18 June 2008

Ice Cores show abrupt climate changes

The climate changed very abruptly and fundamentally in the course of very few years when the ice age ended, shows new, extremely accurate data from the examination of ice cores from Greenland. Researchers from the Niels Bohr Institute at the University of Copenhagen have, together with an international team, analysed the ice cores from the NorthGRIP drilling through the ice cap in Greenland and the epoch-making new results are published in the highly esteemed scientific journal Science and in Science Express.

The Greenland ice cap is formed from snow that year after year remains and eventually is compacted into ice. The annual layers tell about the climate in the years the snow fell, so the ice is an archive of the climate of the past and ice core drillings through the three-kilometer thick ice cap show the climate 125,000 years back in time.

The last ice age in the northern hemisphere ended in violent temperature fluctuations consisting of two warming periods interrupted by a cold period. The first abrupt warming period occurred 14,700 years ago. The temperature in Greenland rose more than 10 degrees and during the milder climate, called the Bølling Period, the first Stone Age people set off towards Northern Europe and Scandinavia. But the good times were not destined to last. The ice age returned again 12,900 years ago with a new severe cold period that lasted until 11,700 years ago when the ice age finally ended. The Greenland ice cores that reflect the climate in the Northern Hemisphere show that the climate changes occurred incredibly quickly.

The ice age ended in one year
“We have analysed the transition from the last ice age to our current warm interglacial period and there is such an abrupt change in climate that it is as if someone just pushed a button”, explains Dorthe Dahl-Jensen, professor at the Center for Ice and Climate at the Niels Bohr Institute at the University of Copenhagen.

The new data from the ice cores show that the climate shifted from one year to the next. The ice’s annual layers have been analysed at very high resolution for a series of substances, each of which tells something about the climate in its own way.

Dust. The amount of dust has been measured. The colder the climate is, the more dust in the Earth’s atmosphere and more dust is blown in and settles on the ice cap.
Oxygen. The amount of the special oxygen isotope O-18, which tells about the temperature where the precipitation falls, is measured. The higher the O-18 content the warmer the local climate at the location of precipitation.
Hydrogen. The amount of the special hydrogen isotope deuterium is also measured.  An excess of deuterium tells about the temperature where the precipitation (water vapor) originated. Air masses circulate in currents around the Earth and the larger the excess of deuterium, the warmer the climate was in the area of origination.

By comparing the amounts of dust, oxygen, and hydrogen in the ice cores annual layers, researchers can now examine how a shift in climate develops from year to year.

First, the dust content begins to change and decrease to a tenth in the course of a few decades. As the dust in the ice mainly comes from Asia, the researchers conclude that climate changes occurred distant from Greenland.

“A few years later the precipitation changes. Our measurements show that the excess of deuterium in relation to O-18 changes over the course of very few years. This means that the water vapor that flows over Greenland comes from other areas. The conclusion is that there occurred lightning quick and dramatic changes in the weather systems over the Atlantic”, explains Sune Olander Rasmussen, ice core researcher at the Center for Ice and Climate at the Niels Bohr Institute.

The new results help show how climate change happens and which processes are critical for the understanding of the changes.

“Even though the climate changes at the end of the ice age appear to have been most violent in the North Atlantic, our measurements suggest that they are triggered by changes in the tropics. We can also see that when the climate changes dramatically, it is the atmospheric circulation that has changed fundamentally”, concludes Dorthe Dahl-Jensen.

The new information about the climate of the past is important knowledge to improve the climate models that are used to predict the climate of the future.

Link: http://www.iceandclimate.nbi.ku.dk/publications/papers/