Glaciers and climate
Climate reflects glacial extension in different areas, and changes in glacial extension tells us something about changes in climate. Since the presence of glaciers is mainly controlled by winter snow and summer temperatures, a glacier will reflect changes in these climatic parameters. Large amounts of snow and low temperatures during the winter favours glacier growth, while low amounts of snow and/or high summer temperatures causes glacier retreat.
Today, both changes in front positions and mass balance is measured at several glaciers in Norway and in the rest of the world. Growth or retreat is a reaction to a positive or negative mass balance. But, due to different reaction time, there may be a delay of several years between a measured positive mass balance and glacier growth. Short and steep outlet glaciers, like Briksdalsbreen or Bøyabreen, reacts relatively fast. Long, large and gentle sloping glaciers, like Tunsbergdalsbreen and Nigardsbreen, has slower reactions to changes in climatic conditions.
There are several methods to calculate pre-historic glacial fluctuations. Valuable information can be detected about previous climates, from dating of moraines and analyses on glacial sediments in lakes and fjords. By the drilling of ice cores, samples from air bubbles can be investigated to discover how the amount of greenhouse gases in the atmosphere, like carbon dioxide and methane, has changed through time. These air bubbles were trapped between ice crystals the last time when snow/ice was in contact with the atmosphere. Several years of snow accumulation transforms snow into ice, through repeated compression and thawing - and freezing processes, creating a glacier.
By looking at the composition of the oxygen isotope in water molecules, ice cores can be studied. The isotopic composition of the precipitation reflects temperature at the time when the snow fell. Thus, temperature changes through time can be interpreted. Ages on the analysed air samples can be determined also by counting snow/ice year layers on the large ice sheets. Dark layers represent summer precipitation while light layers represent winter precipitation. These year layers may also give information on precipitation changes.
Ice core drilling is best suited in areas with continuous snow accumulation and low rates of melting. It is favourable in cold areas with none or minimal ice movement, like the central areas of an ice cap. The most important places for reconstructing temperature, precipitation and air composition back in time, are the large ice covers on Greenland and Antarctica. From these ice covers, over 3000 metres long ice cores has been collected. The longest time data series comes from Antarctica due to the lower amounts of precipitation. Researchers has managed to interpret climate over 120 000 years back in time from the Greenland ice cores, while ice cores from Antarctica, makes it possible to tell something about the climate over 800 000 years ago. According to Lüthi and others (2008), the ice core analysis from Antarctica indicates that the atmospheric carbon dioxide content at present is 30% higher than ever before the last 800 000 years.
The glaciers get much attention regarding climate change since they are one of our best climatic archives, and therefore important in knowledge about climate. They are also important as water reservoirs. Approximately 70% of all fresh water on earth is icebound and the glaciers act as stable water supplies to many human beings. According to the Norwegian Water Resources and Energy Directorate (2004) the glaciers in Norway exists in areas with great hydropower potential. Roughly 15% of already exploited water discharge comes from rivers fed with glacial meltwater. Tunsbergdalsbreen is an example of a glacier that feeds large amounts of meltwater into a power station reservoir.