May 1, 2009

Southern Glaciers Growing Out Of Sync With The North

According to a paper in this week's issue of the journal Science, the vast majority of the world's glaciers are retreating as the planet gets warmer. 

For the last 7,000 years, New Zealand's largest glaciers have moved out of step with glaciers in the Northern Hemisphere, showing strong regional variations in climate, the paper reported.

"This research should provide much more accurate reconstructions of glacial advances worldwide, allowing us in turn to make climate models more accurate," said Paul Filmer, program director in the National Science Foundation's (NSF) Division of Earth Sciences, which funded the research.

During the era of human civilization, the climate has been relatively stable, according to conventional wisdom.  However, the new study challenges this view, showing that New Zealand's glaciers have gone through rapid periods of growth and decline during the current interglacial period, which is known as the Holocene.

"New Zealand's mountain glaciers have fluctuated frequently over the last 7,000 years, and glacial advances have become slightly smaller through time," said Joerg Schaefer, lead author of the paper and a geochemist at Columbia University's Lamont-Doherty Earth Observatory.

"This pattern differs in important ways from the northern hemisphere glaciers. The door is open now towards a global map of Holocene [a geological time period that began about 11,700 years ago and continues to the present] glacier fluctuations and how climate variations during this period impacted human civilizations."

Glaciers are very sensitive to temperature change and snowfall, which makes them well suited for studying past climate.  However, this archive has been widely untapped, due to the difficulty in assigning precise ages to glacier fluctuations.

One way glacial fluxes are measured is by studying the moraines, or rock deposits that glaciers leave behind at their maximum points of advance.  But, until now the methods used for dating moraines, including carbon dating, could be refining the analysis of a method known as cosmogenic dating.  Schaefer and his colleagues were able to assign precise ages to young Holocene moraines for the first time.  They accomplished this by measuring minute levels of the chemical isotope beryllium 10 in the rocks.  This enabled the researchers to pinpoint when glaciers in New Zealand's Southern Alps began to recede, exposing the rocks to cosmic rays.

They constructed a glacial timeline for the past 7,000 years from the results, and compared it with historic records from the Swiss Alps and other places north of the equator.

The team found that glaciers around Mount Cook, New Zealand's highest peak, reached their largest extent in the last 7,000 years about 6,500 years ago, when the Swiss Alps and Scandinavia were relatively warm.  That would put 6,000 years between the times the northern glaciers hit their Holocene peak during the Little Ice Age, between 1300 and 1860 AD.

These results came as a surprise to some scientists that assumed that the northern cold phase happened globally.  New Zealand's record shows other disparities that point to regional climate variations in both hemispheres, such as glacial peaks during classic northern warm intervals such as the Medieval Warm Period and the Roman Age Optimum.

The new chemical and analytical protocols that were developed in Schaefer's cosmogenic dating lab is expected to allow scientists to date glacier fluctuations throughout the Holocene accurately, rounding out the climate picture on the continents.

"With this measure we can go to almost any mountain range on earth and date the moraines in front of the glaciers and produce a similar chronology," said coauthor George Denton, a glaciologist who is a senior professor at the University of Maine and an adjunct scientist at Lamont-Doherty.

Overall, the results showed that glaciers around the world have been declining since 1860, with exception of brief advances in Switzerland in the 1980s, New Zealand in the late 1970s through today, and a few other places.


Image Caption: Scientists have found a record of glacier advances in Mueller Glacier in New Zealand. Credit: George Denton


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