Collecting climate data from ice cores – teachers notes by Adrian Wegeng

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									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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