: A Paleo Perspective on Global Warming
• ACER: Paleo Proxy Data and Temperature Trends