CG7. Collecting climate change data from ice cores - Teachers’ notes. Vicky Wong. Page 1 of 2 Collecting climate data from ice cores – Teachers’ notes Background information Studying ice might not sound very exciting or like it would provide much information – ice is after all just frozen water. Ice cores come from ice sheets, however, and these along with glaciers and snow contain traces of everything which was stable in the atmosphere when they were formed. All the impurities remain as long as the ice persists. Most water molecules are made of 2 atoms of hydrogen with an atomic mass of 1 and one atom of oxygen with an atomic mass of 16. About 1 molecule in 500 contains the heavier isotope of oxygen, 18O. Occuring even less often is water containing 1 atom of deuterium (D) which is hydrogen with an atomic mass of 2. Elements with the same atomic number but different mass numbers are called isotopes. These isotopes are stable (not radioactive) and do not decay. Measuring the relative proportions of them provides a proxy for temperature. The reason for this is explained below. Rain and snow are formed from water which has evaporated from an ocean, condensed as a cloud and then fallen to the Earth. Heavier molecules have lower vapour pressures which means that when water evaporates the vapour is depleted in these molecules and when the vapour condenses out the condensate is enriched in them. As the air moves from the warm oceans towards the poles, water enriched in the heavier isotopes condenses out. As they do so the water vapour becomes more depleted in the heavier isotope. The result of this is that when the air arrives in Antarctica the amount of water remaining and the proportion of the water which contains the heavier isotopes is mainly temperature dependent. The concentration of 18O or D is normally expressed as the change (δ18O or δD) from average ocean values in parts per 1000. In Antarctica, this value is always very negative, as the temperature is always very low. The colder it is, the fewer heavy isotopes there are and the more negative the isotopic value. It is worth emphasising that the data which is collected in Antarctica shows the temperature of Antarctica. This may or may not be representative of what the temperature was like on the rest of the Earth at the time. The snow in Antarctica does not melt, it just gets compressed by the snow falling on top of it. Initially, air can circulate around the ice crystals and it can still diffuse in and out to a depth of about 60-100 m. At this depth the crystals are so compacted together that they form a solid ice matrix and the air is trapped in tiny bubbles which are unable to diffuse away. As a result of this, the age of the air bubbles is different to the age of the ice. The air that is currently being enclosed in bubbles in central Antarctica at maybe 100 m depth is from the late twentieth century, but the ice (containing the temperature data) at the same depth is hundreds or even thousands of years old (depending on the snow accumulation rate). If scientists want to look at the phasing between climate and trace gas changes in old ice, they have to correct for this ice age-air age difference, and this can lead to substantial uncertainty. How science works How scientific data can be collected and analysed How interpretation of data provides evidence to test ideas and develop theories How explanations of many phenomena can be developed using scientific theories, models and ideas That there are some questions that science cannot currently answer CG7. Collecting climate change data from ice cores - Teachers’ notes. Vicky Wong. Page 2 of 2 Recall, analyse and question scientific information and ideas. Answers to questions 1. Isotopes are atoms which have the same number of protons (or are atoms from the same element) but which have a different number of neutrons (or a different mass number). 2. a. 18 b. 20 c. 19 3. a will have the highest concentration as it is made of the most common isotopes. 4. The oldest data is on the right. 5. By about 12˚C. 6. The concentration of CO2 was generally high at around the same time that the temperature was also high. 7. No, it is not possible to say from this graph which rose first. (For more information on this point, see the background info above.) 8. The concentration of CO2 in 2006 was higher than at any time in the last 600 000 years by more than 50 ppmv. It was more than 350 ppmv where at no time in the last 600 000 years had it previously been in excess of 300 ppmv. Possible response to summary Students should include: Past temperatures Impurities in the ice Concentrations of gases such as carbon dioxide This is over very long time scales – over half a million years.
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