SETH BORENSTEIN Associated Press Yahoo News 14 Aug 14;
WASHINGTON (AP) — More than two-thirds of the recent rapid melting of the world's glaciers can be blamed on humans, a new study finds.
Scientists looking at glacier melt since 1851 didn't see a human fingerprint until about the middle of the 20th century. Even then only one-quarter of the warming wasn't from natural causes.
But since 1991, about 69 percent of the rapidly increasing melt was man-made, said Ben Marzeion, a climate scientist at the University of Innsbruck in Austria.
"Glaciers are really shrinking rapidly now," he said. "I think it's fair to say most of it is man-made."
Scientists fault global warming from the burning of coal, oil and gas as well as changes in land use near glaciers and soot pollution. Glaciers in Alaska and the Alps in general have more human-caused melting than the global average, Marzeion said.
The study is published Thursday in the journal Science.
The research is the first to calculate just how much of the glacial melting can be attributed to people and "the jump from about a quarter to roughly 70 percent of total glacier mass loss is significant and concerning," said University of Alaska Fairbanks geophysicist Regine Hock, who wasn't part of the study.
Over the last two decades, about 295 billion tons (269 billion metric tons) of ice is melting each year on average due to human causes and about 130 billion tons (121 million metric tons) a year are melting because of natural causes, Marzeion calculated.
Glaciers alone add to about four-tenths of an inch of sea level rise every decade, along with even bigger increases from melting ice sheets — which are different than glaciers — and the expansion of water with warmer temperatures.
Marzeion and colleagues ran multiple computer simulations to see how much melting there would be from all causes and then did it again to see how much melting there would be if only natural causes were included. The difference is what was caused by humans.
Scientists aren't quite certain what natural causes started glaciers shrinking after the end of the Little Ice Age in the middle of the 19th century, but do know what are human-causes: climate change, soot, and local changes in land use.
There is a sizable margin of error so the 69 percent human caused can be as low as 45 percent or as high as 93 percent, but likely in the middle.
"This study makes perfect sense," said Pennsylvania State University glacier expert Richard Alley, who wasn't part of the research. "The authors have quantified what I believe most scientists would have expected."
Not all of the human-caused melting is from global warming from the burning of fossil fuels, but climate change is the biggest factor, said Ted Scambos, a scientist at the National Snow and Ice Data Center.
The study showed that it took time for global warming and other factors to build up and cause melting. That lag effect means the world is already locked into more rapid melting from the warming that has already occurred, Marzeion and Alley said.
Science: http://www.sciencemag.org
Man-made warming becomes main cause of glacier retreat, study says
Alister Doyle PlanetArk 15 Aug 14;
Man-made warming becomes main cause of glacier retreat, study says Photo: Mariana Bazo
A view of the lake formed by meltwater from the Pastoruri glacier, as seen from atop the glacier in Huaraz, September 19, 2013.
Photo: Mariana Bazo
Man-made greenhouse gas emissions have become the dominant cause of melting in glaciers from the Alps to the Andes that is raising world sea levels, a study said on Thursday.
Human emissions accounted for an estimated 69 percent of loss of ice from glaciers from 1991-2010, overtaking natural climate variations that had been the main driver of a retreat since the mid-19th century, researchers wrote in the journal Science.
Until now, scientists have struggled to quantify the impact of human behavior on glaciers because the frozen rivers of ice take decades, perhaps centuries, to respond to rising temperatures and shifts in snow and rainfall.
The study published on Thursday used historical observations of glaciers around the world, except in Antarctica, twinned with computer models to simulate all factors that could explain the retreat. It found that natural variations were not enough on their own, meaning man-made greenhouse gases played an increasing role.
"This is more evidence of human influence on the climate," Ben Marzeion, of the University of Innsbruck in Austria and lead author of the study, told Reuters.
The scientists estimated that human influences accounted for only about 25 percent of glaciers' total retreat since 1850 - meaning that natural swings in the climate, such as changes in the sun's output, have long been dominant.
LITTLE ICE AGE
Many glaciers grew during a period known as the Little Ice Age from 1350 to 1850, perhaps caused by a natural decline in the sun's output or sun-dimming volcanic eruptions.
Michael Zemp, head of the World Glacier Monitoring Service at the University of Zurich, said snowfall declined after around 1850. Rising temperatures from about the 1890s, when wider burning of coal meant more greenhouse gases, hastened the thaw.
"The big majority of glaciers have been retreating over the past century," he told Reuters. "We even have an accelerated retreat in recent decades." Glaciers have also varied widely - many Alpine glaciers advanced in the 1970s and 1980s.
Thursday's study estimated that water from melting glaciers has contributed a total of 13.3 cm (5 inches) from 1851-2010 to rising sea levels. Without human influences the rise would still have been 9.9 cm (4 inches).
Zemp said that greenhouse gases already in the atmosphere meant that glacier retreat and related sea level rise would continue for decades, even if emissions were to stop now.
Melting glaciers, especially in the Himalayas, also supply water vital to millions of people. A Chinese newspaper said that Tibet was warmer over the past 50 years than at any time in the past 2,000.
Pinning down a human influence on temperatures has been easier. A U.N. scientific panel said last year that it was at least 95 percent probable that mankind was the main cause of higher surface temperatures since 1950.
(Editing by Ruth Pitchford)
