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 per cent 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.
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. Some natural warming followed that period.
So while it may seem obvious that human-caused climate change would be responsible for the melting of glaciers, "it's just a little bit more complicated than that, because the glaciers would have been melting anyway," said Graham Cogley, a glaciologist at Trent University in Peterborough, Ont., who co-authored the paper.
Marzeion, Cogley and their collaborators wanted to know how much they would have been melting with and without human influence.
Natural warming used to be dominant
Based on their analysis, the scientists estimated that human influences accounted for only about 25 per cent 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.
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).
Retreat will continue for decades
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.
Retreating glaciers are a concern for several reasons other than the sea level rise they generate. For instance, in the Himalayas, they supply water vital to millions of people that is usually replenished by snowfall on the glacier in the winter. As they retreat, that water source could dry up for good.
Melting glaciers can also pose an additional problem, Cogley said. The meltwater sometimes pools into "dammed lakes." And when those give way, they may generate devastating landslides and floods that can wipe out entire villages.
The researchers used data on 200,000 glaciers around the world, which are compiled in a database called the Randolph Glacier Inventory that Cogley co-ordinates. He said Antarctic glaciers are not included in the study because they are much bigger and behave very differently from smaller glaciers. For example, they tend to lose most of their mass as they break off into the ocean, rather than from summer melting.
The data was fed into computer models developed by Marzeion and his team.
Compared to the human influence on glaciers, pinning down a human influence on temperatures has been easier. A U.N. scientific panel said last year that it was at least 95 per cent probable that mankind was the main cause of higher surface temperatures since 1950.