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  Un glacier tibétain donne une bonne idée du réchauffement en cours
« le: 21 Novembre 2004 - 12:07:51 » par Mathieu
"Il y a trente ans, il n'y avait de rivière ici"

Ces trente dernières années le glacier Zepu a perdu plus de 100 mètres d'épaisseur... Bien entendu cela est le cas aussi pour tous les autres glaciers. Entre 1850 et 1960 les glaciers ont reculé de 7%, entre 1960 et l'an 2000 ils ont reculé de 7,5%. Rien que dans les années 90 ils ont reculé de 4%.

Wednesday, November 10, 2004 07:50
By Howard W. French

YUREN - Seen from afar, it looked like much of the surrounding landscape, even to the scientists who know these Tibetan wilds intimately: the looming, soot-black shoulder of a mountain.

Close up, though, when one could finally see the base, all thick, glistening white ice, now clearly visible after nearly four hours of hiking through thick, pathless forests, there was no mistaking it.

Mountains, after all, don't melt. This was the glacier the researchers had been seeking, covering 70 square kilometers, or 27 square miles.

Pouring forth from the base of this huge mass of ice nearly 3,500 meters, or 11,500 feet, above sea level was a torrent of melting runoff that formed the powerful new headwaters of a mighty river - an infant river, in geological time, already broad and raging from its first few meters.

"Thirty years ago, there was no river here," said Yao Tandong, the director of China's Institute of Tibetan Plateau Research.

Yao, who has spent the last two decades on expeditions like these to study Tibetan glaciers, added, "Of course, there has always been a river downstream, but up here, everything had always been frozen solid."

The glacier, named Zepu, has lost more than 100 meters of thickness, all in the last three decades, largely because of rising temperatures in the region.

And it is hardly unique.

Working with scientists from Ohio State University, Yao has documented similar losses all over Tibet, the largest and loftiest highlands on earth, and home to the biggest concentration of alpine glaciers anywhere.

Nor are these changes limited to Tibet.

"Make no mistake, what's happening to the glaciers in Tibet is happening around the globe," said Lonnie Thompson, a professor of geological science at Ohio State.

"Our measurements show that between 1850 and 1960, the glaciers retreated 7.5 percent," Thompson said. "Between 1960 and 2000, there was a further 7 percent retreat. In the 1990s alone, the glaciers have shrunk by more than 4 percent."

Peter Clark, an Oregon State University geologist who specializes in glaciers and ice ages, agreed.

"Glacial retreat, which is happening globally, with the exception of one area, in Scandinavia, is a pretty widely understood and accepted phenomenon," he said. "Glaciers advance and retreat in response to two things, precipitation and temperature. Certainly by the standards of the last few thousand years, there has been a marked rise in temperature globally."

To be sure, there are vast stores of glacial ice left in Tibet.

Flying into the province, a traveler passes over densely clustered ice-capped mountain ranges that dwarf the Alps, and ice fields that extend as far as the eye can see.

But the dominant impression of a traveler who spent a week driving over on the muddy back roads of eastern Tibet was of a world of water, not ice.

Streaming white waterfalls fed by melting glaciers pour from mountains around every bend in some areas. Cliff-hugging roads are subject to mudslides and waterborne avalanches every few kilometers, and boulder-strewn whitewater rivers churn with uncommon ferocity.

Thompson said he had documented large puddles of melting ice at 6,000 meters in the Himalayas, where for thousands of years all has been frozen.

Thompson, a craggy 56-year-old who says he has spent at least three years of his life at high elevations researching glaciers, spoke during a recent interview in Beijing. He had just completed the latest of many expeditions to Tibet over the last 20 years.

The essence of his work involves retrieving deep ice samples, or cores, from glaciers. He spends much of his year like a migratory bird, traveling from one glacial mass to another, including the Himalayas and mountains in Peru, Kenya and Alaska. "When we go to retrieve a core, it's usually to a place that no one has ever been, and the beauty of ice, which is really different from any other material, is how much you can learn from it," he said.

"It tells you the history of precipitation, of dust accumulation and of wind strength," he added. "It also gives you a history of the atmosphere, including the presence of sulfates, nitrates and chlorides, which are precisely the factors associated with global warming."

In Peru, he said, the Quelccaya ice cap retreated at a rate of more than 180 meters a year from 2000 to 2002 - up from just 4.5 meters a year in the 1960s and 1970s - leaving a vast lake where none had existed when his studies began.

On Kilimanjaro in Kenya, an 11,700-year-old ice cap is projected to disappear altogether in about 15 years.

"When you see the big picture accumulating from many sites, the evidence of drastic climate change becomes quite compelling," Thompson said.

Climate experts and geologists say the consequences of glacial ice melting on this scale are far-reaching. The most important long-term threat, perhaps, is to the low-lying coastal cities around the world - places like New York and New Orleans, or Tokyo and Shanghai - which could see more frequent flooding as a result of rising sea levels in this century.

In other parts of the Himalayas, large newborn lakes are accumulating behind dams of ice that could break, unleashing deadly flash floods.

For his part, Yao is unwilling to make sweeping predictions.

Instead, fixing his gaze on the leading edge of the fast-melting Zepu glacier, he said, "If you come back here in another 30 years, one thing is for sure: There will definitely be no more ice here."

Experts: more obvious global warming found in Mt Everest region

Dingri region in Tibet, the location of Mt Everest, is subject more obviously to global warming. Some experts exclaimed this after they studied the trend of climate variation in Dingri and Jiangzi, respectively the mother region of Mt Himalayas and Mt. Laguigangri.

Their observation showed that in the past few years, annual precipitation, annual mean temperature, winter mean temperature and relative humidity in both regions had been undergone the greatest annual increase. E.g. data in Dingri presented an about 2.6 ¡æ increase in annual temperature increase from 1971 to 1992, 1.7 higher than the same proxy from 1960 to 1970. The annual temperature rose more obviously since 1993. During 1998 align=2002, the annual mean temperature reached as much as 3.3 ¡æ plus, with the period between 1998 and 1999 to the peak with 4 ¡æ increase annually. Moreover the scale of digression in temperature change turned out to be much larger than Lazi or Rikaze in the range of River Yaluzangbu. Thus the experts confirmed that the west part of Tibet was a region of weak ecology, and that it was subject easily to global warming.

Experts also found that precipitation in this region increased, but the rising temperature meanwhile had doubled the evaporation. They attributed this phenomenon to the weaker ecology there than that in River Yaluzangbu range. As there was little plantation, ecology failed to adjust itself properly to the climate system, thus was more easily subject to various natural disasters. They, thereupon, proposed this region to be the weakest ecological region in Tibet.

But there were experts who were skeptical at this hypothesis. Though the region of Dingri is the mother region of Mt Everest, the temperature change there could hardly confirm the existence of the influence global warming had on the whole mountain.

Institute of Tibetan Plateau Research

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