Photograph by Cyril Ruoso, Minden Pictures/Nat Geo Image Collection
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A glacial lake sits at the base of the Pastoruri Glacier in the Peruvian Andes. Glaciers here are a critical source of freshwater for drinking and agriculture.

Photograph by Cyril Ruoso, Minden Pictures/Nat Geo Image Collection

Melting glaciers contribute a third of sea-level rise

Thousands of glaciers dot the planet’s high mountain regions. Now scientists know how fast they are melting, and how much they are filling the ocean.

Like an ice cube on a hot summer day, many of Earth's glaciers are shrinking.

Last January, a study in Nature Climate Change showed the world's glaciers are the smallest they've been in human history, revealing radiocarbon material that hasn't been exposed for 40,000 years.

Now, new research published in Nature quantifies how much the world's lost glaciers have contributed to rising sea levels.

From 1961, when reliable record keeping began, to 2016, the ocean crawled up 27 millimeters as a result of ice sloughing off the world's non-polar glaciers. Scientists had known that melting glaciers contribute to sea-level rise, but the new study takes a comprehensive look at how much and how quickly they're melting.

They found mountain glaciers contribute roughly a third of measured sea-level rise—the same contribution to sea-level rise as the Greenland ice sheet and more than the contribution of the Antarctic ice sheet. Their research also highlighted that many of the world’s glaciers may disappear in the next century.

In total, NASA estimates that sea levels rise by three millimeters each year. As oceans warm further, scientists estimate thermal expansion will force sea levels up even more.

How do they know?

The study looked at 19 geologically distinct regions that had been previously segmented by the Randolph Glacier Inventory.

For each of these regions, they relied on field data from the World Glacier Monitoring Service. In any of these regions, field measurements are generally only available for one or two glaciers, says Frank Paul, a geographer from the University of Zurich and author on the study.

To get a more detailed analysis, the researchers used data collected from airborne and satellite surveys to calculate changes in a glacier's volume.

Not only did they find that sea levels have risen as much as 27 millimeters in the past 50 years, they also calculated change in glacial mass from 2006 to 2016, finding that sea level roughly increased one millimeter each year.

“The drama is that it's increasingly negative,” Paul says of the measured mass lost from glaciers. Their data showed that in the 1960s and 1970s, glaciers had predictable seasonal changes, losing mass in the summer and regaining it in the winter. In the 1980s, data showed more was lost than regained, and by the 1990s, all the glaciers they measured showed they were losing more volume than they could replace.

To see how much that contributed to rising sea levels, the researchers then divided the total mass lost from glaciers by the surface of the ocean.

Who will it impact?

Coastal towns are already beginning to feel the impacts of sea-level rise. In the Outer Banks in North Carolina, neighborhoods that once looked over the ocean have begun to fall into it. Major cities like Miami are developing adaption plans for when, not if, seas rise.

Melting glaciers will also impact the inland communities that rely on them.

“In Peru, they really are like water towers,” says Paul.

The Peruvian Andes are home to some of the world's most heavily relied upon glaciers. Since the Inca, Peru's glaciers have been a crucial source of freshwater for human consumption and agriculture. A study published last October in the journal Scientific Reports estimated that the Quelccaya Ice Cap, a region that spans more than 9,000 football fields, could reach a tipping point if emissions aren't reduced in the next 30 years.

In nearby Venezuela, the country is expected to soon lose its last glacier, the Pico Humboldt, though scientists have not yet fully studied how the glacier has changed over time and when it might be completely gone.

In the Himalaya, newly released glacier meltwater is forming dangerous lakes that lead to flooding. An assessment published earlier this year estimated that two thirds of the Himalaya could disappear if carbon emissions aren't reduced in the next century.

The researchers concluded their report by stating some glacial regions in Europe, Canada, the U.S., and New Zealand could see their glaciers completely disappear by 2100.