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Kerstin Treydte

Forschungsanstalt WSL

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Article

Author(s): Forschungszentrum Jülich (2006)
Editorial office: WSL, Switzerland
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Tree-rings show an extensive increase in precipitation

Sauerstoff-Isotop-Analyse
Oxygen isotopes being analysed in tree ring cellulose.
Photo: Jülich Research Centre
 
Wacholderbaum im Karakorumgebirge
A juniper tree in the Karakorum Mountains (Northern Pakistan)
Photo: Kerstin Treydte
 
Stammscheibe eines Wacholderbaums
Stem disc of a juniper tree which died in the high age of 1200 years.
Photo: Jülich Research Centre

High tides, major flooding and snow chaos – it seems to be dripping, raining and snowing more than ever. In the issue of “Nature” from 27th April 2006 a research team from The Federal Institute for Forest, Snow and Landscape (WSL) together with scientists from Jülich, Bonn and Potsdam reported that the amount of precipitation in many regions of the world has actually increased over the past 150 years.

In an extensive comparison the team showed that the amount of precipitation, in many regions of the world, has increased in the past 150 years. This development is assumed to be linked to global warming. Humans are obviously responsible for part of this development, because the amount of precipitation clearly increased with the beginning of industrialisation and the ensuing global warming.

Valuable information from tree-rings

Juniper trees on the slopes of the Karakorum Mountains in northern Pakistan have a hard time. The soil is stony, the winters hard and rainfall sparse. Few people live here and that is why the trees have been able to grow undisturbed for over a thousand years. Their only source of water is rainfall as there are no streams or ground water. Along with the rain water the trees also take up oxygen, which they store in the woody cellulose. Within the tree rings the varying relationship between the oxygen isotopes 16O and 18O can be observed.

This was just what the international research team was looking for. From the trunks of the juniper trees the scientists extracted pencil thin cores and isolated the cellulose from the wood of every tree-ring. "The relationship between the two oxygen atoms, which vary in mass, in the cellulose gave us information about the amount of precipitation during that year", explained Professor Gerhard Schleser from the Institute of Chemistry and Dynamics in the Geosphere in Jülich. Tree rings therefore indirectly store climate information and can be regarded as a climate archive.

Precipitation reconstruction and global comparison

During her time as a doctorand at the research centre in Jülich, Kerstin Treydte of the WSL managed to measure oxygen isotopes in tree-rings dating back to 820 A.D. "We have reconstructed around 1,200 years of climate history for the Karakorum area – and have observed a signification increase in the amount of precipitation over the past 150 years", reports Treydte.

Together with the team of Jan Esper at the WSL she has now evaluated and analysed the data. Because the studied trees are so old the scientists were able to produce a 1000 year oxygen isotope series out of the tree rings. It is the first time, worldwide, that such a long and complete series has been produced from tree-rings. Up until now this procedure was mainly used for ice cores.

Using this series it was possible to reconstruct the development of precipitation in the corresponding area. In order to classify this result globally, the WSL research team compared this data with data collected from other world regions, amongst others with data from southern Germany. Precipitation fluctuates greatly in the various regions from year to year, but by comparing data from several hundred years, an overall increase in precipitation in the 20th Century can be observed. This increase can not at present be regionally, where has it rained more and where less and how often and how intensely is not yet clear. It is much more complicated to record long term changes in spatial precipitation distribution than to determine temperature distribution on the earth. "For that we have to be able to evaluate data from many locations" says Schleser. "This is especially important when one takes into consideration the influence precipitation has on humanity and on the various eco-systems, especially on those areas of the world affected by drought."

Apart from the new precipitation reconstructions from high Asia there are only a few similar, long term precipitation time data series available from other climatic areas. Scientists have smoothed out several of these data series and put them alongside the new series. What is interesting is that all of the series show an especially wet period starting in the 19th Century.

Next the team compared the precipitation curve with the temperature curve. They established that this wet period fell within a period of increasing temperatures which started in the 19th Century. The presumption, therefore, is that the earth’s hydrologic cycle is being intensified due to global warming. This has not just been taking place during the past few years but during the past thousand years.

Outlook

The long term objective of this research is to develop models using this climate data which would predict, amongst other things, long lasting droughts or looking more to the future even century floods. This is why scientists as a next step want to improve the data basis as well as to reconstruct precipitation relationships and the dynamic of atmospheric circulation in Europe.

  • Translation: Dawn Meister, Stallikon

Literature

  • The twentieth century was the wettest period in northern Pakistan over the past millennium. Treydte et al., Nature 7088, 1179-1182, News&Views: 1120-1121.

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