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A fridge or freezer can preserve food for weeks, months or years. But that’s nothing compared to glaciers, with layers of ice preserving information about Earth’s climate for thousands of years.

In some places on Earth, such as near the poles and in some mountainous areas, it’s relatively cold all year round. Year after year, more snow falls, burying the snow from previous years. The layers of snow are squeezed together by the weight of later snow falls, forming large volumes of ice called glaciers.

As long as temperatures remain cold and snow keeps falling, glaciers can keep growing. Some glaciers can be hundreds of thousands of years old.

It’s not just snow that gets compacted into glaciers. Other things that get caught include dust particles, pollutants and pollen. And it’s not just solids that are trapped. As snow is compressed, bubbles of gas from the atmosphere are also preserved in the ice. These bubbles are tiny samples of the atmosphere from the past.

These remnants of atmosphere are of interest to scientists. They can be evidence for changes in the atmosphere, climate and environment. The presence of dust or ash particles in the ice might indicate a volcanic eruption or fire.

Scientists can drill down into glaciers to remove cylinders of ice called ice cores. Different layers of the core correspond to different periods in history, with the oldest layers being found further down the core. Analysis of different layers allows a picture of the past to be created.

In the case of gas bubbles, past concentrations of gases such as carbon dioxide can be compared to present day concentrations. This is a significant piece of evidence to show changes in the atmosphere in recent centuries.

An ice core only contains information from the place where it was collected. There are relatively few climate records from places in the southern hemisphere. Hopefully, by analysing more ice cores from places such as Antarctica, scientists will better understand changes in Earth’s atmosphere and climate.