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 behaviour 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 cms (5 inches) from 1851-2010 to rising sea levels. Without human influences the rise would still have been 9.9 cms (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.
(By Alister Doyle, Environment Correspondent; Reporting by Alister Doyle; Editing by Ruth Pitchford)