Assessing natural climate variability during the past 10,000 years from stalagmite records
|Location||International Geological Congress,oslo 2008|
|Holding Date||17 September 2008|
Stalagmites are an archive for the variability of climate in the past. They generally grow rather continuously during periods of time of several millennia. They can be dated very precisely and the profiles of stable isotopes deliver climate information at a centennial to decadal resolution. An additional advantage is that these continental archives are rather widespread, enabling the comparison of meteorological changes throughout the world.
The composition of stable isotopes in stalagmites is driven by three main processes: those occurring in the cave, in the soil and in the atmosphere. Despite that only the two first processes can be assessed with precision at present, our samples show a consistent pattern of centennial and millennial climate variability in Central Europe.
COMNISPA, that was developed from three stalagmites from the Alpine Spannagel Cave, south of Innsbruck (Vollweiler et al., 2006, Mangini et al., 2007), is an example for a well dated an highly resolved climate reconstruction for the last 9,000 years. This record clearly shows the climate variability since the last Glacial: the warmer periods 7,500 - 6,500 years ago (the so called Holocene Climate Maximum), from 3,800 - 3,600 years, as well as 2,200 years ago (the Roman Warm period) as well as 1,200 - 700 years ago (the Medieval Warm Period). These warm periods are opposed by colder periods dated between 7,900 - 7,500 years, 5,900 - 5,100 years, 3,500 - 3,000 years and 600 to 150 years ago (Little Ice age).
These patterns for climate variability in the past are reproduced in several stalagmite samples on a North South transect, from Scandinavia through Romania. They are also reflected in other archives such as the Holocene Section of the Greenland ice cores, as well as in the temperature reconstruction for the last 2,000 years from 18 non tree ring archives by Löehle, 2008. The stalagmite records thus show a significantly larger variability than the curve of the last 1,000 years by the IPCC.
There is no clear explanation for these climate changes in the past yet. But the climate signals of stalagmites throughout the world show a good correlation to atmospheric Delta14C. Assuming that Delta14C reflects changes of the intensity of the sun, this correlation obviously implies an unknown mechanism that amplified the rather small variability of the solar intensity.