Glaciers are a sensitive indicator of climate change – and one that can be easily observed. Regardless of altitude or latitude, glaciers have been melting at a rapid rate since the mid-20th century. So far, however, the full extent of ice loss has only been partially measured and understood. Now, an international research team led by ETH Zurich and the University of Toulouse has written a comprehensive study on global glacier retreat, which was published online in Nature on April 28. This is the first study to include all of the world’s glaciers – around 220,000 in total – excluding the Greenland and Antarctic ice caps. The spatial and temporal resolution of the study is unprecedented – and shows how quickly glaciers have lost their thickness and mass over the past two decades.
Rising sea levels and water scarcity What was once permanent ice has shrunk almost everywhere in the world. Between 2000 and 2019, the world’s glaciers lost a total of 267 gigatonnes (billion tonnes) of ice per year on average – an amount that could have submerged Switzerland’s entire land area under six meters of water each year. The loss of ice mass also accelerated sharply during this period. Between 2000 and 2004, glaciers lost 227 gigatons of ice per year, but between 2015 and 2019, the lost mass amounted to 298 gigatons per year. Melting glaciers caused up to 21 percent of the observed sea level rise during this period – about 0.74 millimeters per year. Almost half of sea level rise is attributable to thermal expansion of water as it warms, meltwater from the Greenland and Antarctic ice caps and changes in the storage of l land water representing the remaining third.
Some of the fastest melting glaciers are those of Alaska, Iceland and the Alps. The situation also has a profound effect on the mountain glaciers of the Pamir Mountains, the Hindu Kush and the Himalayas. “The situation in the Himalayas is particularly worrying,” explains Romain Hugonnet, lead author of the study and researcher at ETH Zurich and the University of Toulouse. “During the dry season, meltwater from glaciers is an important source that supplies major waterways such as the Ganges, the Brahmaputra and the Indus. At present, this increased melting acts as a buffer for the people of the region, but if the shrinking of the Himalayan glaciers continues to accelerate, populated countries like India and Bangladesh could face shortages of water. water or food in a few decades. “The results of this study can improve hydrological models and be used to make more accurate predictions on a global and local scale – for example, to estimate the amount of meltwater from Himalayan glaciers that can be anticipated at the over the next few decades.
To their surprise, the researchers also identified areas where melt rates slowed between 2000 and 2019, such as on the east coast of Greenland and in Iceland and Scandinavia. They attribute this divergent pattern to a weather anomaly in the North Atlantic that caused higher precipitation and lower temperatures between 2010 and 2019, thus slowing the loss of ice. Researchers have also found that the phenomenon known as the Karakoram Anomaly is disappearing. Prior to 2010, glaciers in the Karakoram Mountain Range were stable – and in some cases even growing. However, the researchers’ analysis revealed that the Karakoram glaciers are also losing mass.
Study based on stereo satellite images As a basis for the study, the research team used images captured on board NASA’s Terra satellite, which has orbiting the Earth every 100 minutes since 1999 at an altitude of 700 kilometers. Terra is home to ASTER, a multispectral imager with two cameras that record pairs of stereo images, allowing researchers to create high-resolution digital elevation models of all the world’s glaciers. The team used the full archive of ASTER images to reconstruct a time series of glacial rise, which allowed them to calculate changes in ice thickness and mass over time.
Main author Romain Hugonnet is a doctoral student at ETH Zurich and at the University of Toulouse. He worked on this project for almost three years and spent 18 months analyzing satellite data. To process the data, the researchers used a supercomputer at the University of Northern British Columbia. Their findings will be included in the next assessment report of the United Nations Intergovernmental Panel on Climate Change (IPCC), due to be released later this year. “Our findings are politically important. The world really needs to act now to avoid the worst-case scenario of climate change, ”says co-author Daniel Farinotti, head of the glaciology group at ETH Zurich and the Swiss Federal Forestry Institute, Snow and Mountain Research. WSL landscape.
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