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  The poles, archives of the world’s climate

Studying the past climate of our planet is like detective work. The smallest clue is of importance, the slightest irregularity in the composition of ice, submarine sediments or the soil can provide crucial information which highlights ancient jolts to the Earth's climate. But it is still necessary to know where to look, and how to decipher this information. This is what battalions of scientists have been doing for many years in the polar regions.

Polarised light projected through a slice from an ice core reveals samples of air from the past as well as aerosol particles deposited by snowfalls – some of which could be from volcanoes which erupted far from the Antarctic
Polarised light projected through a slice from an ice core reveals samples of air from the past as well as aerosol particles deposited by snowfalls – some of which could be from volcanoes which erupted far from the Antarctic
© CNRS/LGGE
At the highest latitudes, vast quantities of information can be found imprinted in ice. Continental ice such as can be found in Greenland or the Antarctic is often very thick and results from an accumulation of precipitations going back anything up to a million years. Indeed, throughout their formation, these ice caps have trapped gases, molecules and dust which provide clues to historic and prehistoric environmental conditions.

A similar phenomenon also attracts researchers to the floors of our oceans, where our climate and its history can also be studied. There, the stratification of sediments which have slowly accumulated over millions of years reads like a book to those who know how to decipher it. 

These data can be further refined by analysing the pollen found in other locations, such as polar tundra, at the bottom of lakes, in the soil or even underground. In temperate regions such as Europe, natural caves which have formed in limestone landmasses also bear the scars of ancient climatic fluctuations and thus supplement the findings from polar regions.

A gigantic history book
Ice cores, and the tiny bubbles of air they contain, have enabled scientists to read the past like a book.
Ice cores, and the tiny bubbles of air they contain, have enabled scientists to read the past like a book.
© CNRS/LGGE
Reaching thicknesses of almost 3,000 metres in Greenland and 5,000 metres in Antarctica, the gigantic polar ice sheets constitute some of the most precious tools used by paleoclimatologists.

Throughout their formation, these sheets have accumulated countless seasons of snowfall. Each year, during its passage through the atmosphere, this snow has trapped ambient gases, different types of dust or even pollen, and these are compressed every time new layers of snow cover the old ones. Over the centuries, the ice sheet has thus taken the form of a layer cake. It can also be considered as a gigantic book, the pages of which are thin layers in the ice cores extracted during major drilling campaigns. For example, it is possible to detect major volcanic eruptions, or even traces of lead arising from Roman industrial activity 2,000 years ago.

The researchers look for different types of data. For example, analysis of ice core strata also reveals the extent of precipitation through different seasons. The air bubbles trapped by snow and then found in the ice provide clues to the composition of different atmospheric gases in the past.

The isotopic composition of samples makes it possible to determine the temperature at which a layer formed. Study of entrapped particles, their type and their size, provides information on the circulation of the atmosphere. Other physicochemical parameters of the ice, such as its electrical conductivity, provide researchers with further indicators, and by cross-checking these parameters they can reliably reconstitute ancient climatic conditions.

The relationship between the levels of atmospheric CO2 trapped in the form of bubbles and the temperature at which the ice formed can also demonstrate the extent of the greenhouse effect. Over the past 200 years, this greenhouse effect, exaggerated by the worldwide demographic boom and coupled with growing industrialisation and development of the combustion engine, has been recorded in polar ice. Also recorded are the different nuclear tests performed during the 1950s and, of course, major natural cataclysms which have affected the atmosphere.