Today climate changes have become a big issue. Our climate seems to be changing fast. What is happening? What mysteries are behind the change? Has it happened in the past, and, if so, how do we know about it?

A major source of climate information is information derived from drilling out ice cores from the Greenland glaciers. Cores with a diameter of 10 cm and 3000 m [3 km!] deep have been drilled out from both Greenland and Antarctic glaciers. Each core is composed of ice layers formed annually. The layers can be counted and their content analyzed.

In the Arctic, a new layer of ice forms yearly from falling snow. The snow contains elements deposited with the snow, such as dust and sand, and in years of volcanic eruptions, volcanic ash from other parts of the world, which can sometimes be used to date a layer. The snow is the product of the evaporation of sea water which, when blown towards the Arctic, cools, condenses and falls to the ground. Water (and snow) is created by the combination of 2 atoms of hydrogen (H) with one of oxygen (O) which we indicate as H2O. As the temperature cools, the oxygen in the water changes slightly: there is less of the heavy O18 isotope [an isotope is a form of an element differing slightly from other forms in atomic mass and physical and chemical properties]. Consequently snow and ice deposited in the summer have a higher promille of O18 than that formed in the winter when it is colder. Since every year contains a winter and a summer, their respective layers distinguishable in the ice, the two together form deposits which can be translated into years. One can also melt selected samples, collect the gases trapped in them and subject them to radiocarbon dating. Using the radiocarbon-dated layers one can date the other layers. From the O18 content we also have an indication of the shift in temperature from year to year. From the glacial cores we have a climate record that goes back 200,000 years. It not only tells us about the climate of the Viking Age but also is a key source of information about global warming today.

Another way to find out about past climate is by the study of marine sediment cores taken from the bottom of the ocean. The content of the sediments changes with the presence of sea ice and the temperature of the water and changes in currents.

Yet another method that tells us a great deal of the climate is dendrochronology, or tree ring dating. A tree forms a new layer yearly, visible as a ring if one cuts a cross-section through the tree. The thickness of each ring depends on the growing conditions. Warm years generally produce thick rings; in cold years the tree grows more slowly. In Scandinavia there is now a tree ring record of the climate, year by year, reaching back 1400 years in time, in Labrador about 600 years.

Finally, there are also historical records of temperature changes. Iceland has the oldest ones in the western hemisphere. They go back to the time of the first settlements. Usually it is the cold years that have been noted, or the years with unusually abundant sea ice. Iceland has a lot of volcanic action and the immediate years after a volcanic eruption were usually famine years because the ash entering the atmosphere acted as a screen against the sun. The prevailing winds also blew the ashes towards Greenland, so also Greenland was also affected even though it has no volcanoes.

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