Glaciers Account for More Sea Level Rise Than Previously Thought

A new study published April 8 in the journal Nature found that glacier melt is occurring more rapidly than previously thought and accounts for 25-30 percent of observed sea level rise since 1961. The research used a new approach to produce more precise and accurate measurements, improving upon previous studies of glacier contribution to sea level rise.

The international research team, based at the World Glacier Monitoring Service at theUniversity of Zurich, says glaciers lost more than 9,000 billion tons of ice since 1961, raising ocean levels by 27 millimeters. The team used field observations and satellite measurements from over 19,000 glaciers to reconstruct changes in ice thickness.

Columbia Glacier, in Alaska, has been in “catastrophic” retreat since 1982 (Source: NASA/USGS/Google)

The study’s principal author, Michael Zemp, leads the World Glacier Monitoring Service and is involved with various scientific projects in the Department of Geography of the University of Zurich. “Glaciological measurements made in the field provide the annual fluctuations, while the satellite data allows us to determine overall ice loss over several years or decades.” Zemp said in a press release from the University of Zurich. “By combining these two measurement methods and having the new comprehensive dataset, we can estimate how much ice has been lost each year in all mountain regions since the 1960s.”

Glaciers in Alaska were the largest contributors, followed by melting ice fields in Patagonia and Arctic glaciers. Glaciers in different parts of the world make their contributions to sea level rise in different decades. A glacier’s input to sea level rise is determined by their mass and rate of loss. Alaskan and Patagonian glaciers, for example, are not as far poleward as some other glaciated regions. They are melting faster and contributing the most to sea level rise due to their large glacier area. Conversely, Antarctica’s periphery glaciers, situated near the south pole, contributed least to sea level rise during the study period. While glaciers in the western US, Canada, and Iceland, located in even warmer climates than Alaska, lost the most mass. Due to their small total glacier area of those regions, however, they contributed little to sea level rise.

Regional contributions to sea level rise (Source: Michael Zemp/Nature)

Sea level rise is a direct result of climate change, though its local and regional extent and impact varies, and depends on geologic, oceanographic, and atmospheric influence. The primary contributors to ocean volume and mass are from thermal expansion (water expands as it warms) and the addition of melt water from ice sheets and glaciers. Glaciers are made up of fallen snow that, over many years, compresses into large, thickened ice masses, and due to their mass, flow like very slow rivers. As they melt, their runoff contributes to sea level rise. Ice sheets, which cover most of Greenland and Antarctica,  are a mass of glacial land ice extending more than 50,000 square kilometers (20,000 square miles), whose meltwater raises sea levels. An ice shelfis a portion of an ice sheet that spreads out over water. Because ice shelves are already on the water, they do not contribute to sea level rise as they melt.

Understanding the physical processes behind glacier mass loss and its effect on sea level rise is crucial to projecting the impacts of climate change for society. According to the Fourth National Climate Assessment, a congressionally mandated report issued by the US Global Change Research Program, sea level rise this century and beyond will pose a growing challenge to coastal communities, infrastructure, and ecosystems from increased (permanent) inundation, more frequent and extreme coastal flooding, erosion of coastal landforms, and saltwater intrusion within coastal rivers and aquifers. Glaciers are not just icons of climate change; their rate of retreat is an indicator of warming and accurate accounting of their melt is necessary for calibrating models of sea level rise.

Zemp and his colleagues aimed to use updated methods to provide a clearer view of the extent of global glacier loss. “Over 30 years suddenly almost all regions started losing mass at the same time,” said Zemp.  “That’s clearly climate change if you look at the global picture.”

The most significant improvement from the Intergovernmental Panel on Climate Change’s Fifth Assessment Report (IPCC AR5) in 2013, according to the authors, is the volume and accuracy of remote sensing data. Sampling increased from a few hundred glaciers to more than 19,000 globally, with an observational coverage exceeding 45 percent of the glacier area in 11 out of 19 glacier regions. Studies included in that report had to rely on data from 2003–2009, while earlier years had to be estimated. IPCC AR5 documented the sea level contribution of all glaciers globally to be 0.71 millimeters per year.  Zemp’s study found that glaciers contribute 18 percent more than was reported in IPCC AR5, around one millimeter of sea level rise per year.

Matthias Huss, a Swiss glaciologist from the University of Fribourg and Secretary for Glaciers at the Cryospheric Sciences of the European Geosciences Union, was also involved in the study. Huss told GlacierHub, “In comparison to the knowledge included in the last assessment report of the IPCC the increase in remotely-sensed information on glacier mass change is tremendous.” He added, “our study has now attempted to combine all data, also including development of new approaches for optimally combining the available measurements.”

Zemp’s study will be included in the IPCC Special Report on Oceans and the Cryosphere chapters on high mountains and sea level rise, to be published in September of this year. The next IPCC report will be issued in 2021.

Read More on GlacierHub:

Glaciers Get New Protections with Passage of Natural Resources Act

Drying Peatlands in the Bolivian Andes Threaten Indigenous Pastoral Communities

Measuring the Rise and Fall of New Zealand’s Small and Medium Glaciers

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