Clues to the past

Go back in time 50 million years, and Earth looked incredibly different. Crocodiles lived near the North Pole, and the Antarctic coast looked more like our West Coast beech forest than the frozen expanse it is today. At least, that is what we know from fossil evidence. More recent and more detailed records of climate in past centuries to hundreds of thousands of years ago come from ice cores – frozen history.

But how do scientists investigate what Earth was like in the past? Ask a paleoclimatologist – they are the scientists who fit the ancient climate puzzle pieces together.

Climatology versus paleoclimatology

Climatology is the study of weather over a period of time. Since the mid 1800s, climatologists have been using instruments to directly measure climatic patterns. These methodical records provide valuable information, but they only tell us about a very short part of Earth’s overall history.

Paleoclimatology is the study of climate conditions in the geologic past – hundreds of thousands to millions of years ago. Paleoclimatologists are interested in ancient climate conditions along with their causes and effects.

Paleoclimate proxies – how they work

Paleoclimatologists cannot travel back in time to make direct measurements of atmospheric conditions so they use proxies instead. A proxy is a substitute or a representation of something. Experts use clues from the past as proxies to build a picture of what the world once looked like.

For example, climatologists use anemometers to measure wind speed and direction. Paleoclimatologists can use ancient ice samples instead. Salt, originating from sea spray, provides information about the direction and strength of Antarctic winds. By measuring salt concentrations from different depths on an ice core, scientists can learn about wind patterns and variability over long periods of time. In this case, the salt concentrations in the ice core are a proxy for wind speed and direction.

Proxies come from the land, sea and air

Modern climatologists use a variety of measurements when trying to understand the current climate, and paleoclimatologists do the same. They use proxies from the land, sea and air to measure chemical, physical and biological factors to build a picture of past climates.

Ice cores

Ice sheets in the Arctic and Antarctic have built up over hundreds of thousands of years. Each summer and winter season produces a snow layer with its own unique chemistry and structure. Scientists use hollow drill bits to obtain ice cores about 1–2 metres long and several centimetres in diameter. Each ice core is like a vertical timeline – the snow bands tell a story about the weather. In addition, scientists use air bubbles in the ice cores to collect information about the concentration of greenhouse gases in the atmosphere at the time the ice was formed. (Read about this process in Trapped in ice.)

Sediment cores

Sediment cores also provide vertical timelines about the past going back hundreds of thousands of years. Sediments are materials that settle to the bottom of lakes, bogs (wet, muddy areas) and oceans. Geological materials like dust and minerals provide clues about wind patterns, water currents and volcanic events. Biological materials like microscopic shelled organisms, algae, pollen grains and spores provide clues about temperature, nutrient levels, ice cover, oxygen levels and salinity. The article Ice ages unearthed describes how Dr Marcus Vandergoes used sediment cores and pollen to develop a picture of climate change in southern New Zealand.

Fossils

Microfossils like pollen and marine organisms are just part of the fossil story. Macrofossils – such as plant leaves – also help to build up a picture of past climates. The number of stomata on a fossilised leaf provides clues to CO₂ concentrations. Plants need fewer stomata when CO₂ is abundant. Paleobotanists like Professor Jane Francis use this type of information when building up a picture of climate. She says, “What fascinates me is finding all these little bits of evidence and then putting it together. From understanding what the plant was like to understanding what the flora was like, then trying to find out what the whole environment was like.”

In the online citizen science project Fossil Atmospheres scientists are comparing some features of fossilised plants with the same features of plants living today to learn more about the effect of changing carbon dioxide (CO₂) levels.