Glaciers – large slow-moving rivers of ice – are sensitive indicators of the climate. They become shorter when the climate warms and lengthen when it cools. Research recently published on dating glacier moraines has come up with a puzzling picture of the global climate system.
Dating glacier moraines
Arcs of rock debris called moraines mark where glaciers advance to and stop before receding.
Many scientists believe that the Northern and Southern Hemispheres are climatically linked during periods of global warming and cooling (ice ages). In other words, it is believed that they behave in same way during periods of prolonged warming or cooling.
If the moraines of different glaciers around the world could be dated, comparisons would provide information about this connection. Moraine dating could also then be used to identify the beginning of global warming.
A group of 9 scientists, including 3 from New Zealand, have come up with a way to tell precisely how old these moraines are using an isotope called beryllium-10.
Surface rocks are continually bombarded by cosmic rays. Over time, cosmic radiation causes the build-up of certain isotopes in the rock. By measuring the amounts of beryllium-10 in rocks on the moraines, the scientists have been able to pinpoint dates when glaciers started to recede.
They have been using this dating technique to ascertain a reliable timeline of advances and retreats of glaciers in the Mount Cook region over the past 7,000 years.
To their surprise, the scientists found that the growth and retreat of New Zealand glaciers differs from glacier movements in the Northern Hemisphere. Over the past 7,000 years, the Mt Cook glaciers achieved their maximum length 6,500 years ago. They have been smaller ever since. By contrast, Swiss glaciers advanced to their maximum length in the last 700 years.
The dating revealed no simple pattern of variation. Some warm periods in Europe coincide with well advanced glaciers in New Zealand. At other times, glaciers were well advanced in both areas.
This challenges some widely held beliefs about the global climate system and how it functions, says geologist Dr David Barrell of GNS Science, 1 of the 3 New Zealanders involved in the project.
"The New Zealand findings point to the importance of regional shifts in wind directions and sea surface temperatures, such as El Nino-Southern Oscillation. These regional weather patterns are superimposed on global climate trends that are reflected in the behaviour of glaciers."
The new dating technique can now be used on glacier moraines around the world. This information should help improve the accuracy and usefulness of climate models.
The findings were published in the prestigious international journal Science in May 2009.
Find out more about glaciers in this article.
Read the findings published in Science in May 2009.