Human activity was responsible for two-thirds of glacial melt, which increased sea levels by 69 millimeters across the world in the past two decades, according to a new study published in Science yesterday.

The rest of the observed glacial melt was due to natural causes.

This is the first time scientists have attributed a certain portion of the glacial melt and the resulting sea-level rise to humans. As such, the study is a landmark as well as a preliminary effort; the scientists are just 85 percent confident of their values.

"They have done an interesting analysis that confirms a big part of our intuition that anthropogenic forcing is important in guiding the glacier response to climate change," said Shad O'Neel, a glaciologist at the U.S. Geological Survey in Alaska who was not affiliated with the study.

Retreating glaciers are a powerful visual of climate change and a key contributor to global sea-level rise together. Over the past decade, glaciers have contributed as much to the oceans as the ice sheets in Antarctica and Greenland. Over the past century, melting glaciers have accounted for half of the global sea-level rise and will continue to play a role, according to the Intergovernmental Panel on Climate Change.

But the reason behind glacier melt has been difficult to resolve.

"It seems so obvious that it is getting warm because of anthropogenic emissions and if it's getting warmer, glaciers are melting," Ben Marzeion, a climate scientist at the University of Innsbruck in Austria and lead author of the study, said over the phone. "The problem with that is, nobody had shown robustly that these glaciers are melting because of anthropogenic emissions."

Worldwide inventory of glaciers
This is because glaciers respond to changes in climate over many decades or even centuries. Some of the melt observed today could be an artifact of events that played out in the distant past.

Even more confusingly, individual glaciers can melt at different rates depending on the topography, elevation and local climate. There is no one equation that could explain the behavior of all the glaciers of the world. Scientists have so far studied in detail only about 350 glaciers out of the 200,000 that exist today.

A major step toward resolving some of the unknowns came last year when scientists released the Randolph Glacier Inventory of all the glaciers in the world.

The inventory was a huge step forward and was used by the IPCC in its latest report released last year, Tobias Bolch, a senior glaciologist at the University of Zurich who helped assemble the inventory, said over the phone.

But the inventory also has its uncertainties, especially in regions like China and India, Bolch said. Glaciers tend to cross over political boundaries and give rise to contested water supplies, which means nations do not readily share information about their glaciers.

Marzeion and his colleagues used the inventory to create a model of how glaciers behave. They then input the results of the glacier model into 12 different climate models. This allowed the scientists to explore the impacts of climate change on glacial evolution.

They found that from 1851 to 2010, glaciers contributed 99 millimeters of global sea-level rise without anthropogenic influence. And with the influence, glaciers contributed 133 millimeters.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500