Global glacier retreat has accelerated
An international research team with participation of ETH Zurich shows: Nearly all glaciers worldwide are thinning and losing mass - and at an accelerating rate. The study is the most comprehensive and accurate of its kind to date.
Glaciers are a sensitive and obvious indicator of climate change. Regardless of altitude or latitude, glacier ice has been melting rapidly since the mid-20th century. But the extent of ice shrinkage has been patchily recorded and incompletely known until now. Now, an international research team led by ETH Zurich and the Université de Toulouse is presenting a comprehensive study of global glacier retreat, published online April 28 in the journal Nature. This study is the first to include all of the world's glaciers - some 220,000 - with the exception of the Greenland and Antarctic ice sheets. It is of unprecedented spatial and temporal resolution - and it shows how quickly glaciers have lost thickness and mass over the past two decades.
Rising sea levels and water scarcity
Almost everywhere, the volume of once-eternal ice shrank. Between 2000 and 2019, glaciers worldwide lost a total of 267 gigatons (billion tons) of ice per year on average. This volume could have submerged the land area of Switzerland six meters every year. During this period, the loss of mass has also accelerated sharply: While glaciers lost 227 gigatons of ice per year between 2000 and 2004, mass loss between 2015 and 2019 was 298 gigatons per year. Glacier melt caused up to 21 percent of the measured sea level rise in the process, or about 0.74 mm annually. Nearly half of the sea level rise is due to thermal expansion of warming water, with the remaining third due to melt water from the Greenland and Antarctic ice sheets and changes in water reservoirs on land masses.
Among the fastest melting glaciers are those in Alaska, Iceland or the Alps. The high mountain glaciers of the Pamirs, Hindu Kush and Himalayas are also severely affected. "The situation in the Himalayas is particularly worrying. The major rivers such as the Ganges, Brahmaputra and Indus are fed to a large extent by glacial meltwater during the dry season. Currently, the increase in meltwater acts as a buffer for the people of the region. However, if the Himalayan glaciers continue to shrink at an increasing rate, populous countries such as India or Bangladesh could face water shortages or food shortages in a few decades," says first author Romain Hugonnet of ETH Zurich and the University of Toulouse. The results can now be used to improve hydrological models and make more accurate predictions on a local and global scale, for example, to estimate how much melt water from Himalayan glaciers can be expected in the coming decades.
To their surprise, the researchers also identified areas where melt rates slowed between 2000 and 2019, such as the east coast of Greenland, Iceland and Scandinavia. The researchers attribute this to a weather anomaly in the North Atlantic. This caused higher precipitation and lower temperatures locally from 2010 to 2019, which slowed ice shrinkage. The research team also uncovered that the so-called Karakoram anomaly is disappearing. Before 2010, glaciers in this mountain range were stable or even increasing. The current study shows that the Karakorum glaciers are now also losing mass.
Stereo satellite images as a basis
The researchers used images taken by the multispectral instrument ASTER on board Nasa's Terra satellite from an altitude of 700 kilometers as the basis for this study. The satellite has been orbiting the Earth once every 100 minutes since 1999. The ASTER instrument uses two cameras to capture pairs of so-called stereo images, which allow researchers to create digital elevation models of all the world's glaciers with high temporal and spatial resolution. Using the ASTER image archive, researchers were able to reconstruct time series of glacier elevations and use them to calculate changes in ice thickness and mass over time.
First author Romain Hugonnet, a doctoral student at ETH Zurich and the University of Toulouse, worked on the project for about three years. For 18 months, he analyzed the satellite data. To process the data, the researchers used a high-performance computer from the University of Northern British Columbia. The results will be incorporated into the IPCC's next state of the world report, due out later this year. "On a policy level, our findings are important. The world really needs to take action now so that we can avert the worst in terms of climate change," says co-author Daniel Farinotti, head of the glaciology group at ETH Zurich and the Swiss Federal Institute for Forest, Snow and Landscape Research WSL.
In addition to the University of Toulouse, the ETH and the WSL, researchers from Ulster University (UK), the University of Oslo and the University of Northern British Columbia, Canada, among others, were involved in the study (for a complete list of participating institutions, see reference).
Press release ETH
Literature review: Hugonnet R, McNabb R, Berthier E, Menounos B, Nuth C, Girod L, Farinotti D, Huss M, Dussaillant I, Brun F, Kääb A. Accelerated global glacier mass loss in the early twenty-first century, Nature, published online 28 April 2021. DOI: 10.1038/s41586-021-03436-z
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