Dr. Graham Cogley, professor of geography at Trent University, and an international team of scientists led by Dr. Ben Marzeion, of the University of Innsbruck, have found unambiguous evidence for an increase in the loss of glacier mass in recent decades due to human activities. In a paper published today in the prestigious academic journal Science, the team reports that about one-quarter of the global glacier mass loss from 1851 to 2010 is anthropogenic, or caused by humans. But the fraction attributable to human activity has increased steadily, reaching almost two-thirds of the overall loss between 1991 and 2010.
“Everybody knows that if the temperature goes up, ice will melt. But we don’t use thermometers when we measure glacier mass balances, and the glaciers would give us reliable information about climatic change even if the thermometer had never been invented,” said Professor Cogley. “So our finding is an independent nail in the coffin of the belief that climatic change is not mostly our fault. People who still believe that are running out of places to hide.”
Glaciers have been shrinking since about the middle of the 19th century, when the Little Ice Age came to an end. Glaciers respond both to natural climatic influences, such as the changing output of radiation from the sun, and to human alteration of the climate. But the extent of the human contribution has been unclear until now.
Glacier mass loss causes sea levels to rise, changes the seasonal availability of water resources, and increases hazards such as landslides and floods. Melting glaciers have become emblems of anthropogenic climate change, but glacier extent responds only slowly to climatic influences. “It takes glaciers decades or centuries to adjust to climate changes,” said Dr. Marzeion.
Anthropogenic glacier change
The team of scientists simulated glacier changes in Dr. Marzeion’s computer model of glacier evolution. “We [found] unambiguous evidence of a growing human impact on glacier mass loss,” Dr. Marzeion said. “The results are consistent with measured glacier mass balances.”
All glaciers in the world outside Antarctica were included in the study. The recently released Randolph Glacier Inventory, a complete worldwide inventory of all glaciers, enabled the team to run the model. “The inventory provides data for nearly all glaciers on the earth in machine-readable format,” said Dr. Cogley, one of the coordinators of the Randolph Inventory.
Climate models can include different factors driving climate change. By switching the different factors on or off, climate modellers can distinguish between natural and human influences on climatic change, and glacier modellers can use the climate-model outputs to simulate glacier mass changes. “We keep factors such as solar variability and volcanic eruptions unchanged, but then we re-run our models driven also by land use changes and greenhouse gas emissions,” Dr. Marzeion said.
Significant increase in recent decades
The scientists show that only about one-quarter (25+/-35%) of the global glacier mass loss between 1851 and 2010 is attributable to human activities. However, between 1991 and 2010, the human fraction is about two-thirds (69+/-24%). “Up to about 1950, glacier mass loss attributable to human activity is hardly noticeable, but the attributable percentage has increased steadily since then,” Dr. Marzeion said. “We are very confident that it is now dominant, regardless of the fact that the glaciers would have lost some mass anyway.”
The authors of the study also looked at model results on regional scales, but found that, in many regions, measurements are too few to allow natural and human factors to be distinguished clearly. However, in some well-observed regions, such as western North America and the Canadian Arctic, there is indeed a clear human contribution to glacier mass loss.
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