Climate change is man-made shows arctic research – Niels Bohr Institute - University of Copenhagen

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03 September 2009

Climate change is man-made shows arctic research

New research shows that the temperature of the arctic region fell steadily from over 2000 years ago all the way up to a 100 years ago. The cooling was caused by less solar radiation during the summer and the cold temperatures would have continued undisturbed. But around the year 1900 there occured a dramatic increase in temperature and the new research results therefore provide further evidence of man’s influence on the climate. The results are published in the scientific journal, Science.

Icecore drilling through the icesheet in
Greenland. Photo: Centre for Ice and
Climate, Niels Bohr Institute.

There are various methods for determining the temperature of the climate of the past: Ice cores, sea sediments and the annual rings of trees. Ice sheets, like that on Greenland, are formed when snow remains year after year and with time is pressed together into a thick ice cap. The ice contains air bubbles and provide information about the temperature during the year in which the snow fell. By drilling an ice core all the way through the ice cap researchers can determine the temperature for every single annual layer going back thousands of years. Sea sediments form layers in the same way and can tell about the temperature conditions in and around the sea. Annual layers show the growing conditions of the tree for every single year and they also provide information about the climate.

An international team of researchers have collected data from 23 locations arount the Arctic. The data are from north of 600 N in Greenland, Canada, Alaska, Northern Scandinavia, and Siberia.

”The idea is that all the different data is used to see how the climate in the Arctic has developed in the last 2000 years”, explains the Danish ice core researcher Bo Vinter, Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen.

Cold course
The collected results show that the temperature has fallen from 2000 years ago and up through the entire Middle Ages. In this period the summer temperature fell steadily at around 0,20 C pr. 1000 år. The cooling was the result of less solar radiation in the Arctic due to slow changes in the tilt of the Earth and its orbit around the Sun. The cooling culminated during ’the Little Ice Age’ between the years 1600 and 1850, which was the coldest period in the last 2000 years.

The research is based on data from 23 locations as
shown on the map. Blue is icecores, green is
treerings and blue is sea sediments. Map: Bo Vinther.

But even though the amount of summer sun continues to decrease, and the cooling of the temperature in the Arctic therefore should have continued unchanged, the new research shows, that from around the year 1900 the temperature begans to rise 0,70 C.  This coincides with industrialisation in the 20th century and results in four of the five warmest decades in the entire 2000 year long period. Furthermore, the period from 1999 to 2008 was record hot with a temperature 1,40 C higher than expected according to the observed cooling trend.

The temperatures in the Arctic have increased nearly three times faster than the temperatures in the rest of the Northern Hemisphere. This is due to the special phenomenon in the Arctic in that the white ice and snow, which normally reflects the sun, gradually melts away and exposes the landscape, which is dark and absorbs the solar heat. This gives a reinforcing effect, the result of which, is that the processes in the polar regions are especially sensitive and are therefore a parameter for the Earth’s climate.
”Our results are in complete agreement with the climate models and everything indicates that the observed increase in temperature in the Arctic is man-made”, explains Bo Vinther.