When drilling down through the glacier to bedrock, each of the layers of snow and ice that we pass through represents the state of the climate and the atmosphere over a successive period of years. The deepest layers are therefore the oldest. In Antarctica or Greenland, the great thickness of the glaciers stretching down thousands of metres and the limited snowfall each year mean that those old layers date far back into the past – as far as hundreds of thousands of years ago.

Mountain glaciers, on the other hand, are not as thick and there is a great deal more annual snowfall. What’s more, the glacier itself flows downwards over the steep slopes. For all of these reasons, the ice found deep down can range from a century to a few millennia old. Sometimes, at the base of these glaciers, you can even find layers formed during the Last Glacial Maximum approximately 20,000 years ago.

What does the future hold for ice core science?

This area of science is still in its infancy. First emerging in the early 1960s, the resulting discoveries (such as the close connection between natural climate change and the amount of greenhouse gases in the atmosphere) were made possible thanks to advances in technology. This progress suddenly enabled the measurement of previously inaccessible chemical elements using new measuring instruments. The advancement of knowledge has also led researchers to study signals present in the ice, which is something they had never even considered before then. The language of the ice has an extremely broad alphabet, and we continue to discover new letters.

At present, nobody can yet accurately confirm what sort of discoveries lie in store for us within the ice archives. However, we can just about make out a few clues, one of which involves biology. While the snow layers lock away solid particles as they are deposited, they also contain living organisms such as bacteria and viruses attached to aerosols and particles and carried by the wind. Once deposited, these living organisms remain in the ice, as if frozen in time. With analysis techniques continuing to be developed, it is safe to predict that, one day, researchers will be able to study the change in the genome of such bacteria or viruses over time. Not only that, but also learn about the mechanisms which lead to the changes seen. On the strength of such progress, perhaps ice core science might even benefit the world of medical research.

Recording the environment

As layers of snow are deposited on the surface of a glacier, they record a wealth of information about the state of our environment. Made up almost entirely of water molecules, these snow layers also contain impurities, which sometimes represent just a few millionths of billionths of the mass of snow deposited.

These impurities may be solid, such as dust released by soils or deserts. As a result, Alpine glaciers contain dust-enriched layers originating from intense sandstorms in the Sahara Desert. This dust has crossed the Mediterranean Sea before being partly deposited on the surface of the glaciers. Successive layers of fresh snow then carry these layers of dust down into the icy depths. Among the solid particles, those released by human activities can also be found, such as heavy metal particles (e.g. lead, zinc, copper, platinum, rhodium, palladium).

The impurities may also be liquid. The term ‘aerosols’ is used to refer to tiny droplets comprising, for example, acids: sulphuric acid from coal combustion, nitric acid emissions from agricultural soils or the conversion of nitrogen oxides emitted by cars or heating sources, hydrofluoric acid emitted by certain industrial activities and organic acids resulting from natural emissions of organic compounds from plants, the burning of those plants or the burning of fossil fuels, etc.

Radioactive pollutants can also leave traces in high-altitude snow. Dissolved in snowflakes or attached to tiny clay particles carried by the wind, these pollutants are seen in the diffuse emission of beta or gamma radioactivity. Consequently, radioactive elements produced by thermonuclear tests conducted during the 1950s and 1960s can be found. Closer to home, the Chernobyl disaster also left its mark on the Alpine glaciers, causing a peak in caesium-137. Other naturally occurring radioactive elements can also be measured in the ice – for example, beryllium-10. It is an extremely rare element, with measurements showing only around 10,000 atoms per gram of ice. It is produced in the upper atmosphere from the interaction of solar wind with nitrogen molecules. Its presence in greater or lesser quantities gives us an insight into the quantity of snowfall over time, as well as changes in solar activity or the intensity of Earth’s magnetic field.
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Recording the climate

The oxygen and hydrogen of the water molecules in snow come in different forms, known as ‘isotopes’, some of which are heavier than others. Accurately measuring the proportions of heavy and light isotopes in the water molecules offers a window to the temperature conditions or the amount of precipitation at the time the snow was deposited.
When the snow sinks deeper into the glacier, it is made denser by the weight of the successive layers above it. As it densifies, the porosity between the snow grains gradually decreases, trapping a sample of atmospheric air. In that gaseous fraction of the layers of ice, we can measure the concentration of trace gases. Such is the case with greenhouse gases: carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, etc.